Liquid water is one of the most important media and solvent for chemical reactions, which is also the main object in scholarly investigation of the chemical reactions occurring in condensed (liquid) matter. The composition, structure, and characteristics of water may vary significantly under different conditions, especially under extreme conditions, which may change both the rates (kinetics) and favorability (thermodynamics) of individual chemical reactions in solution. Thus, the condensed matter chemistry under (normal) mild conditions, hydrothermal conditions, and supercritical water conditions may differ considerably. In this review, we discuss the influences of the composition, structure, and characteristics of liquid water and solution on the chemical reactions within, which includes the state and reactivity of the reactants, processes and mechanisms of reactions, compositions and structures of the intermediate and final products. Examples of these reactions are dissolution and crystallization, double decomposition reaction of salts, acid-base reaction, precipitation, gelation and crystallization, hydrolysis, redox reaction, and coordination reactions. In our discussions, we emphasize that it is essential to consider the chemical reactions occurring in aqueous solutions at the level of condensed matter physical science; and similarly it is also essential to investigate chemical reactions occurring in other types of liquids such as organic solvents, ionic liquids, and molecular substances in molten state. We well understand that this review will result in more in-depth discussions and possibly criticisms about the topics of condensed matter chemistry as well as our perspectives among our peer researchers, which will definitely advance this emerging science in liquids and possibly serve as a base for establishing the new discipline of condensed matter chemistry.
Contents
1 Introduction
1.1 Property and structure of liquid water
1.2 Composition and structure of aqueous solution
2 Dissolution and crystallization
2.1 dissolution and conversion of salts
2.2 Solubility under thermodynamics equilibrium state
2.3 Condensed matter chemistry in crystallization
3 Double decomposition reaction
4 Acid-base reaction
5 Precipitation reaction
5.1 Crystalline precipitation
5.2 Non-crystalline precipitation
5.3 Non-crystalline nickel borate
5.4 Silica gel and amorphous SiO2precipitation
6 Gelation and crystallization
6.1 Polymerization of aluminosilicate
6.2 Hydrated aluminosilicate gel and crystallization of zeolites
7 Hydrolysis reaction
8 Redox reaction
8.1 Redox reactions of media water
8.2 pH and redox potential
8.3 Disproportionation reactions
8.4 Influence of precipitation on oxidation-reduction(redox)reactions
8.5 Influence of complexation on redox potential
8.6 Influence of trace amount of oxygen on oxidation-reduction(redox)reactions
Wenfu Yan, Ruren Xu.
Chemical Reactions in Aqueous Solutions with Condensed Liquid State*.
Progress in Chemistry, 2022, 34(7): 1454-1491 DOI:10.7536/PC220325
Liquid water is one of the most important media and solvents for chemical reactions, and it is also the main object for studying chemical reactions in condensed (liquid) state. Under different external conditions, especially under extreme conditions, the composition, structure and properties of liquid water will change greatly, which makes the chemical reactions in it show different characteristics, thus forming three types of condensed matter chemistry under mild conditions, Hydrothermal conditions (Hydrothermal conditions) and Supercritical hydrothermal conditions (Supercritical water condition). This paper mainly discusses the chemical reactions in liquid and aqueous solutions under general conditions.
1.1 Properties and structure of liquid water
Liquid water has many excellent physical and chemical properties, so it can be used as a solvent and medium for a large number of chemical reactions. These properties include a high dielectric constant (associated with distortion and breakage of hydrogen bonds); Higher conductivity (associated with a structure in which H3O+ and OH- are held together by hydrogen bonds); High thermal stability and specific heat capacity; Large surface tension; A certain viscosity. In addition, water itself has many chemical activities, such as hydration, dissociation, hydrolysis, oxidation-reduction properties and coordination properties of water. Therefore, the chemical reaction in liquid water is not only affected by the composition (such as H2O, H3O+, OH- and a large number of water clusters) and structure of liquid water, but also affected by the participation of water molecules in the reaction, which makes the reaction in liquid water more complex. Finally, it is worth noting that water is an essential chemical for all life processes. In this paper, eight main reactions are introduced, including: 1) dissolution and crystallization reactions; 2) metathesis of salts; 3) acid-base reaction in aqueous solution; 4) precipitation reaction in aqueous solution; 5) gelation and crystallization in aqueous solution; 6) hydrolysis reaction; 7) oxidation-reduction reaction in aqueous solution; 8) Coordination chemical reaction in aqueous solution. This paper mainly describes the influence of the composition, structure and properties of liquid water and aqueous solution on the reaction, such as the existing state and structure of reactants, chemical properties, the process and mechanism of the reaction, as well as the composition, structure and properties of products and intermediates, and expounds the main laws and theories. The in-depth study and understanding of reaction chemistry in liquid water (solution) provides an important theoretical basis for the development of modern marine chemistry and salt lake chemistry, and more importantly, for promoting the construction of condensed matter chemistry (reaction chemistry in condensed (liquid) water (solution)), which is an important field in chemical science[1,2].
Many models have been proposed for the structure of liquid water, among which the most practical one is the polyhedral water cluster model with short-range order. In liquid water, molecules are constantly in thermal motion, and the relative positions of molecules are constantly changing. Water cannot have a single, defined structure like a crystal. Because there are still a large number of hydrogen bonds between water molecules, it is generally believed that there are a large number of dynamic equilibrium and incomplete polyhedral connections between molecules, in addition to the irregular distribution of structural fragments and the fragments appearing in the ice structure. Polyhedra take the form of pentagonal dodecahedra and other incomplete polyhedra composed of five-membered rings and six-membered rings. Fig. 1A is a structural model of a pentagonal dodecahedron (512),b and C tetradecahedron (51262) and a pentadecahedron (51263), respectively, formed by water molecules arranged through hydrogen bonds[3].
1.2 Study on the Structure of Aqueous Solution
The structure of liquid water (solution) has a rich and novel physical picture, such as dispersion structure, average vibration structure and transient structure. Pitzer's ionic interaction model is a landmark modern electrolyte solution theory developed after the Debye-Huckel limit law[4,5]. In recent years, the theory of solution structure based on experiments has been greatly developed due to the progress of X-ray diffraction, neutron scattering, EXAFS, Raman spectroscopy, NMR and other characterization techniques. In addition, liquid structure and liquid phase transition are of great significance to the study and understanding of chemical reactions in condensed (liquid) state, mainly liquid water, and are one of the important directions of chemical development in the 21st century.
In ionic solutions, solvent molecules approach the central ion in a specific number and orientation, forming so-called "hydrated ions" or "solvated ions". Such as Hydrated anion and Hydrated cation, "Shared ion-pair" formed by sharing water molecules, "Water molecule split ion-pair", "Contact ion-pair" formed by direct Contact between anions and cations, etc. Aggregates of more than two anions and cations are called Hydrated "ion clusters"[4,5]. The first hydration layer of hydrated ions often shows short-range order. Due to the attraction of the central ion, the ligands in the first hydration layer have a certain number and orientation, and this short-range order will lead to the deterioration of the order of the ligands in the second coordination layer. Of course, molecular thermodynamics will lead to frequent exchange of water molecules. Direct measurement of solution structure can give detailed information on the first and second coordination spheres of short-range order, such as bond lengths, bond angles, and steric configurations within the molecule, discrete distances, angles, and coordination numbers between ion and ion, ion and solvent, and solvent and solvent for solvated ions, solvent-separated ion pairs, and contact ion pairs. If the coordination bond is formed between a water molecule and a metal ion, it can be described by bond length, bond angle, and coordination number; Otherwise, it is better to use the internuclear distance or the distance between the nearest/next nearest neighboring atoms, the angle and the number of nearest/next nearest neighboring atoms or the frequency factor or the hydration number to describe, which can show the image of the static structure of the solution. At the beginning of the 21st century, Fang Chunhui and Fang Yan used fine XRD technology to study the liquid structure of Li2SO4 and MgSO4 at 25 ℃ in detail.It was found that the dominant species in the Li2SO4 solution are two hydrated ions, Li(H2O and SO4(H2O ,In MgSO4 solution, Mg2+ and exist in the form of four hydrated chemical entities shown in Scheme 1 in liquid aqueous solution due to hydration with water molecules[6][7].
In recent years, Fang Chunhui and Fang Yan have continued to combine theoretical calculations with X-ray diffraction, Raman spectroscopy, EXAFS, NMR and other characterization techniques to study the Cl- and of IA group element M+, as well as the composition and structure of hydrated chemical units of borate ions with different polymeric States in liquid aqueous solution.This provides a useful basis for further studies of dissolution-crystallization reactions in liquid water (solution)[8][910][11⇓~13]. Recently, Feng Jiandong et al. Of Zhejiang University invented a microscopic technique that can directly image monomolecular chemical reactions in solution, and achieved ultra-high spatio-temporal resolution imaging, which provides a better basis for studying and understanding the impact of chemical reactions in water on the structure of water and liquid aqueous solution[14].
2 Dissolution and crystallization reaction
2.1 Dissolution and Transformation of Salts in Water
Water has the formula H2O and is a polar molecule. When water exists in liquid form, water molecules will form a huge hydrogen bond network, resulting in stronger interaction between water molecules and higher boiling point of water, as shown in Figure 2. Salts (usually crystalline) consist of cations and anions with ionic bonds between them, such as NaCl, which consists of Na+ and Cl-. When salt solid enters the interior of liquid water, its surface will be covered by water molecules, the negative part (O) of polar water molecules will move closer to cations (Na+), and mutual attraction will occur, while the positive part (H) of water molecules will move closer to anions (Cl-), and mutual attraction will also occur.When these interactions are sufficient to overcome the electrostatic interactions (ionic bonds) between the cations and anions in the solid, the cations (Na+) and anions (Cl-) on the solid surface are detached from the solid surface, enter the water phase, and are surrounded by multiple water molecules (also called polarization or hydration). The electronegative part (O) in the water molecule will be closer to the cation (Na+) than the electropositive part (H), and the electropositive part (H) in the water molecule will be closer to the anion (Cl-) than the electronegative part (O), as shown in Fig. 2. The active sites where the crystal begins to dissolve are often interfaces, defects, and specific grain boundaries, where the absence of symmetric interactions results in atoms with higher energy and easier dissociation from the solid.
The dissolution process of salt solids in liquid water involves three interactions: solute-solute (ionic bonding), solvent-solvent (hydrogen bonding), solute-solvent (polarization). Solids dissolve by overcoming solute-solute and solvent-solvent interactions (endothermic processes), and solute and solvent interactions release energy (exothermic processes, also known as polarization or hydration processes). Dissolution occurs if the released energy is sufficient to offset the absorbed energy.
2.2 Thermodynamic equilibrium solubility
When a chemical species such as salt reaches thermodynamic equilibrium in water, the concentration of salt in a saturated solution is called thermodynamic equilibrium solubility, or equilibrium solubility for short.
Lattice enthalpy affects solubility and also affects the crystallization process from solution when conditions change, that is, it determines which phase crystallizes from the liquid phase first. But the analysis of this problem is much more difficult than the analysis of the reaction process. A widely applicable rule is that salts formed from ions of widely different radii are generally soluble in water; However, the salts formed by ions with similar radii are poorly soluble, that is, compounds with mismatched ion sizes are easily soluble in water.
This trend is illustrated by the use of two familiar classes of compounds, alkaline earth metal sulfates whose solubility decreases from Mg to Ba, and the gravimetric use of Ba2+ to precipitate quantitatively. While the solubility of alkaline earth metal hydroxides increases from Mg to Ba, :Mg(OH)2 is insoluble "magnesia milk", while Ba(OH)2 is a soluble hydroxide used to prepare OH- solutions. The former case indicates that large anions require large cations for precipitation, and the latter case indicates that small anions require small cations for precipitation. Experience has shown that MX-type ionic compounds with M+ ionic radii less than about 0.88 Å (or more) of the X- tend to be very soluble solid crystals.
Before explaining this, it should be noted that the solubility of ionic compounds depends on the Gibbs free energy of the process described below.
The interactions that originally contributed to the lattice enthalpy are replaced in this process by hydration (solvation). The difference is that the contribution of enthalpy effect and entropy effect in the dissolution process is generally balanced and difficult to estimate accurately, especially the entropy change also depends on the order of solvent molecules near the solute. Fig. 3 shows the relationship between the enthalpy of solution of the halide and the difference between the enthalpies of hydration of the two ions, indicating that the enthalpy effect is important at least in some cases, and the enthalpy of solution is related to the difference between the enthalpies of hydration of the constituent ions. If the enthalpy of hydration of the cation is greater than the enthalpy of hydration of the anion (reflecting the difference in radii of the two ions) or vice versa, the dissolution process of the salt is exothermic.
The change of enthalpy can be explained by the ionic model, and the lattice enthalpy is inversely proportional to the distance between ions:
The enthalpy of hydration is the addition of the individual contributions of the two ions:
If the radius of an ion is relatively small, the value of the term related to the ion in the hydration enthalpy is relatively large. In the expression of lattice enthalpy, a small ion cannot make the denominator smaller by itself, that is, a small ion can lead to a large hydration enthalpy, but it may not lead to a high lattice enthalpy. Therefore, the volume mismatch of anions and cations can lead to exothermic dissolution. If both ions are small, both the lattice and hydration enthalpies may be large, resulting in a weaker exothermic effect during dissolution.
2.3 Condensed Matter Chemistry in Crystallization Reaction
2.3.1 The composition and structure of liquid aqueous solutions and the hydration composition and type of crystallization reaction products
The precipitation of crystals with specific composition and structure from aqueous solution by crystallization reaction is a very complex condensed matter chemistry problem, which is closely related to the following three aspects: (1) the formation of crystal nuclei in mother liquor with specific composition (H3O+, OH- and ion hydration unit) and structure; ② crystal growth; ③ Solubility and supersaturation in liquid water. The solubility and solubility curve in aqueous solution are the basis for the growth and precipitation of crystals. Dissolution equilibrium (positive temperature coefficient of solubility) of MgCl2·6H2O in water is illustrated below as an example.
In the binary system of MgCl2-H2O, the solubility of MgCl2 increases with the increase of temperature. Fig. 4 shows the solubility curve and phase diagram of the binary system at different temperatures, and the specific concentration values and solid phase composition are shown in Table 1. As can be seen from fig. 4, there are two kinds of solubility, equilibrium solubility and metastable solubility. In Fig. 4, the part plotted with a solid line represents the thermodynamically stable equilibrium solubility of this system. Curve BD represents the equilibrium solubility curve of MgCl2-12H2O.
The phase regions of various hydrates in the figure are denoted as MgCl2·12H2O(S12), MgCl2·8H2O(S8), MgCl2·6H2O(S6), MgCl2·4H2O(S4) and MgCl2·2H2O(S2), respectively. The line segments DE, EK, KL, and LM represent the thermodynamic equilibrium solubility curves of MgCl2·8H2O(α), MgCl2·6H2O, MgCl2·4H2O, and MgCl2·2H2O, respectively. The equilibrium solubility temperature coefficients of all hydrated salts of magnesium chloride are positive.
What kind of MgCl2·xH2O crystals can be precipitated at a certain temperature mainly depends on the structure of hydrated anions and cations in the liquid mother liquor, which is determined by the formation of "nuclei" with specific structures under different conditions. A MgCl2·12H2O crystallized at a low temperature (< -16.3.degree. C.) will be described as an example. Fig. 5 is a MgCl2·12H2O of a layered structure composed of a square [Mg·6H2O]2+ and a rhombic [Cl(H2O)6]- sharing a common vertex.
The structure of MgCl2·12H2O is that Mg2+ and Cl- are hydrated in liquid water at low temperature to form very regular Mg(H2O)6 octahedron and distorted Cl(H2O)6 octahedron (Cl-O: 3.11 ~ 3.2626 Å), forming the crystal nucleus as shown in Fig. 5. The coordination groups of each octahedron share four vertices, that is, the Mg(H2O)6 is shared with four Cl(H2O)6, and the Cl(H2O)6 is shared with two Cl(H2O)6 and two Mg(H2O)6[18]. The composition of the layers is MgCl2(H2O)12, and there are hydrogen bonds between water molecules between the layers, which pull the layers together. The water molecule attached to the Mg2+ has two neighboring species (H2O or Cl-),The other half of the water molecules have four nearby species (2 Cl- and 2 H2O or 1 Cl- and 3 H2O) in a tetrahedral distribution. This structure perfectly illustrates the behavior of water molecules in the formation of crystal nuclei of such hydrates and the formation of macroscopic crystals through crystal growth.
It can be seen from the EK section of the solubility curve of magnesium chloride in the range of 3.4 ~ 116 ℃ that due to the increase of temperature, in the saturated mother liquor, the Mg2+still exists in the state of Mg(H2O , while the Cl- loses hydrated molecules due to the increase of temperature and becomes the state of non-hydrated Cl-.In this temperature range, the Mg(H2O in the mother liquor recombines with Cl- to form the "crystal nucleus" of Fluorite structure (in which each water molecule is associated with two Cl- anions), and then crystallizes to precipitate as MgCl2·6H2O crystals[19]. As the temperature continues to rise, the degree of hydration of the Mg2+ in the mother liquor also decreases, so that the composition of the solid phase corresponding to the solubility curves KL and LM also changes to MgCl2·4H2O and MgCl2·2H2O.
In 1975, Wells summarized the MxXy·zH2O of inorganic salts crystallized from liquid aqueous solution with specific composition and structure under different temperature and other conditions (the hydration coordination number of cation M is n,The anion X is a halide or oxysalt (, , , anion, and OH-, etc.) hydrate. They have different hydration compositions and structures (Z/xn), and are divided into two categories (fully hydrated and incompletely hydrated) according to the hydration degree of cations. The specific compositions, structures, crystallization reactions and conditions are shown in Table 2[20][21].
In the dissolution-crystallization reaction under different external conditions, the change of external conditions such as temperature, concentration and the participation of impurity components will affect or even determine the dissolution-crystallization reaction in liquid water with different compositions and structures.A large number of hydrated crystals (metastable phases) with different structures will appear, the main reason is that the composition and structure of hydrated ions in solution will inevitably change with the change of temperature. These changes will lead to the changes of hydration enthalpy and hydration entropy, which will affect the solubility, the formation of core structure and the growth of crystals, thus precipitating the hydration state of crystals and causing metastable phase and supersaturation.
Dissolution of double salt in liquid aqueous solution. With the development of research on the dissolution of crystal salts, it was found that when the double salt with crystal structure, such as carnallite (MgCl2·KCl·6H2O) composed of potassium chloride and magnesium chloride, was dissolved in water, the concentration-temperature (C-T) curve of KCl appeared a great turning point[22]. Another example is schoenite (K2SO4·MgSO4·6H2O), many sulfate double salts of Na2SO4·MgSO4·4H2O, and ulexite, etc., which are all "disproportional dissolution" when dissolved, and will be transformed. After a certain salt reaches supersaturation, it will gradually reach equilibrium, which becomes the characteristic of dissolution and solubility curve[23].
The effect of hydrogen bonding on solubility in liquid aqueous solutions. The two main chemical processes of the Solvay or Ammonia-soda process are shown in Equations 4 and 5.
NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
2NaHCO3 → Na2CO3 + CO2↑ + H2O
The above reaction can proceed because the solubility of the NaHCO3 (such as 15℃8.8 g/100 g,30℃11.1 g/100 g H2O) is much lower than that of the Na2CO3 (such as 30℃50.5 g/100 g H2O) at the same temperature,The reason is that the following form of H bond is generated between the HC ions :,(HCO3 polarization deformation between the macroanion and Na+ leads to the increase of lattice energy,The solubility of the NaHCO3 is greatly reduced, thereby greatly improving the yield of the Na2CO3.
This soda route is based on the following metathesis reaction (Equation 6), and the metathesis product, NaHCO3, can be decomposed by heating to produce soda ash (Equation 5).
Since the raw materials of brine, NH3 and CO2 are cheap, and the NH3 and CO2 can be recycled, the above alkali production process is of great economic value.
2.3.2 Crystallization reaction in supersaturated solution
Another important problem in the precipitation of crystals by crystallization reaction is the supersaturation phenomenon. Liquid solutions have different degrees of supersaturation, and supersaturation is a prerequisite for the precipitation of crystals from solution. The supersaturated state is thermodynamically unstable, and the degree of instability is different in the whole supersaturated region. The closer to the solubility curve, the more stable the solution state is, and the farther away from the curve, the more unstable it is. The relationship between spontaneous crystallization and supersaturation can be shown in Fig. 6. In addition to the solubility curve, there is an oversolubility curve, which divides the supersaturated solution into metastable and unstable regions. The oversolubility curve is not as definite as the solubility curve and is easily affected by some external factors, and this view has been controversial.
However, whether the oversolubility curve really exists or not, there is no doubt that there is a metastable region of liquid mother liquor near the solubility curve in the supersaturation region. The whole solution state diagram (Fig. 6) can be divided into three regions: stable region, metastable region and unstable region, in which the stable region is determined, and the metastable region and unstable region are variable to a certain extent, which is difficult to strictly distinguish. The characteristics of these regions are as follows: the stable region, that is, the unsaturated region, where crystallization is impossible; In the metastable (supersaturated) region, spontaneous crystallization is generally not easy to occur; Unstable (supersaturated) region in which the mother liquor nucleates spontaneously and crystallization occurs.
There are two other important reasons for attaching importance to the study of supersaturation in crystallization reactions.
First,The object of the crystallization reaction from the aqueous solution is generally the Cl- of the group IA elements Li+, Na+ and K+,Ionic compounds composed of N , , , and B4. They all have high solubility and can form different types of hydrated chemical units after dissolving in water, such as the four hydrated ions in MgSO4 solution listed in Figure 1. It is a rather complex chemical process to make these complex hydrated chemical units react and nucleate. Fang Chunhui and Fang Yan et al. Have further theoretically calculated the structures (conformation, internuclear distance and phase angle) of the "shared ion pair" and "contact ion pair" of water molecules between the two chemical individual :Mg2+ and the of the above MgSO4 in the supersaturated solution at 25 C.Compared with the data in the crystal structure of the possible hydrate MgSO4·xH2O(x=7,6,4), the following conclusions are obtained: when there are more "shared ion pairs" of water molecules in the solution, it is beneficial to the formation of MgSO4·7H2O crystal nuclei; if there are more "shared ion pairs" of water molecules in the solution, it is beneficial to the formation of metastable phase MgSO4·6H2O crystal nuclei[25]. This is why it is often encountered that even if the solution is supersaturated, it is not easy to form "specific" crystal nuclei and crystallize, especially for the crystallization process of polyborates with complex structures, which are not easy to crystallize even in the case of large supersaturation[26].
Secondly, generally speaking, the growth rate of crystal always increases with the increase of the supersaturation of solution, but it is difficult to maintain the same supersaturation of the whole crystal plane and keep it constant with the increase of the supersaturation, and at the same time, the dissolved impurity components are easy to enter the crystal and lead to the destruction of crystal uniformity. As a result, the growth rate of the damaged crystal face is always greater than that of the smooth face, so that a change in the relative growth rate occurs, thus affecting the growth morphology of the crystal. It has been experimentally demonstrated that the crystal morphology changes when the supersaturation of the solution exceeds a critical value. For example, in the case of low supersaturation, the reaction precipitates sodium chloride that crystallizes into a { 100 } simplex; However, in the case of high supersaturation, it crystallizes into a { 111 } simplex. For example, cesium chloride crystallizes into { 110 } simplex at low supersaturation and { 100 } simplex at high supersaturation. The effect of supersaturation on the morphology of crystal growth may be due to the different effects of different crystal planes on water molecules. For example, in the case of high supersaturation, the { 111 } plane of sodium chloride is a polar plane, and the electrostatic field perpendicular to this plane is particularly strong, so the adsorption of water molecules is very strong. However, in the same case, the electrostatic field of the { 100 } surface is very low, so it has a weak effect on the adsorption of water molecules. In this way, the repulsion of adsorbed water molecules becomes the main factor limiting the growth rate, resulting in the decrease of the growth rate of { 111 } simplex[27].
2.3.3 The composition and structure of liquid and aqueous solutions and the hierarchical structure of crystalline products
Function-oriented research and understanding of the relationship between the function of crystalline substances and the specific multi-level structure is the basis for the creation of new functional materials. By summarizing and developing the crystallization reaction in aqueous solution with specific composition and structure under certain conditions, including the effect of supersaturation on the crystallization reaction described in 2.3.2.It is an important content and approach in "crystal formation science" to synthesize "synthetic crystals" with multi-level structures, such as grain morphology, size (nanometer, micron) and uniformity, interface, various types of defects, intergrowth and twins of crystallization, etc., which contains rich condensed matter chemistry[28].
3 Metathesis of salts in aqueous solution
When two salts are dissolved in liquid water, the salts with a specific molar ratio will undergo metathesis reactions such as hydration, dissociation, dissolution, and recombination between ions in liquid water. The conditions of the system (including the water content of the reaction system, reaction temperature, dissolution of solid state, composition and structure of liquid state, type and concentration of ions, etc.) will affect and determine the formation and structure of "nucleus" in liquid water, as well as the rate of crystal growth, resulting in different types of hydrated salts, double salts and solid solutions with different complete crystal States. Finally, according to the solubility of solid products (including supersaturation solubility phenomenon), the formation and precipitation of metathesis reaction products from aqueous solution were determined[29,30]. The reaction of soluble NaCl with MgSO4 in water is discussed below as an example. In order to fully understand the progress and results of the reaction, it is necessary to carefully study the phase equilibrium relationship and phase diagram (including equilibrium and metastable conditions) of the quaternary interactive hydration system of Na+, Mg2+‖Cl-, -H2O at different temperatures, as well as the role and influence of the composition and structure of liquid water in the metathesis reaction.
Example 1 metathesis reaction and temperature
Many scholars have studied in detail the isothermal phase diagrams of the hydrated system of Na+, Mg2+‖Cl-, -H2O at different temperatures, including thermodynamic equilibrium solubility and non-equilibrium solubility. The systematic study of the above phase diagram system is helpful to comprehensively understand the metathesis reaction of NaCl and MgSO4 in liquid water. Fig. 7A shows the phase diagram at 0 ℃, and the crystal product of mirabilite Na2SO4·10H2O (Formula 7) can be obtained by the metathesis reaction of 2NaCl-MgSO4 with a wide range of reactant molar ratios on the diagonal[31].
Fig. 7 is the phase diagram of Na+, Mg2+‖Cl-, -H2O quaternary interactive system at 0, 25, 55 and 75 ° C. When the temperature increases, the metathesis reaction between NaCl and MgSO4 is not as good as that at 0 ℃, but as the temperature increases to 25 ℃, 55 ℃ and 75 ℃, the metathesis products are often a variety of double salt phases, such as Na2SO4·MgSO4·4H2O(Ast), 3Na2SO4·MgSO4(Van) and 6Na2SO4·7MgSO4·15H2O(Loe), in a specific range of reactant molar ratios. This is because when the temperature rises, the solubility of salts in the three phase regions at 0 ℃ will increase, and the concentration of related ions in the liquid phase will increase greatly, which will make the ions diffuse and transport to the low concentration, and then recombine with each other to form complex salts with different compositions and structures.Products such as Ast type at 25 ℃, Van type and Ast type at 55 ℃, and Loe type double salt at 75 ℃ (Fig. 7 B Formation of double salt phase region in 25 ℃, 7 C 55 ℃, 7 d 75 ℃ phase diagram).
Example 2 Metastable phase chemistry study of the quaternary interactive system (Na+, Mg2+‖Cl-, -H2O) at 35 ° C[33]
The supersaturation phenomenon described in the previous section is the metastability phenomenon, which is ubiquitous in nature. In the process of natural evaporation of salt lake brine or seawater in salt pan, the structure of the "nucleus" formed and the crystal growth behavior are often changed due to the change of natural conditions, resulting in the change of the crystallization sequence and the stable equilibrium phase diagram of salts.It presents a metastable equilibrium and has the following characteristics: (1) Some phase regions in the stable phase diagram may change in the metastable phase diagram, such as expansion, contraction, or even disappearance, or even the appearance of new phase regions, but some phase regions may not change; (2) Some solid phases in the metastable phase diagram do not appear because they are difficult to "nucleate" in the mother liquor, and their phase regions will be divided by adjacent regions, but the new boundaries formed still obey the thermodynamic equilibrium relationship between them, that is, the number of divisions can be determined.
The quaternary interaction system of Na+, Mg2+‖Cl-, -H2O has been well studied. The following also takes this system as an example to study the stable phase diagram of this system at 35 ℃ (Fig. 8), which includes five eutectic points (see Table 3 for the chemical formula and Chinese name represented by the abbreviation): (NaCl + Ast + Hex), (NaCl + Bis + Tet), (Ast + Eps + Hex), (NaCl + Th + Ast), and (NaClHex + Tet); seven crystalline regions[33]. However, there are few studies on the metastable solubility of the system at 35 ° C, and Jin Zuomei et al. Reported that the metastable quaternary system has only three eutectic points containing sodium chloride at 35 ° C: (Th + Ast + NaCl), (Ast + Hex + NaCl) and (Tet + Bis + NaCl) when studying the quinary system (Na+,K+,Mg2+‖Cl-, -H2O)[34].
Moreover, it can be seen from Fig. 8 that a new crystallization region, Ko crystallization region, appears in the metastable phase diagram, while the crystallization regions of Ast and Th become smaller, and the crystallization regions of Eps and NaCl expand. The metastable total saturation point data obtained by Jin Zuomei et al. Are close to the stable total saturation point data in other literatures, but quite different from the metastable total saturation points obtained by simulating the natural evaporation conditions of salt lake brine in the Qaidam Basin[34]. This may be due to the rapid evaporation of salt lake brine in Qaidam Basin, which leads to obvious metastable phenomenon and obvious difference from the stable equilibrium phase diagram.
4 Acid-base reaction in aqueous solution
According to the acid-base proton theory, also known as the Br Brönsted-Lowry acid-base theory, the essence of the acid-base reaction is the transfer of a hydrogen ion (H+) from an acid to a base. At the same time, Lewis, an American chemist, put forward the acid-base electron theory, which holds that acid is any molecule or ion that can accept electron pairs, is the acceptor of electron pairs, and must have an empty orbital that can accept electron pairs. A base is a molecule or ion that can donate an electron pair. It is the donor of an electron pair and must have an unshared lone pair of electrons. The acids and bases are bonded by coordinate bonds without electron transfer. Many examples show that Lewis's electronic theory of acids and bases has a wider range of applications, but it is not perfect, for example, it can not be used to compare the relative strength of acids and bases. At present, there is no universal acid-base theory that is applicable to all situations.
According to the acid-base proton theory, the strength of acid and base refers to the ability of acid to give protons and the ability of base to accept protons. The strength of acids and bases depends first on their ability to give or accept protons, and secondly on the ability of solvents to accept or give protons. Strength and weakness are relative. In the same solvent, the relative strength of acid and base depends on the nature of each acid and base, while the relative strength of the same acid or base in different solvents depends on the nature of the solvent. For example, acetic acid is a weak acid in water, becomes a strong acid in liquid ammonia, and behaves as a base in anhydrous sulfuric acid. This is because liquid ammonia has a stronger ability to accept protons (alkalinity) than water, which promotes acetic acid to give protons. Anhydrous sulfuric acid has a stronger ability to give protons (acidity) than water, which promotes acetic acid to accept protons. It can be seen that under the action of different solvents, the acidity and alkalinity of substances can be weakened, weakened and strengthened, acid can be changed into alkali, and alkali can be changed into acid. Therefore, in order to compare the strength of various acids and bases, it is necessary to fix a solvent as a reference standard (generally water as a reference standard) to compare the ability of various acids and bases to transfer protons. By measuring the dissociation constant Ka or Kb of various acids and bases in water, their relative strength can be compared according to their size. The dissociation constant Ka or Kb is mainly determined by the composition and structure of acid-base molecules, and is also subject to the external environment (the structure and temperature of aqueous solution, etc.). For the dissociation constants of oxoacids, Pauling summarized a semi-quantitative empirical rule: (1) For oxoacid OpE(OH)q, the pKa≈8-5p, where p is the number of bonds between the central element and O, and Q is the number of bonds between the central element and OH; (2) For polybasic oxoacids (Q > 1), two adjacent dissociation constants differ by about 5 orders of magnitude[35]. When there is no bond between the central element and O in the molecule, but only the bond between the central element and OH, that is, p = 0, the first order pKa≈8, when the central element forms a bond with O, the first order pKa≈3, and when it is regarded as two bonds, its first order pKa≈-2. Sulfuric acid O2S(OH)2, for example, has p = 2 and Q = 2, so its is ≈ -2 (strong acid) and its is about + 3, but its experimental value is 1.9, indicating that Pauling's rule is still an approximate rule. Taking phosphoric acid OP(OH)3 as an example, three P-OH can be dissociated into three protons, which is a tribasic acid. According to Pauling's rule, the pK1≈3,pK2≈8,pK3≈13 can be calculated, which is not much different from the corresponding experimental value. Under the action of the + 5 P center ion with strong polarization ability in the phosphoric acid molecule, the OH bond associated with it will have strong polarity. When the phosphoric acid is dissolved in water, under the action of the strong polar water molecule, the layered structure will be disintegrated, and the proton in the P-OH will be dissociated and transferred[36]. When the first H+ of H3PO4 dissociates (pK1=2.15), the surrounding H2O molecules will rearrange.A new hydrogen bond network is formed around H2, affecting the dissociation of the second H+ (pK2=7.21). Similarly, when the second H+ dissociates, the surrounding H2O molecules form a new hydrogen bond network with , which affects the dissociation of the third H+ (pK3=12.36).
In general, for a pair of conjugate acids and bases, the acid dissociation constant Ka, the dissociation constant of the conjugate base Kb, and the proton self-transfer constant of water (i.e., the ion product of water) are related by :Ka×Kb=Kw.
In aqueous solution, the ionization process of acid is actually the process of transferring H+ to H2O. After the acid transfers H+ to H2O, the species formed is hydronium ion H3O+(Hydronium) as shown in Fig. 9[37].
H3O+ ion is also known as proton hydrate, Oxonium ion, oxonium ion, or Hydronium ion. Various experimental studies have identified the pyramidal H3O+ ion as a stable entity. The O — H bond has the same length as in ice, and the cone angle is between 110 ° and 115 °.
Protons in water migrate rapidly from one oxygen atom to another, and the lifetime of a single H3O+ ion in water is about 10-13s[38]. The rate of reaction of H3O+ with a base in water (such as OH-) is very fast, but the process is controlled by diffusion.
In the H2O molecule, the distance between H and O atoms is about 100 pm, hydrogen bonds exist between water molecules, and the distance between O-O is about 276 pm. If the distance between the H atom and the two O atoms changes, moving closer to the O atom with which it is hydrogen-bonded, the water molecule dissociates, also known as hydrolysis, to form H3O+ and OH-. Its ionization equation is :H2O+H2O≒H3O++OH-, abbreviated as H2O≒H++OH-, which is an endothermic process. Water is an extremely weak electrolyte, and the ionization process is extremely difficult to occur. Only one water molecule in about 55.5×107 water molecules is ionized to produce equal numbers of H+ and OH-. In aqueous solution, the H+ concentration is often expressed in terms of pH, which is defined as :pH=-lg[H+], where [H+] is the H+ activity, which can be considered to approximate the molar concentration of H+ in a very dilute solution. The experimental results show that only 1×10-7mol of water molecules are ionized in 1 L pure water at 25 ℃, and the concentrations of H+ and OH- are both 1×10-7mol/L.The ion product of water, Kw=[H+][OH-]=1.0×10-14M2, is a constant that depends only on temperature, so the pH value of pure water at 25 ℃ is 14 7,pKw. Its value is also different when the temperature is varied, with Kw being 0.113×10-14 and 5.474×10-14 at 273 and 323 K, respectively. When an acid, base, or salt is dissolved in pure water, the solute can undergo ionization or hydrolysis, resulting in a change in the concentration of H+ or OH-, but not in the pKw of the solution.
It is shown that the hydrated proton may exist not only in the form of H3O+, but also in the form of H(H2O (positive integer n > 1) clusters.Such as H5 (called Zundel cation) and H9 (called Eigen cation)[39,40][41⇓~43]. In that of the H5, two wat molecules are symmetrically position on both sides of a proton, and in the of the H9, three water molecules symmetrically polarize a H3O+ ion,The structural formula is H3O+(H2O)3, and there is a hydrogen bond between H3O+ and H2O, as shown in Fig. 10.
A freezing point depression study has shown that hydrated ions in the form of H3O+(H2O)6 also exist in cold water, that is, each H3O+ is surrounded by six H2O molecules, because these water molecules have strong interactions with H3O+, so they no longer have polarization ability for other solute molecules[44].
In addition, ESI-MS studies of water structures have shown that some of the hydrated structures in the gaseous state are quite large in size, such as the "Magic" hydrated ion H3O+(H2O)20 (called "Magic" because the hydrated ion has better stability than the hydrated structure involving the same number of water molecules), and the H3O+ may be located in a dodecahedron[45]. But an ab initio MD simulation study showed that H3O+ is located on the surface of the H3O+(H2O)20 cluster[46].
In 2007, Markovitch and Agmon first calculated the enthalpy and free energy of various hydrogen bonds around H3O+ in liquid protonated water at room temperature, and discussed the mechanism of proton migration[47]. According to the theoretical calculation results, they found that the hydrogen bond around the H3O+ (the hydrogen bond formed between the H3O+ and three H2O molecules in the first polarization layer around it) is significantly stronger than that in the water environment.
In addition to the liquid and gaseous hydration structures, there are also a variety of hydration structures in the crystalline hydrates of strong acids, in addition to the H3O+,Also present are larger hydrated ions such as H5O2+, H7O3+, H9O4+, H14O62+, as well as the hydrated hydroxyl ion H3O2-[48][49][50][50][51][52,53].
In fact, it is somewhat oversimplified to write it as H3O+ because it forms a large number of hydrogen bonds with H2O molecules in an aqueous environment, forming clusters with different structures.
Dilute hydrofluoric acid solution is a very weak acid, which exists in the form of "tight ion pair (tight ion pairs)F--H+-OH2”" in aqueous solution. With the increase of HF concentration, the ion pair H3O+F- will dissociate according to formula (8), and will be formed at the same time:
X-ray structural analysis shows that in the crystalline H2O·nHF, n = 1, 2, 4, that is, there are H3O+F-,H3O+H3F4- and H3O+H3[54,55].
The typical acid-base reaction is neutralization reaction, and secondly, there are abundant types of polymerization reactions between oxyacid groups in liquid aqueous solutions with different compositions and structures. The key factor affecting these reactions is the acidity and basicity of the aqueous solution, that is, the concentration (pH) of [H+] or [OH-]. These two reactions are taken as examples to illustrate the effect of the acidity and alkalinity (pH) of the solution on the reaction and the state and distribution of species in the solution.
4.1 Neutralization reaction
The acid-base neutralization reaction generally refers to the reaction in which H+ and OH- in solution are recombined into H2O, which can be considered as the reverse reaction of the dissociation reaction of H2O and is carried out in a stoichiometric manner.That is, one H+ and one OH- combine to form one H2O molecule, if the product of the concentrations of [H+] and [OH-] in solution is greater than the product of ions at that temperature,Then H+ and OH- combine to generate H2OThe speed of the molecule will be greater than the speed at which the H2O dissociates to produce H+ and OH-,Appears to consume H+ and OH- in solution until the product of the concentrations of [H+] and [OH-] is equal to the product of ions at that temperature; While if the product of the concentrations of [H+] and [OH-] in the solution is less than the ion product at that temperature,Then H+ and OH- combine to generate H2OThe speed of the molecules will be less than the dissociation of the H2O to produce H+ and OH-,Appears as the production of H+ and OH- until the product of the concentrations of [H+] and [OH-] is equal to the ion product at that temperature. Except for strong acids and bases, H+ and OH- are completely dissociated from acid or base molecules in aqueous solution and are in a free state, most other acids and bases, especially weak acids or bases, exist in a molecular state in solution, and whether dissociation occurs depends on the concentration changes of [H+] and [OH-] in solution. For weak acid molecules, if the concentration of [OH-] in the solution increases, that is, the pH value increases, the dissociation equilibrium of H2O will move to the left, and at the same time, OH- will combine with H+ dissociated from acid molecules to neutralize and generate H2O molecules, which will promote the dissociation of acid molecules and form negatively charged acid ions. For example, with the progress of neutralization reaction, that is, with the increase of pH value, H3PO4 molecules are contained in the neutralized H3PO4 solution which belongs to medium-strong acid with a given concentration.The distribution of H2 anions, anions, and anions will change, and is controlled by the pH value of the liquid aqueous solution and its dissociation constant.For example, when pH<pKa1, the main species in the solution is H3PO4,pH>pKa3, then the solution is mainly , when the pH value is between pKa1 and pKa3,Then the intermediate species exist with different contents, as shown in Fig. 11, and the solute content expressed by complete H3PO4 molecules can be calculated according to Equation 9.
f(H3PO4)=
Thus for acid-base neutralization reactions, the species distribution in solution and the reactions associated with it are determined by the liquid pH as well as other compositions under the constraints of the stepwise ionization constants pKa1, pKa2, pKa3 at that temperature.
4.2 Polymerization of oxyacid and oxyacid radical in aqueous solution
Except for oxyacids with very strong acidity, most other oxyacids (including transition elements with high oxidation States such as Ti, V, Mo, W, etc.)Polymeric structural units with different degrees of polymerization, different dimensions and different distributions are often formed through different types and mechanisms of polymerization, and even polymerized into sol particles, resulting in gelation. The progress of the different polymerization reactions and the formation of the products are governed by the rich condensed matter chemistry in liquid and aqueous solutions. The polymerization of silicic acid, boric acid and phosphoric acid and their acid radicals will be described as an example.
4.2.1 Polymerization of Silicic Acid and Silicate in Aqueous Solution
The molecular formula of silicic acid is Si(OH)4. Because of its structure and properties, it will undergo shrinkage polymerization in acidic solution and dissociation (pK1=9.60,pK2=11.8) in neutral or slightly alkaline solution.[Si(OH)3O]-, [Si(OH)2O2]2- and H+ are generated, and under the catalysis of H+ or OH-, silicate anions and silicic acid molecules undergo the following "oxygen bonding reaction". In 2007, Kinrade et al. Studied the existing state and distribution of polysilicate ions in strong alkaline potassium silicate solution by 29Si-29Si COSY NMR technique, and found 48 kinds of oligomeric silicate species[57]. It was also found that when the concentration of silicon in the aqueous solution of silicic acid was low, the polymerization product of silicic acid was silica sol, and when the concentration of silicon was high, the silica sol could be further polymerized to form silica gel clusters.The whole process includes rich condensed (liquid) state chemistry: silicic acid will form structures with different characteristics in liquid aqueous solutions with different acid and alkaline compositions.These structures will polymerize by different mechanisms until the formation of silica sol colloidal particles, which will spontaneously polymerize to form micelles, and then the micelles will stack according to a certain coordination number under the action of ion-water molecular clusters[58⇓⇓⇓~62]. The composition (mainly H3O+, OH-, positive and negative ions of salts), structure and properties of aqueous solution will have a decisive impact on the above process, and the whole process constitutes the condensed state reaction chemistry of gel precipitation. There are many factors affecting the polymerization reaction, the most important of which is the acidity of the aqueous solution, which not only affects the form and reactivity of silicic acid, but also determines the process and mechanism of the polymerization reaction (see Examples 1 and 2).
Example 1 Shrinkage Polymerization in Acidic Aqueous Solution
Example 2 Oxygen Bonding Reaction under Slightly Acidic, Neutral and Weak Basic Conditions
According to Dai Anbang et al., the existing state and polymerization mechanism of silicic acid are closely related to the pH of the aqueous solution. In the following, it is explained that monosilicic acid exists in two structural States I and II in neutral or slightly alkaline solution and can spontaneously carry out "oxygen bonding" polymerization at room temperature. Monosilicic acid is polymerized according to formula 10. The resulting dimeric silicic acid further reacts with the liberated OH- according to formula 11. A dimeric silicate anion is generated and can be further polymerized with a neutral silicic acid molecule according to formula 12.
The generated OH- will continue to promote the polymerization in one-dimensional, two-dimensional and even three-dimensional directions until the sol particles with a size of 10 ~ 100 nm are formed. See Section 5.4 of this paper for a discussion of the whole process. Silica sol is a dispersion system containing hydrated SiO2 colloidal particles.Silica sol is a thermodynamically unstable system because of its huge surface free energy.When there are soluble inorganic salts in the liquid aqueous solution, the colloidal particles will automatically coalesce into hydrated silica gel micelles.
4.2.2 Polymerization of Boric Acid and Borate in Aqueous Solution
From the B2O3-H2O phase diagram (Fig. 12), it can be seen that boronic acid molecules can be crystallized from aqueous solution by hydrogen bond polymerization under different temperature conditions, and two kinds of orthoboronic acid (H3BO3) and metaboric acid (HBO2) with different structures are shown in Fig. 13 and Fig. 14. When the concentration of boric acid in aqueous solution is higher than 2. 5 mol/L, it exists in the form of H2B4O7 polymerization with strong acidity.
The boronic acid H3BO3 net layered structure is formed by the polymerization of B(OH)3 molecules (B — O bond length of 1.3636 Å) by hydrogen bonds O — H … O (bond length of 2.7272 Å) (Fig. 13). The metaboric acid HBO2(B3O3(OH)3) network structure (fig. 14) is mainly formed by the polymerization of (B3O6)3- with a ring structure through hydrogen bonds.
Both H3BO3(5.56 mol%) and metaboric acid are soluble in water, and H3BO3 is a very weak acid (pK1=9.236) in liquid aqueous solution. However, compared with the proton transfer reaction characteristic of simple oxyacids in the post-P region, the ionization equilibrium of H3BO3 in liquid water is more
Complex. Because on the one hand, boronic acid with three-coordinate OH structure is actually a weak L-acid, it can form H2OB(OH)3 complexes with H2O molecules in liquid water, and deprotonation occurs due to the polarization of the central atom of B3+, as shown in Formula 13.
When in an acidic solution, the [B(OH)4]- polymerizes with undissociated H3BO3 molecules through a "hydroxyl bonding" reaction to form a series of polyborate ions containing both tricoordinated B atoms and tetracoordinated B atoms, as shown in Formula 14.
The structures of [B3O3(OH)4]- and [B4O5(OH)4]2- are shown in Fig. 3.
The H3BO3 also has the typical characteristics of the lighter elements in the P block, that is, the oxyanion has the tendency of shrinkage polymerization (polycondensation), and in neutral or concentrated alkaline aqueous solution, the polycondensation reaction occurs to form a polyanion, as shown in Formula 15.
3B(OH)3⇔[B3O3(OH)4]- + H+ +2H2O pKa=0.85
In particular, the change of acidity and alkalinity in the liquid aqueous solution, the change of substance concentration and the existence of different types of inorganic cations (namely the composition of the liquid aqueous solution) can affect the progress of the polycondensation reaction and generate various types of borate polymeric species,As shown in fig. 15, B can be changed from three-coordinated to four-coordinated, and the coordinated O atom outside the ring can be either O or OH group. In his book Structural Inorganic Chemistry (1975), Wells detailed the results of research in this area and cited the original literature[65].
4.2.3 Polymerization of Phosphoric Acid and Phosphate in Aqueous Solution
Phosphoric acid molecule H3PO4 and phosphate radical can polymerize in acidic aqueous solution through two ways: polymerization by H-bond and shrinkage polymerization of in acidic aqueous solution.
Example 1 Polymerization by H-bond
Phosphoric acid is a kind of medium-strong acid. In aqueous solution, phosphoric acid molecules will dissociate step by step.Therefore, there will be H3PO4 with a specific concentration distribution in the aqueous solution.H2 species such as , , , as well as H3O+. In concentrated solution, phosphoric acid molecules can be gradually precipitated from the aqueous solution by hydrogen bond polymerization to form a crystal with a layered structure. In the layered structure, each PO(OH)3 molecule forms six hydrogen bonds with two bond lengths with the surrounding PO(OH)3 molecules, as shown in Figure 16.
Another example is the acidic solution of KH2PO4, when the conditions are suitable, it will also precipitate from the aqueous solution in the form of crystals, and the PO2 tetrahedron will also form an infinite three-dimensional network under the organization of hydrogen bonds, as shown in Figure 17.
The results of the above two examples show that when the components in the liquid aqueous solution are mainly H+ and OH- and the structure changes, the existing state of the substances dissolved in them will be affected and different types of polymerization will occur.
Example 2 Shrinkage Polymerization of P in Acid Solution
In phosphoric acid aqueous solution, orthophosphate ions can be polycondensed into pyrophosphate ions P2O74- under the action of H3O+, but the polycondensation actually goes through two steps, the first step is the protonation reaction of orthophosphate to produce , and the second step is the shrinkage polymerization of two , as shown in Formula 16[68].
This kind of polymerization process exists in a large number of acidic aqueous solutions with high-valence oxyacid radicals, including d-block elements such as Ti, V, Nb, Ta, Cr, Mo and W with high oxidation States.A large number of isopolyacids and heteropolyacids are formed through the polymerization of acid radicals. The polymerization of oxyacids in liquid aqueous solutions with different compositions and structures is the chemical basis for the formation of polyacids, while the composition and structure of polymerization products are subject to the composition of liquid condensed state.
The above studies show that when oxyacids with different compositions and structures are polymerized in liquid aqueous solution, the composition and structure of liquid aqueous solution will affect or even determine the existing state and properties of reactants, the polymerization mechanism and the structure and properties of reaction products.
5 Precipitation reaction in aqueous solution
In the precipitation reaction, the nature, composition and structure of the chemical species in the liquid solution are the main factors that determine whether the precipitation reaction can proceed and the composition and structure of the product.
5.1 Formation of crystalline precipitate
The first type of "precipitation reaction" refers to the precipitation of solid precipitates with certain composition and structure from liquid water or organic solvents by chemical reaction nucleation under certain external conditions. The solid precipitates are generally insoluble crystalline solid compounds composed of ionic bonds, such as AgCl, BaSO4, CaC2O4·H2O, MgNH4PO4, etc. When the product of the related positive and negative ion concentrations entering the liquid water is greater than the solubility product constant Ksp of the solid precipitate at a certain temperature,Such as [Ksp of Ag+][Cl-]>1.77×10-10(AgCl), [Ba2+] ] >1.08×10-10,[Ca2+][C2 ]>2.3×10-9 、[Mg2+ ] ]U UN>2.5×10-13AgCl, BaSO4, U CaC2O4·H2O U N MgNH4PO4 were crystallized and precipitated from water[69]. Selecting appropriate conditions to obtain crystalline precipitates with high crystallinity from aqueous solution is the basis for the determination of chlorine, sulfur, calcium and magnesium by Gravimetric analysis developed in the first half of the 20th century[70].
Of course, the solid structure of the crystalline precipitate can also be affected by adjusting the crystallization conditions, such as the generation of crystal plane dislocation and lattice distortion, to control the morphology of the crystalline product, the generation of lattice defects and twins, so as to meet the functional requirements. Controlling the crystallization conditions is actually controlling the composition, structure and properties of liquid water and solution.
The following example shows that if the constituent ions of the crystalline precipitate can react with water (which is an example that can often occur in aqueous solution), it will affect this kind of precipitation reaction, or even make the reaction completely impossible.
Example: When neutral or slightly acidic lead salt (Pb2+) solution is mixed with dichromate (Cr2),The insoluble PbCr2O7 precipitate was not formed, but the yellow PbCrO4 crystalline precipitate (Ksp=2.8×10-13) was precipitated. The reason for this result is that the Cr2 dissolves in water and first reacts with the H2O according to Equation 17,A small amount of was produced (pK1 for H2CrO4 was 6.488 for 0.74,pK2)[71].
Cr2 + H2O ⇔ ⇔ 2H + +
Because the Ksp of the PbCrO4 is far less than the Ksp of the PbCr2O7, even if a small amount of is generated in the solution due to the existence of the above equilibrium,The solubility product of [] and [Pb2+] can exceed its Ksp, so that precipitation occurs, and the above equilibrium is driven to the right direction, which promotes the continuous generation and precipitation of PbCrO4 precipitates. It also makes it possible to understand that the PbCrO4 can be dissolved by the acid due to the existence of the above-mentioned equilibrium in water.
5.2 Precipitation of amorphous precipitate
In ionic crystalline precipitation reactions, a problem worthy of attention is that the Ksp values of sulfides and hydroxides of a large number of transition elements or high-valence elements are very small.As Cu2S(Ksp=2.5×10-48), Ag2S(Ksp=6.3×10-50), HgS (red) (Ksp=4.0×10-53),HgS (black) (Ksp=1.6×10-52) and Pb(OH)4(Ksp=3.2×10-66), ZrO(OH)2(Ksp=6.3×10-49), Sn(OH)4(Ksp=1.0×10-56), Te(OH)4(Ksp=3.0×10-54).
When the precipitation reaction of the above metal sulfides and hydroxides occurs, due to their extremely small Ksp values, the solution has an extremely high degree of supersaturation since the start of the reaction, which inevitably leads to the formation of precipitates with a very large interface, which facilitates the adsorption of H2O molecules and reverse phase ions in the solution on the surface.As a result, the Aggregation velocity of particles greatly exceeds the Orientation velocity of crystalline motifs, resulting in amorphous structures such as disorder or partial disorder and distortion, and even the formation of colloidal particles, which then gel and precipitate in a colloidal state. Such precipitates are often non-equilibrium products[72].
Effect of hydrogen bonding on solubility product Ksp of poorly soluble M(OH)3. Hereinafter referred to as Group IIIB Sc(OH)3,Y(OH)3 and Yb(OH)3 and Lu(OH)3 of the last two elements in the lanthanide "contraction" are used as examples to illustrate the effect of hydrogen bonding on the M(OH)3 solubility product Ksp. Table 4 shows the Ksp of the above four hydroxides and the radii of the metal ions. It can be seen that the ionic radius of M3+ in crystalline precipitates is not much different, but the Ksp of M(OH)3(M=Y, Yb, Lu) is 7 to 10 orders of magnitude different from the Ksp of ScOH3.
A careful study of the Sc(OH)3structure (Fig. 18) reveals that the main reason for the large difference in the above Ksp may be the formation of a ReO3-type structure composed of distorted octahedral Sc(OH)6 common vertices under certain conditions when the precipitation reaction is carried out.Due to the close distance between some adjacent OH, internal hydrogen bonds are formed in the structure, which leads to the structure being thermodynamically more stable and easier to generate, so the solubility product constant Ksp is significantly smaller[21][73]. A similar situation has been observed in group IIIB element hydroxide structures such as In(OH)3(pKsp=33.2), Ga(OH)3(pKsp=35.14), etc[74].
5.3 Synthesis and Structure of Amorphous Nickel Borate
NiCl2·6H2O and Na2B4O7·10H2O were dissolved in water with molar ratio of 1 ∶ 2 and 1 ∶ 8, respectively, and the pH of the above two reaction solutions was adjusted to 8.8 and 7.9 with H3BO3(10%) solution, respectively, and two kinds of nickel borates were precipitated after the reaction was completed (4 H). Liu and Fang used chemical analysis and thermogravimetric-derivative thermogravimetric (TG-DTG) analysis to determine the molecular composition of the two amorphous nickel borates as NiO 0.8B2O3·4.5H2O and NiO·B2O3·3H2O, respectively[75]. The results of their laser Raman experiments showed that the boronoxide anions (polyborate ions) in the two nickel borate precipitates were different. They used the synchrotron radiation extended X-ray absorption fine structure (EXAFS) method to analyze the structure of the sample, and obtained the species, coordination number and interatomic distance of the neighboring coordination atoms around the Ni atom by data fitting, and found that the local structure around the Ni atom in the sample had good similarity with that in the ICSD No. Crystal (ICSD No.31387) (Fig. 19). The coordination atoms around the Ni atom are O, B and Ni, and for NiO·0.8B2O2·4.5H2O, the coordination numbers are 5.7, 3.8 and 3.8, and the coordination distances are 0.208, 0.263 and 0.311 nm, respectively; For NiO·B2O2·3H2O, the coordination numbers are 6.0, 4.0 and 4.0, and the coordination distances are 0.207, 0.262 and 0.310 nm, respectively[75]. The conditions for the formation of the two amorphous nickel borate precipitates are mainly different in the pH of the liquid mother liquor, that is, the concentration of H3O+ and OH-, the ratio of nickel to boron and the total concentration of B in the mother liquor. In addition, the presence of other metal cations (such as Na+) in the aqueous solution can also affect the state and form of borate[76]. The pH value has a greater influence on the existence form and distribution of polyborate, as shown in Fig. 20[76]. Therefore, a variety of metastable amorphous nickel borate can be formed in aqueous solution under different composition conditions.
5.4 Structure of Silica Gel and Amorphous SiO2 and Formation of Precipitate
The basic particles of silica sol, silica gel and amorphous SiO2 are all spherical colloidal particles of SiO2, but they aggregate in different ways. Silica sol is dispersed in liquid aqueous solution, silica gel is a soft object with colloidal particles closely combined into a three-dimensional network structure, and amorphous SiO2 precipitate is a silica gel fragment, as shown in fig. 21.
5.4.1 Polymerization of silicic acid and formation of silica sol
The existing state of silicic acid, the type and mechanism of polymerization, and the formation of polymerization products are controlled by the composition of liquid and aqueous solutions, especially the pH value of the system, and have rich condensed matter chemistry, which has been explained in Section 4 of this paper. Whether through polycondensation reaction or oxygen bonding reaction, the polymerization reaction can proceed spontaneously, resulting in one-dimensional linear, two-dimensional and even three-dimensional polymerization, until the formation of sol particles with a size of 10 ~ 100 nm. The whole process is shown in Figure 22. Silica sol is a dispersion system containing hydrated SiO2 colloidal particles. The most important feature of silica sol is its huge surface free energy. Therefore, silica sol is a thermodynamically unstable system. Colloidal particles, especially when there are soluble inorganic salts in the liquid phase, will automatically coalesce into large particles or hydrated silica gel micelles.
5.4.2 Structure of silica sol particles
Fig. 23 is a schematic diagram of a colloidal particle structure and an electric double layer of an alkaline silica sol. The center of silica sol particles is the colloidal core, which is a compact aggregate composed of a large number of SiO2 molecules and is usually negatively charged. The colloidal core is generally insoluble in water (and may sometimes adsorb some water) and can selectively adsorb positive ions from the surrounding aqueous solution to form a compact layer, while the counterions of the ions are distributed around it to form a diffusion layer. When the colloidal nucleus moves, the water molecules in the compact layer move together. The colloidal core and the compact layer make up the colloidal particle, which has an electric charge. The whole composed of colloidal particles and their surrounding diffusion layers is called micelle, which is electrically neutral. The potential difference between the colloidal particle and the solution is the electrokinetic potential (ζ) of the silica sol. There are two types of silica sols: alkaline and acidic. The ζ of alkaline silica sol is negative, and the ζ of acidic silica sol is positive. Because the surface of the same kind of silica sol particles has the same electrical potential, they are stable due to electrostatic repulsion. The higher the ζ, the better the stability of the silica sol. The stability of acidic silica sol is better than that of alkaline silica sol.
5.4.3 Gelation of silica sol
Hydrated gels of silicon are often prepared by gelation of silica sols. The transformation of spherical particles in the sol into a homogeneous hydrogel containing clusters of water molecules is very rapid, as shown in fig. 24. During gelation, there are Si — OH groups on the surface of the spherical particles in the silica sol. When these colloidal particles collide with each other, especially under the base catalysis of OH-, the — OH groups on the surface of the colloidal particles undergo condensation polymerization with the Si — OH groups of the adjacent colloidal particles, forming shengcheng Si — O — Si ≡ bonds at the interface, which connect the colloidal particles of the sol into micelles. The contact of these colliding spherical colloidal particles has an infinitesimal negative radius of curvature, which makes the solubility there very small, almost zero, so some monomers deposit around these contacts, making the sol particles grow up until the solubility advantage there basically disappears.
The surface of the colloidal particles of the sol is subjected to polycondensation to form ≡ Si — O — Si ≡ bonds, which connect the colloidal particles together and continuously grow. Due to different connection modes, chain structures can be formed, and further developed into rod-like or fibrous structures, which are polymerized and further linked to form network and three-dimensional structures, forming SiO2 gels containing a large amount of aqueous solution, and colloidal precipitates are separated from the aqueous solution.
5.4.4 Structure of silica gel
It is generally believed that the micelle of SiO2 gel is formed by the aggregation of primary SiO2 colloidal particles with a certain size, and this aggregate has a three-dimensional structure. Some researchers have studied the as-formed SiO2 gel by small angle X-ray scattering, and found that it is an aggregate composed of 3 – 6 nm SiO2 spherical colloidal particles. The structure of the gel is very complex because of the different sizes and compositions of the colloidal particles. To study the structure of silicone gel is mainly to understand the size and shape of the micelle of the SiO2 gel, and the size and shape of the micelle depend on the coordination number and arrangement regularity of the colloidal particles. Therefore, the structure of silica gel micelles is generally described by the packing density or coordination number of the arrangement between the colloidal particles in the gel micelle, the porosity, pore volume and pore diameter between the gel micelles, and other parameters.
The packing density (S) of colloidal particles in a gel micelle is expressed by the volume fraction of SiO2 colloidal particles contained in the total volume of a certain amount of SiO2 gel. The relationship between packing density (S) and packing coordination number (C. N.) of normal spherical particles is shown in Table 5. Therefore, the packing density or packing coordination number can be calculated by measuring the intermicellar pore volume in a certain amount of SiO2 gel.
The structural parameters of SiO2 gel can be measured by gas adsorption method and adsorption isotherm, and the results are shown in Table 6.
It can be seen from Table 6 that the bulk density of the compound in these SiO2 gels ranges from 0.331 to 0.522. The most tightly packed is sample no.2, which has a colloidal packing coordination number of roughly 6 (Table 5). The most relaxed arrangement is sample no.1, which has a packing density of 0.331 and a packing coordination number of 4 (Table 5).
There are also studies on the SiO2 gel structure with more relaxed packing, and the coordination number is measured to be about 2 to 3. This gel has a larger pore volume and specific surface area, as shown in figs. 25 and 26.
5.4.5 Regularity of formation and precipitation of hydrated gel in aqueous solution (composition of H3O+ and OH- in gelling mother liquor)
(1) Effect of pH
The relationship between the gelation time of silicic acid and the pH of the solution in the precipitation process of hydrated silica gel is in the shape of "N". In the past, scholars mostly studied in a certain pH range, so the relationship between gelation time and pH was a part of the "N" shape[60]. Based on a large number of experimental work, Dai Anbang et al. concluded that the polymerization of silicic acid is closely related to the pH value of the liquid mother liquor (Fig. 27). In the solution with higher pH, the oxygen bonding of neutral molecules and negative ions of silicic acid occurs, and the isoelectric point of silicic acid, the highest point of the "N" curve, is the watershed of this oxygen bonding mechanism. Dai Anbang et al. proved that in the solution on the right side of the peak of the curve, the alkalinity gradually increases, and the negative ion of silicic acid gradually increases. When its concentration is equal to that of the neutral molecule of silicic acid, the polymerization rate is the fastest (the lowest point of the "N" curve in Fig. 27).
(2) Effect of concentration of silicic acid in gelling solution on gelling
As shown in Fig. 27, the gelation time of silicic acid with different concentrations at different pH shows an N-shaped curve, and with the increase of the concentration of silicic acid in the solution (see the curves 1, 4, 5 and 6 on the right side in Fig. 27), the gelation rate will be doubled, that is, it is easier to precipitate hydrated silica gel.
(3) The presence of inorganic salts promotes the precipitation of hydrated silica gel.
The promoting effect of inorganic salts on the precipitation of hydrated silica gel is evident from fig. 22.
6 Gelation and crystallization in aqueous solution
The gelation of inorganic species in liquid aqueous solution and the crystallization of colloidal species are important reactions in liquid aqueous solution. The formation process of colloidal SiO2 includes the polymerization of silicic acid, the formation of colloidal particles of silicic acid sol, and the aggregation reaction (gelation) of colloidal particles of SiO2. The composition and structure of liquid aqueous solution have an effect on the gelation reaction and the formation of water molecular clusters and specific coordination structures in hydrated gel. Complex condensed state reactions run through the gelling process. The composition and structure of liquid water (including gel-forming mother liquor) are the main factors that affect or even determine all the above processes. The crystallization of colloidal species, including the process and mechanism (the structure and reaction of polymeric components, the crystallization reaction under the action of templates, etc.), is an important content in liquid condensed matter chemistry.
In this paper, the gelation and crystallization reaction of SiO2 hydrogel in aqueous solution is introduced by taking the hydrothermal reaction of pure silicalite-1 (Si-ZSM-5) with three-dimensional cross microporous structure from an aqueous solution (SiO2:0.08(TPA)Br:0.04 NH4F:20 H2O) with specific composition at 200 ℃ for 15 days in the presence of trace tetrapropylammonium bromide (TPABr) and NH4F as an example. The crystallization of colloidal silica or aluminosilicate, especially the crystallization of zeolites with regular microporous structure under hydrothermal conditions, is accompanied by a series of condensed chemical reactions. Taking the crystallization reaction of silicalite-1 as an example, under strong alkaline hydrothermal conditions, colloidal SiO2 (mainly hydrated gel) was dissolved to generate silicate ions in different polymerization States, which were rearranged and condensed around the TPA+ with sp3 structure in the mother liquor to form silicalite-1 with two-dimensional cross channels, as shown in Fig. 28[84].
In the framework of silicalite-1, the 10-membered ring channel parallel to the [100] direction is S-shaped, and the 10-membered ring channel parallel to the [010] direction is linear, and the two are vertically crossed. The structure-directing agent tetrapropyl ammonium cation (TPA+) is located at the intersection of the channels, and its four propyl groups extend to both sides of the S-shaped channel and the straight channel, respectively (Fig. 28). There is a symmetry plane perpendicular to the [001] direction through the N atom in the structure-directing agent, and the two propyl groups of the TPA+ extending to both sides of the straight channel are symmetrically distributed on both sides of the N atom, as shown in fig. 28b, while there is no such symmetry for the two propyl groups of the TPA+ extending to either side of the S-shaped channel in the symmetry plane, as shown in fig. 28a. The fact that the four propyl groups in the structure-directing agent extend to the four directions of the cross channel indicates that the small silicon-oxygen building blocks dissolved from the SiO2 gel assemble around the structure-directing agent to form the silicalite-1 channel structure.
The condensed state chemical reaction in the process of gelling and crystallization of aluminosilicate is discussed by taking the crystallization of aluminosilicate molecular sieve, which is widely used and has a complex structure, as an example. Firstly, the gel-forming reaction and mechanism of silicoaluminic acid in strong alkaline solution are introduced.
6.1 Polymerization and Mechanism of Aluminosilicate in Aqueous Solution
Sodium silicate solution (Na2O·xSiO2), sodium aluminate solution (NaAl(OH)4) and sodium hydroxide solution (NaOH) were mixed uniformly at room temperature according to a certain molar ratio. Because the Na2O·xSiO2 solution contains a large number of polysilicate ions with different compositions and structures, a large number of parallel silicon-aluminum polycondensation reactions can occur after the polysilicate ions are contacted with NaAl(OH)4 in a strong alkaline solution to generate various aluminosilicate colloidal particles with an electric double layer structure, and the aluminosilicate colloidal particles are uniformly dispersed in the aqueous solution to form sol[57,85,86][87,88]. The concentrated silica-alumina sol is a thermodynamically unstable system, and the silica-alumina colloidal particles will spontaneously aggregate into colloidal particle aggregates containing water molecules (water-silica-alumina gel micelles), and further aggregate to form silica-aluminum hydrogels with a certain Si/Al ratio. Zhadanov has studied the quaternary phase diagram of the Na2O-Al2O3-SiO2-H2O system with 85% water content, and Fig. 29 shows the ternary projection of the phase diagram on the Na2O-Al2O3-SiO2[89]. It can be seen from fig. 29 that the silica-alumina gel can only be formed in a limited composition region (indicated by a dotted line in the figure). When the silica-alumina ratio is relatively small and the alkali content is low, the gel cannot be formed. The meaning of the five lines in the figure is the four connecting lines between the bottom line of the :SiO2-Al2O3 and the Na2O of the vertex, which means that the ratio of silicon to aluminum (n) is equal to 1/3, 1, 2 and 5, respectively. The molar ratio of the Na2O/Al2O3 on the line passing through the vertex SiO2 and the point on the Na2O-Al2O3 composition axis where the molar ratio of the Na2O/Al2O3 is equal to 1 (the point labeled 50 on the Na2O-Al2O3 composition axis) is equal to l. The composition range of the raw material is wide, while the composition range of the generated gel skeleton solid phase after washing is very narrow (expressed by O), which is on the straight line with the Na2O/Al2O3 equal to 1. Microporous crystals such as NaA, NaX and NaY can be formed only when the gel framework phase with Na2O/Al2O3 equal to 1 is crystallized under hydrothermal conditions at a certain temperature (such as 100 ℃).
6.2 Composition and Structure of Hydrated Silica-alumina Gel and Crystallized Zeolite
NaA (molecular sieve structure code LTA), NaX and NaY (molecular sieve structure code FAU) are three zeolite molecular sieves that have achieved large-scale industrial applications. The composition of the silica-alumina hydrogel used to synthesize these molecular sieves, the crystal composition of the molecular sieve and the silica-alumina ratio information are shown in Table 7, and the detailed structural information is shown in Fig. 30 (LTA) and Fig. 31 (FAU). The silica-alumina hydrogel may be ordered in the short range but disordered in the long range, while the crystallization product zeolite molecular sieve is ordered in the long range. Therefore, under hydrothermal conditions, the silica-alumina hydrogel must have undergone an ordering process to produce the final zeolite molecular sieve crystals. However, due to the lagging development of in-situ characterization technology, there are still two viewpoints on how the ordering process occurs, namely, solid phase mechanism and liquid phase mechanism. According to the solid phase mechanism, due to the large surface free energy of hydrated gel, silicon and aluminum structural units can self-assemble around the structure-directing agent in the presence of a specific structure-directing agent to form a microporous zeolite molecular sieve with a specific structure.According to the liquid phase mechanism, the silicate and aluminate ions in the silica-alumina hydrogel with large surface area and thermodynamically metastable state can be gradually dissolved into water under the action of water molecules and hydroxide ions, and polymerize and crystallize again under the action of a structure-directing agent to form a microporous zeolite molecular sieve with a specific structure[87]. Either mechanism involves the polymerization of nonstoichiometric silicate and aluminate.
Among the 255 types of microporous molecular sieves that have been discovered so far, there are more than 110 types of aluminosilicate molecular sieves, which are formed by gelation of aluminosilicate and then hydrothermal crystallization. Although the formation mechanism is not yet fully understood, it is believed that the role of structure-directing agents (templates) in the hydrothermal crystallization process is a major scientific issue in the formation of porous materials such as micropores and mesopores in liquid aqueous solutions, and is also the key to exploring and developing materials with specific microporous structures. With the increasing understanding and exploration of the structure-directing mechanism, the formation of pore structure and the structural association of templates have become the focus of scientific research.
It should be particularly pointed out that among the more than 100 kinds of silica-alumina zeolite molecular sieve structures, more than 40 kinds are natural zeolite molecular sieves (ANA, BEA, BIK, BOG, BRE, CHA, -CHI, DAC, EAB, EDI, EON, EPI, ERI, FAU, FER, GIS, GME, G00, HEU, LAU, LEV, LOV, LTL, MAZ, MER, MFI, MON, MOR, NAB, NAT[94].
It has been found that the introduction of organic structure directing agents is not necessary for the formation of some natural silica-alumina zeolite molecular sieves and the synthesis of some zeolite molecular sieves that do not exist in nature (in fact, it also includes aluminophosphate molecular sieves, silica-phosphorus-aluminum molecular sieve, boron-aluminum molecular sieve and other microporous molecular sieves with various types of heteroatoms). For example, there is no organic structure directing agent involved in the crystallization process of the NaA, NaX and NaY zeolite molecular sieves,Therefore, this phenomenon raises an important scientific question: what substances replace the general concept of structure-directing agents as "templates" in these crystallization processes, leading to the crystallization of zeolite crystals with specific pore structures under hydrothermal conditions? Special attention should be paid to the role of the composition, structure and properties of the two levels of water and crystallization mother liquor in pore formation, such as the existence of cationic hydrated molecular clusters and their role in pore formation. As early as the 1970s, Flanigen proposed that in the process of hydrothermal crystallization,Inorganic cation Li+, Na+, K+, Rb+, Cs+,The hydrated ions of Ca2+, Sr2+, Ba2+, etc. May have played a "template" role,The formation of structural units and pore structures of some zeolite crystals has been proposed in detail, such as hydrated Na+(7.2Å) and hydrated K+(6.6Å, which may contribute to the formation of SOD and CAN cages in zeolite molecular sieve structures[94]. This is a good inspiration for understanding the template-crystallization reaction in liquid condensed matter chemistry.
7 Hydrolysis reaction
Chemical reactions in liquid and aqueous solutions, not only as described above, but also the composition and structure of the two levels of water and aqueous solution, will affect the progress and results of chemical reactions. In the hydrolysis reaction, water participates in the chemical reaction as the "identity" of the main reactant, which makes this kind of condensed matter reaction in the liquid state more abundant.
When a large number of salt MX3 of M3+ (including transition metals and some M2+ with strong polarization performance (ne/r2)) enter water, the hydrated Mn+ ions are separated from the original salt crystal lattice and dissolved into the solution. Because of their high valence and small radius,Polarization ability is very high, so when it enters water, this kind of ion with high Polarization performance will polarize and deform the H2O molecules in the first coordination sphere, and can repel and dissociate the H+ in the molecules to generate OH-, that is, hydrolysis. If the polarization performance of Mn+ is further improved (polarization index increases), the OH- ions closely connected with it can be structurally different. An example of a Ca2+(1.8),La3+(2.0),Al3+(9.2) with different Polarizing Power (fig. 32) is described below.
For example, when the hydrolysis reaction is carried out under different conditions (such as concentration, temperature, pH, etc.), different cations and OH- with different structures aggregate to form different structures such as amorphous, amorphous, colloidal and crystalline insoluble hydroxide (Hydoxide) with H bond (Oxyhydoxide). The structures of Al(OH)3, α-AlO (OH) and γ-FeO (OH) are introduced below as examples, which lead to strong hydrolysis reaction, shift the dissociation equilibrium of water, and affect the acidity and alkalinity in aqueous solution and the related chemical reactions. The hydrolysis of aluminum salt (AlX3) and iron salt (FeX3) is described below as an example[96].
Example 1 Hydrolysis of aluminum salt (AlX3) can produce poorly water-soluble AlO(OH)4 and Al(OH)3(pKsp=32.89) with the structure shown in Fig. 33, or simultaneous hydrolysis of anions (H2S∶pK1=6.97,pK2=12.90):
It is also worth pointing out that the hydrolysis of aluminum salts can lead to changes in solution composition and acidity and alkalinity, which tend to promote and affect other chemical reactions that occur in water, as shown in Equations 21 and 22.
Example 2 Hydrolysis of iron salts (Fe+, Fe+3) can produce Fe(OH)2(pKsp=16.3) and FeO(OH)(Fe(OH)3)(pKsp=38.55), which are poorly soluble in water, and promote and regulate other related reactions (such as reactions 27 and 28) in water.
The structure of A1(OH)3 is very rich, but α (bayefite) and γ (gibbsite) phases are dominant. These structures are all made of exactly the same layers of AX3, as shown in Fig. 33, with the difference in the way of stacking. This layer can be thought of as consisting of AX6 octahedra sharing three edges. In the bayefite phase, the O atoms are approximately hexagonal close-packed (as in Mg(OH)2), while in the hydrargillite (a monoclinic gibbsite) phase, the OH groups on the upper side of the layer are directly opposite to the OH groups on the lower side of the layer, as shown in fig. 34.
Fig. 35 is a front view of the structure of boehmite α-AlO (OH) and lepidocrocite γ-FeO (OH). Fig. 35a shows a point-to-point arrangement of double chains and close-packed layers of oxygen atoms. The H atom is attached to the O atom, and the distance between O — H — O is 2.6565 Å (see the dotted line in Fig. 35a). The H atom is not on the line between the O atom pairs, as shown in Fig. 36.
It can be seen from Figure 35a that the OH group and three Al atoms are pyramidal, with three Al atoms on one side (Al-3OH, 1.9898 Å) and H atoms on the other side, while the O atom is bonded to three adjacent coplanar Al atoms (Al-3O, 1.8686 Å), and the O — H — O hydrogen bond points in the direction of the fourth tetrahedral bond. In the α-MO (OH) structure, each OH group is hydrogen-bonded to the same O atom, but not in the γ-MO (OH) structure (Fig. 35b).
8 Oxidation-reduction reaction in aqueous solution
A large number of inorganic compounds and some organic species can undergo oxidation-reduction reactions in water. In addition to the general law of oxidation-reduction reaction, the oxidation-reduction reaction in liquid aqueous solution will also be affected by many factors such as the composition and structure of condensed liquid water, including the oxidation-reduction reactivity of water itself, the acidity and alkalinity of aqueous solution (the influence of pH on electrode potential E),The structure of water (including the influence of solvated electrons in a specific water structure, i.e., hydrated e, on the gain and loss, transfer and transfer of electrons e), the influence of the composition of aqueous solution on the state of existence of reactants and products (such as the formation of precipitates or complexes),As well as the influence of the presence of oxygen (O2,O) species in aqueous solution, etc.), which are important research contents of oxidation-reduction condensed matter chemical reactions in aqueous solution.
8.1 Oxidation-reduction reaction of water in aqueous solution
Water can be reduced to H2 as an oxidant or oxidized to O2 as a reductant, and the electrode potential of the species that can exist stably in water must be between the potentials of the above two processes. Secondly, many reactions in aqueous solution involve not only electron transfer but also H+ transfer, so the electrode potential is also closely related to the pH of the solution.
8.1.1 Reaction of oxidation by water
The reaction of a metal with water (or an acid in aqueous solution) is actually the oxidation of the metal by water or an H+, and the overall reaction can be expressed in the following form.
M(s) + H2O (1) → M+(aq) + (1/2)H2 (g) + OH-(aq)
M(s) + H+(aq) → M+(aq) + (1/2)H2 (g)
A similar pattern exists for the generation of higher charged metal ions. From the thermodynamic point of view, this kind of reaction can occur in other s-block metals except Be and the metals from the fourth to the seventh group of the first transition system (Ti, V, Cr, Mn).
8.1.2 Reduction reaction by water
Water is the reducing agent in the following half-reaction:
2H2O(1) → 4H+(aq) + O2(g) + 4e-
The oxidative half-reaction is the reverse of the following reductive half-reaction:
O2(g)+4H+(aq)+4e-→2H2O(1 )
EƟ= + l.23 V
Such a high positive value of the EƟ indicates that the acidified water is a very weak reducing agent, which can be reduced only when it meets a strong oxidizing agent, for example, the Co3+(aq)(EƟ(Co3+/Co2+)=1.92 V) is reduced to release O2, and the reaction equation is as follows:
Only a few oxidants (another example is Ag+) oxidize water and release O2 at appreciable rates. Another example is Ce4+/Ce3+(+1.76V),The formation and release of Cr2 /Cr3+(+1.38 V) in acidic solution and Mn /Mn2+(+1.51 V),O2 in acidic solution can be retarded by kinetic factors,Because the activation energy required to transfer four electrons and the activation energy required to form an oxygen-oxygen double bond are both higher.
8.1.3 Stability range of water
Both reducing agents, which rapidly reduce water to H2, and oxidizing agents, which oxidize water to O2, are difficult to stabilize in aqueous solution. The stability range of water is defined by the two coordinates of redox potential and pH (Figure 37), and the water in the region defined by the coordinates is thermodynamically stable to both oxidation and reduction.
The boundary of the stability range can be found by the dependence of the E value of the relevant half-reaction on pH. The half-reaction of the O2/H2O pair is:
O2(g) + 4H+(aq) + 4e- → 2H2O(l) n=4
Q=
Where p(O2) is the partial pressure of oxygen, thus:
E=EƟ+ ln(p(O2)[H+]4)
If the partial pressure of the O2 is 1 bar and the temperature is 25 ° C, Equation 36 can be written as:
E = 1.23 V-0.059 V×pH
Any species with a potential above this value can oxidize water to release O2, so the equation defines the upper boundary of the stability range. The half-reaction for the reduction of H+(aq) to H2 is:
2 H+(aq) + 2 e-→ H2(g) n=2 Q=
As a result,
E=EƟ- ln
If the partial pressure of the H2 is 1 bar and the temperature is 25 ° C, Equation 39 can be written as:
E = -0.059 V × pH
Any species with a potential below this value can thermodynamically reduce the H+(aq) to release the H2, so the equation defines the lower boundary of the stability range. To simplify the discussion, chemists often refer to the reduction of H+(aq) in water as "reduction of water".
The pH value of natural water is usually between 4 and 9. The area defined by two vertical lines and two oblique lines at the positions of pH = 4 and pH = 9 represents the stable zone of natural water. After that, we can recognize and understand the redox reactions and the results of these patterns in environmental chemistry and mineral chemistry. Pourdaix diagram and its application in environmental chemistry are discussed in detail[100].
8.2 pH of aqueous solution versus oxidation-reduction electrode potential (EƟ)
E-pH diagram, when the electrode reaction is carried out in liquid water, H+ or OH- in liquid water will participate in and affect the reaction, and the change of pH in the system will affect the electrode potential E.
Many reactions in aqueous solution involve not only electron transfer but also H+ transfer, so the electrode potential is also related to the pH of the solution. The electrode potential of many reactions in aqueous solution changes with pH because the Br Brönsted basicity of the reduced species in the couple is much stronger than that of the oxidized species. For an oxidation-reduction couple that transfers ve electrons and vp protons, the equation is as follows.
Ox+vee-+vpH+→Red Q=
E=EƟ- ln = EƟ- ln + ln[H+]
Combining the concentrations of reducing and oxidizing species with the EƟ, the potential E 'can be defined as:
E'=EƟ- ln
Using the two relations of ln[H+]=ln 10×log[H+] and pH=-log[H+] to transform, the electrode potential can be written as:
E=E'- pH
在25 ℃,E= E'- pH
The potential decreases (becomes more negative) as the pH increases (the solution becomes more alkaline). For example, the half-reaction of electricity to / is:
(aq) + 2H+(aq) + 2e- → (aq) + H2O(l)
When pH = 0, EƟ=+1.201 V; At pH 7, the reduction potential of the couple is 1.201 V- (2/2) (7 × 0.059) V = 0.788 V. Therefore, the perchlorate anion is a strong oxidant under acidic conditions.
The standard potential in neutral solution (pH = 7) is denoted by the symbol . Because cell sap is buffered to a pH near 7, data are particularly important in biochemistry. The condition of pH = 7 corresponds to the Biological standard state; It is sometimes expressed in biochemistry as' Midpoint 'potential or Em7,“m7” to indicate the' Midpoint 'potential at pH = 7. For example, to determine the reduction potential of the H+/H2 couple at pH = 7 (with the other species in the standard state), one needs to keep in mind that E'=EƟ(H+,H2)=0. At this time, the reduction half-reaction is:
2H+(aq) + 2e- → H2(g)
Thus ve=2,vp=2, the corresponding standard potential of the biological system is:
E⊕=0-(2/2)(0.059 V)×7.0=-0.41 V
The relationship between pH and potential is illustrated by two examples.
Example 1 Chromium in Different Chemical States
The oxidation state of element chromium is complex, and different chemical species exist at different pH. The pH-potential diagram (Fig. 38) can be used to study the relationship between solution pH and the existence of chemical species in a specific oxidation state and the related potential.
The electrode reactions (or reactions) represented by some line segments in Fig. 38 are as follows.
AD Cr2 +14H++6e-=2Cr3++7H2O
+4H2O+3e-=Cr(OH)3+5OH-
+2H2O+3e-=Cr+4OH-
The above 3 lines are not parallel to the coordinate axes because they represent electrode reactions with H+ or OH-.
BE Cr3++e-=Cr2+
The BE line is parallel to the horizontal axis because it represents an electrode reaction in which no H+ or OH- is involved.
GH Cr2 +H2O-=2 +2H+
GH is parallel to the vertical axis because it does not represent an oxidation-reduction electrode reaction.
The stability regions of various oxidation States of chromium are also reflected in the figure, for example, the stability region of Cr2 is quadrilateral ADHG, while the stability region of Cr(OH)3 is polygonal DEFJH.
Example 2 As
The electrode reaction of H3AsO4/H3AsO3 is:
H3AsO4+2H++2e-=H3AsO3+H2OEƟ = 0.56 V
The Nernst equation for the electrode reaction is
E=EƟ+ lg
When both H3AsO4 and H3AsO3 are in the standard state, we have:
E=EƟ+0.059 V×lg[H+]
即E=0.56 V-0.059 V×pH
It is obvious that the E-pH diagram of the electrode reaction is a straight line. In Fig. 39, the E-pH diagrams of the two pair electrode reactions of H3AsO4/H3AsO3 and I2/I- are represented in the same rectangular coordinate system by As and I lines, respectively.
In the E-pH diagram shown in fig. 39, if E of a certain electrode pair is above the As line, that is, the electrode potential is higher than that of the H3AsO4/H3AsO3 of the electrode pair, then the oxidation type of this electrode pair can oxidize the H3AsO3 to H3AsO4, so above the As line is the stable region of H3AsO4; On the contrary, if the E of a pair is below the As line, that is, the electrode potential is lower than that of the H3AsO4/H3AsO3 of the pair, the reduction type of the pair can reduce H3AsO4 to H3AsO3, so the stable region of H3AsO3 is below the As line.
From the above discussion, it can be concluded that above the E-pH line is the stable region of the oxidation form in the electrode reaction; Below the E-pH line is the stable region of the reduction form in the electrode reaction.
The electrode reaction of I2/I- is:
I2+2e-=2I-EƟ=0.54 V
There is no H+ or OH- in the reaction equation, so the electrode potential of the couple is not affected by the pH of the system, and the E-pH diagram is a straight line parallel to the abscissa axis, such as the I line in Fig. 39.
It can be seen from the above figure that H3AsO4 can oxidize I- in strong acid medium with low pH:
H3AsO4+2I-+2H+=H3AsO3+I2+H2O
In weak acid or alkaline medium with high pH, I2 can oxidize H3AsO3:
H3AsO3+I2+H2O=H3AsO4+2I-+2H+
8.3 Disproportionation in aqueous solution
Since both the EƟ(Cu+,Cu)(+0.52 V) and the EƟ(Cu2+,Cu+)(+0.16 V) are in the stable region of water, water is neither oxidized nor reduced by Cu+. Nevertheless, Cu (I) is not stable in water because it is prone to disproportionation. A disproportionation reaction is a reaction in which the oxidation number of an element increases and decreases at the same time, in other words, the element itself that disproportionates is both an oxidizing agent and a reducing agent:
Cu+(aq)+Cu+(aq)→Cu2+(aq)+Cu(s )
Because of the EƟ=0.52 V~0.16 V=+0.36 V, the reaction is spontaneous, and its equilibrium position can be quantitatively determined by the relation EƟ=(RT/nF)lnK:
0.36V= lgK
One electron is transferred in the reaction, so K=1.3×106 can be calculated.
In addition, hypochlorous acid can also be disproportionated in water:
5HOCl (aq) → 2Cl 2(g) + (aq)+2H2O(l) + H2O (aq)
The reaction is the difference between two half-reactions:
EƟ=1.63 V-1.43 V=+0.20 V of the overall reaction, i.e., K=3×1013.
Another example is the disproportionation of into and Mn2+: in acid solution.
(aq) + 3H+(aq) → (ag) + Mn2+(ag) + 4H2O (1)
The above reaction can be regarded as the difference between the following two reductive half-reactions:
[HMn4]- (aq) + 7H+ (aq) + 4e → Mn2+(ag) + 4H2O (1) EƟ = +1.66 V
(ag) + 4H+(aq) + 4e → 4 Mn (aq)EƟ = +0.90 V
The difference between the two is + 0.76 V, so the disproportionation process tends to be complete (K=5×1052), so in fact, high concentration of X can only be generated from alkaline solution, but not from acidic solution.
8.4 Effect of precipitation in aqueous solution on oxidation-reduction reaction
It is very likely that there are some precipitants or impurities with coordination properties in the aqueous solution of the oxidation-reduction reaction, which will affect the reaction. Some examples are given below.
Because of the existence of precipitant, the concentration of substance in the electrode pair changes due to the formation of precipitate, which will inevitably lead to the change of electrode potential. According to the Nernst equation, if the concentration of oxidized form decreases, the electrode potential decreases; If the concentration of the reduced form becomes smaller, the electrode potential increases.
As a simple example, KCl is added to the solution of a standard Ag-Ag+ electrode such that c(Cl-)=1.0×10-2mol·dm-3, then the E value can be calculated as follows:
AgCl=Ag++Cl-
Ksp(AgCl)=c(Ag+)c(Cl-)=1.8×10-10
C(Ag+)= = =1.8×10-8
故E=EƟ+ lg
Therefore, it is calculated that:
E = 0.08 + 0.059 lg (1.8×10-8) = 0.34 V
8.5 Effect of Complex Formation on Potential in Aqueous Solution
In addition to the precipitation reaction, the impurity composition in the aqueous solution has an effect on the reaction of complex formation and the potential, as illustrated below.
Example 1 Since the ability of the ML6 of a complex to accept and lose electrons is different from that of the corresponding hydrated ion, the reduction potential must be affected when the hydrated ion of a metal forms a complex with a chemical entity in solution that is present as an impurity and has coordination properties. For example
[Fe(H2O)6]3+(aq)+e-→[Fe(H2O)6]2+(aq) EƟ=+0.77 V
[Fe(CN)6]3-(aq)+e-→[Fe(CN)6]4-(aq)EƟ=+0.36 V
The EƟ of the two half-reactions showed that [Fe(CN)6]3- was more difficult to reduce than [Fe(H2O)6]3+. In order to understand this difference, the reduction of the cyanogen complex may be considered as an addition of the following three reactions:
[Fe(CN)6]3-(aq)+6H2O(l)→[Fe(OH2)6]4+(aq)+6CN-(aq)
[Fe(OH2)6]3+(aq)+e-→[Fe(OH2)6]2+(aq)
[Fe(OH2)6]2+(aq)+6CN-(aq)→[Fe(CN)6]4-(aq)+6H2O
The coordinated water molecules in the two hydrated ions ([Fe(OH2)6]3+ and [Fe(OH2)6]2+)) both become more stable when replaced by CN-,However, the stability of [Fe(CN)6]3- is greater than that of [Fe(OH2)6]3+, that is, [Fe(CN)6]3- is more difficult to restore.
This example shows that when the complex formed in the high oxidation state of the metal is more stable, it is beneficial to the reductive oxidation (the reduction potential is more negative); When the complex formed in the low oxidation state of the metal is more stable, the oxidation reduction is favored (reduction potential correction).
Example 2 2,2 '-bipyridine can replace the CN- to replace the water molecule in the [Fe(OH2)6]3+ to form Fe (Ⅲ) chelate:
Mv+(aq)+e-→M(v-l)+(aq) EƟ(M)
MLv+(aq)+e-→ML(v-l)+(aq) EƟ(ML)
Fig. 40 is a thermodynamic cycle of the above reaction, and it can be seen that the chelating agent L can promote the formation of ML, as shown in Equations 71 and 72, thereby changing the standard electrode potential of the Mv+/M(v-1)+. Since the sum of ΔG of the whole reaction thermodynamic cycle is zero, in the following equation, Kox and KRed represent the equilibrium constant of Mv+/M(v-1)+ chelating L (K = [ML]/ ( [M] [L])), respectively, and the integral formula can be expressed as:
-FE(M)-RTlnKred+FEƟ(ML)+RTlnKox=0
E(M)-EƟ(ML)= ln
At 25 ° C, and ln X = ln 10 × log X, then:
E(M)-EƟ(ML)=(0.059 V) log
8.6 Effect of Slightly Dissolved Oxygen in Aqueous Solution on Oxidation-Reduction Reaction
For solution exposed to air, the possibility of oxidation reaction between solute and dissolved oxygen (O2 or atomic O) must be considered, for example, the hydrolysis of Fe (II) salts in aqueous solution often produces oxidation reaction:
In addition, when the aqueous solution of Bi(NO3)3 is exposed to air, the liquid surface tends to form a floating film of bismuth oxynitrate:
(a) Bi(NO2)3 → BiO(NO3) + 2NO↑
(b) 2Bi(NO2)3 + O2 + 2H2O → 2BiO(NO3) + 4 HNO2
For another example, when Pd enters the aqueous solution, Pd(OH)2 film is often formed on its surface after a period of time:
2 Pd + H2O + O2 → 2 Pd(OH)2↓
The main reason for this kind of oxidation reaction is dissolved oxygen, and its electrode potential is (EƟ):.
2H2O = O2 + 4H+ + 4e -1.23 V
4OH- = O2 + 2H2O + 4e -0.40 V
4OH- = 2O(g) + 2H2O + 2e -1.59 V
Fe(OH)2,Bi3+,Pd and the like can be oxidized, however, the overpotential required for a rapid reaction cannot be provided, and such oxygen reactions proceed rather slowly.
Many chemical substances, in addition to the oxidation-reduction reaction in aqueous solution, can also undergo oxidation-reduction reaction between solid substances, but there are often obvious differences between the two.
By grin, that above reaction can be easily carried out even if the reaction is carry out by merely mixing the above reactants homogeneously even at room temperature sometimes because the. Delta. G of the reaction is reduce, whereas the above two reactions cannot be carried out in an aqueous solution.
This is because in aqueous solution, the hydration and hydrolysis of ions and the transmission of e are difficult, resulting in difficult collision. Moreover, since:
I-/I2=0.535V (EƟ)。
[Fe(CN)6]4-/[Fe(CN)6]3-=0.36 V(EƟ)
This oxidation-reduction reaction cannot proceed even after the collision of the reactants.
8.7 Hydrated electron
In 1863, Weyl discovered that dissolving alkali metals in liquid ammonia would produce a light blue solution, which gradually changed from blue to copper with the increase of the amount of alkali metals dissolved (when the concentration of alkali metals was greater than 3 mol/L)[104]. Later studies have shown that alkali metal atoms dissociate electrons in liquid ammonia to produce metal cations and Solvated electrons, which polarize the surrounding ammonia molecules, and the hydrogen in the ammonia molecules points to the electrons, due to the repulsion of the lone pair of electrons on the N atom in the ammonia molecules.A certain distance is kept between the electron and the NH3, and a plurality of NH3 molecules surround the electron to form a cavity with a diameter of 300 to 340 pm, and the electron is bound in the cavity to form an Ammoniated electron, and the solvated electron is also regarded as the smallest anion. Solvated electrons can be generated in many systems. Except for the metal-liquid ammonia system, the lifetime of solvated electrons formed in other systems (such as aqueous solution) is very short and difficult to be observed directly[105]. The solvated electrons formed by aqueous solution are also called Hydrated electrons, which are surrounded by water molecules. The H atoms in the H2O point to the electrons, and the negative charges are distributed in a cavity with a radius of 250 ~ 350 pm, that is, the electrons are bound in the cavity formed by water molecules, as shown in Figure 41[106]. When the metal Na is in contact with water, the outermost electron of the Na atom is detached from the Na atom to form a Na+ and a hydrated electron, as shown in formula (91). The hydrated electron can migrate in aqueous solution and react when brought close enough to another species.
Na → Na + +
If the hydrated electron and the H2O molecule are close enough for some reason, they will also react to form atomic H and a hydrated OH-, as shown in Equation 92, but the reaction is very slow.
+ H2O → H +
The resulting H atom will quickly combine with another H atom to form a H2, which is a very fast reaction.
Therefore, it can be inferred that as long as the electron transfer reaction occurs in solution, hydrated electrons will be generated, which has a high diffusion coefficient and mobility, and is a strong reducing agent. The stability (lifetime) of solvated electrons is affected by the nature of the solvent, impurities, and light. The dielectric constant, acidity and basicity, and oxidizability of the solvent are the three main factors affecting the stability of solvated electrons. Generally speaking, the larger the dielectric constant is, the better the stability of the formed solvated electrons is. The stronger the acidity of the solution, the more H+ in the solution, the easier it is to react with hydrated electrons to form H atoms, and then to form H2. In acidic solutions, the half-life of hydrated electrons is about 10-11s, while in alkaline solutions, it is about 10-3s. The more oxidizing the solvent is, the less stable the solvated electrons are. If the solution contains catalytic species, such as transition metal salts, it can also reduce the stability of solvated electrons, which is manifested by the acceleration of chemical reactions. In addition, illumination can also promote the decomposition of solvated electrons and reduce the stability of solvated electrons. Further study on the hydrated electron and its relationship with the electron gain, loss, transfer and transmission between reactants in the oxidation-reduction reaction will certainly promote the understanding of the oxidation-reduction reaction process in aqueous solution.
9 Coordination reaction in aqueous solution
9.1 Formation thermodynamics of coordination reaction
The basic chemistry of coordination reactions in liquid aqueous solution is the substitution of a ligand for a coordinated water molecule in a hydrated cation in liquid aqueous solution. The strength of interaction between a ligand (relative to a solvent molecule, usually a H2O molecule, as a ligand) and a metal atom is generally expressed in terms of a "formation constant". The stepwise formation constant is the formation constant of each solvent water molecule substituted in the formation of the complex (complex), and the total formation constant is equal to the product of the stepwise formation constants.
(1) Substitutional coordination equilibrium and formation constant in aqueous solution
The reaction equilibrium constant Kf is the Formation constant of the product complex. The concentration of solvent (usually H2O) does not appear in the expression because it is constant in dilute solution and the activity is taken to be unity. The Kf value represents the bonding strength of the ligand relative to the H2O, and the :Kf value is large, which means that the binding force of the ligand with the metal ion is stronger than that of the solvent (H2O) molecule with the metal ion; On the contrary, it is weak. Since the variation range of Kf is too large (see Table 8), it is often expressed by its logarithm (logKf).
The discussion of stability becomes more complicated if more than one ligand is substituted. For example, the reaction of [Ni(OH2)6]2+ to [Ni(NH3)6]2+:
Even if the cis-trans isomerization reaction is not considered, at least six steps are included. For complexes of the general formula MLn, the overall reaction is:
M + nL → MLn
The stepwise formation constant is:
M + L ↔ ML 1 Kf1=
ML1 + L ↔ ML 2 Kf2=
The general formula is:
MLn-1 + L ↔ ML n Kfn=
The stepwise formation constant can help us understand and explore the relationship between structure and reactivity.
The overall formation constant (Overall formation constant)βn: is used to calculate the concentration of the final product (complex MLn).
M + nL ↔ ML n β n =
The total formation constant is equal to the product of the stepwise formation constants:
βn = Kf1Kf2…Kfn
Since the Kd and the Ka forms of the acid are the same (M instead of H+), this provides a convenient way to compare the metal complexes with Br Brönsted acid. If protons are simply regarded as general cations, Kd and Ka can be listed in one table. For example, HF can be seen as a complex of a H+(B- acid) with a F-(B- base) that acts as a ligand.
(2) Change trend of stepwise formation constant
The magnitude of the formation constant is also a direct reflection of the magnitude and sign of the Gibbs free energy of formation under standard conditions (because ΔfGƟ=-RTlnKf), and the order of the stepwise formation constant usually seen is Kf1>Kf2>…>Kfn. This sequence is not difficult to explain: as the formation reaction proceeds, both the number of ligands and the number of H2O molecules that can be substituted decrease. It is not difficult to see this from the comparison of the following two reactions:
[M(OH2)5L](aq) + L(aq) ↔ [M(OH2)4L2](aq) + H2O(l)
[M(OH2)4L2](aq) + L(aq) ↔[M(OH2)3L3](aq) + H2O(l)
The decrease in the stepwise formation constant reflects the fact that the statistical factor decreases as the ligand displacement process proceeds. Also consider the fact that as the number of bonded ligands increases, the possibility of a reverse reaction also increases. The above explanation is more or less correct with that data of the Serie of complexes (from [Ni(H2O)6]2+ to [Ni(NH3)6]2+)) lis in Table 9, in which the enthalpy change of the reaction in six consecutive steps is less than the 2 kJ·mol-1.
The inversion of the relationship generally indicates an important change in the electronic structure of the complex as more ligands coordinate. For example, the Tris (bipyridine) complex of Fe (II) ([Fe(bpy)3]2+) is significantly more stable than the bis (bipyridine) complex ([Fe(bpy)2(OH2)2]2+). This phenomenon may be related to the change of the electronic configuration: the bis (bipyridine) complex (note that there is a weak field ligand H2O molecule) is converted from a high spin (weak field) configuration to a low spin (strong field) configuration, and the ligand field stabilization energy (LFSE) increases significantly.
The Kf3 anomaly in Example 2 is significantly lower than the Kf2, indicating a major change in the properties of the complex, which is generally believed to be due to the change of the complex from a six-coordinate configuration to a four-coordinate configuration.
(3) chelation effect and macrocyclic effect
A comparison of the Kf1 of complexes of bidentate chelating ligands (e.g. 1,2-diaminoethane, en) with the β2 of related diammine complexes shows that in general the former are larger than the latter:
[Cd(OH2)6]2+(aq) + en (aq) ↔ [Cd(en)(OH2)4]2+(aq) + 2H2O(l)
Both reactions form two similar Cd — N bonds, but the former forms a complex containing a chelating ligand. The phenomenon that chelating complexes are more stable than non-chelating analogues is called Chelate effect. The chelation effect mainly occurs in the difference of reaction entropy between the two complexes (chelating and non-chelating) in dilute solution. The chelating reaction results in an increase in the number of independent molecules in solution, while the net change in the number of non-chelating molecules is zero (compare the above two chemical equations). The former therefore has a more positive reaction entropy and is therefore a favorable process. The above interpretation is supported by the results of the determination of the reaction entropy in dilute solution.
The favorable effect of entropy on chelation is not limited to bidentate ligands and in principle applies to any multidentate ligand. In fact, the more donor sites a multidentate ligand has, the greater the entropic benefit of replacing a monodentate ligand. Complexes formed upon coordination of macrocyclic ligands containing multiple donor atoms [such as phthalocyanines and crown ethers] are even more stable than expected. This is the so-called Macrocyclic effect, which is believed to be caused by the entropy effect and the additional energy contribution term in the chelation effect. The energy term contributes because the coordinating group is preorganized into a macrocycle, and no additional strain is introduced when the ligand coordinates.
The chelating effect and the macrocyclic effect are of great practical importance. Most reagents used in complexometric titrations are multidentate chelating agents (e.g., EDTA4-); Most biochemical binding sites for metal atoms are chelating ligands or macrocyclic ligands. A sign that the chelating effect and the macrocyclic effect are operative is a formation constant of up to 1012~1025.
In addition to the thermodynamic explanation for the chelation effect, kinetic factors are at play. Once one binding group of the multidentate ligand is coordinated to the metal, it will cause the other binding groups to be closer to the metal atom, thus facilitating the subsequent coordination. Therefore, the formation of the chelate is also kinetically more favorable.
9.2 Ligand substitution reaction and substitution reaction rate
The rate span of the substitution reaction is very large and depends on the structure of the complex. Complexes with fast reaction are called active complexes. Complexes that react slowly are called inert or inactive complexes.
For coordination chemistry in solution, reaction rate and reaction equilibrium are equally important. Many isomers of Co (Ⅲ) and Pt (Ⅱ) ammonia complexes have played a vital role in the development of coordination chemistry, but if the rate of ligand substitution reaction and isomer interconversion is very fast, it will be difficult to isolate these isomers. The following is a discussion of what factors determine that some complexes are stable for a long period of time, while others react rapidly.
The rate of conversion of one complex to the other is determined by the height of the activation energy barrier between the two. Thermodynamically unstable complexes that are stable for a long time (traditionally at least 1 min) are often referred to as "inert" complexes, but the term Non-labile may be more appropriate to express this inertness. Complexes in which rapid equilibrium occurs are called Labile complexes. For example, [Ni(OH2)6]2+ is an active complex with a half-life of only milliseconds when the coordinated H2O molecule is replaced by another H2O molecule or other stronger bases. However, the half-life of the replacement of the coordinated H2O molecule in the inactive complex [Co(NH3)5(OH2)]3+ by a stronger base is as long as several hours.
Fig. 42 shows the characteristic lifetimes of some important hydrated metal ion complexes. The species at one end of the span has a lifetime of only 1 ns (roughly the time it takes a molecule to diffuse a molecular diameter distance in solution), while the species at the other end has a lifetime of several years. Even so, the map does not show the time of the longest life (equivalent to geological time).
The following two points can now be summarized: (1) Metal complexes without other additional stabilizing factors, such as ligand field stabilization energy (LFSE) and chelation effect, are the most active complexes. Any additional stabilizing factor will increase the activation energy of the ligand substitution reaction, thereby reducing the activity of the complex. (2) Very small ions tend to be unreactive. This is not only because the strength of the M-L bond is relatively high; From steric considerations, it is also more difficult for the incoming ligand to get close to the metal atom.
In addition to the above two points, other important points that can be concluded are that the complexes of all s-block ions are extremely active except for the smallest ions (Be2+ and Mg2+), which form inactive complexes; The complexes of M (Ⅲ) ions in f region are very active. The complexes of d10 ions (Zn2+, Cd2+ and Hg2+) are usually very active. The complexes of 3D M (Ⅱ) ions in the d block usually show moderate activity, and the distorted Cu (Ⅱ) complexes appear to be the most active. The complexes of d-M (Ⅲ) ions are obviously less active than those of d-M (Ⅱ) ions. The d-metal complexes with d3 and low-spin d6 configurations [such as Cr (Ⅲ), Fe (Ⅱ), and Co (Ⅲ)] are generally inactive because of their large LFSE. Metal chelate complexes with the same configuration (such as [Fe(phen)3]2+)) appear to be more inactive; The fact that complexes of 4D and 5d series metals are generally inactive reflects their high LFSE and strong metal-ligand bonding forces.
In addition to the above points, the structure of the ligand in the complex also affects the rate of the coordination-substitution reaction. The formation constant describing the equilibrium of the coordination reaction and the substitution reaction rate constant of the ligand are important factors that affect or even determine the progress of the coordination reaction in aqueous solution. This is not only limited to ligand substitution reactions, but also to almost all important coordination reactions carried out in aqueous solution, such as hydration isomerization reactions,This is true of hydrolysis reactions (including acid and base hydrolysis, as well as the host chemistry of hydrolysis reactions of octahedral complexes), and even of coordination oxidation-reduction in aqueous solution characterized by electron gain and loss, transfer and transmission (inner-sphere and outer-sphere mechanisms)[110][111]. In addition to the above two points 1 and 2, which are important factors affecting or even determining the coordination reaction in aqueous solution, the composition, structure and properties of liquid water can also have an important impact on the coordination chemical reaction in liquid water (including reactants, reaction mechanism, structure and state of intermediate and final products).
9.3 Effect of the composition, structure and properties of liquid water on substitution-coordination reaction
When the "substitution-coordination reaction" is carried out in liquid and aqueous solution, it is necessary to pay careful attention to and understand the chemical effects and effects of "water molecules" on ligands, metal ions, intermediate coordination products of stepwise substitution coordination reactions and final coordination products in liquid state.Such as hydration, hydrolysis, dissolution and precipitation in water, oxidation and reduction in aqueous solution, thus affecting the complexation-coordination reaction in liquid aqueous solution. This is an important description of condensed matter chemistry of the composition, structure and properties of the liquid state in liquid chemical reactions. Several reactions are described below as examples.
Example 1 Ligand Substitution in Aqueous Solvent
The use of ligand substitution reactions in aqueous solution is one of the most common methods for the synthesis of complexes. The synthesis of acetylacetone, ammonia, cyanogen, amino acid and amine complexes is mostly carried out in aqueous solution. For example, the classical complex [Cu(NH3)4]SO4 can be prepared by the reaction of CuSO4 aqueous solution with excess ammonia.
At room temperature, the coordinated water of the [Cu(H2O)4]2+ is immediately replaced by NH3 to form a dark blue solution, and when ethanol is added to the solution, the dark blue [Cu(NH3)4]SO4·xH2O crystallizes. This method is also suitable for the synthesis of ammine compounds such as Ni (II), Co (II) and Zn (II), but it is not suitable for the formation of ammine compounds of hard acid metal ions such as Fe (III), Cr (III)
NH3 + H2O ↔ + OH-
There is a competition between the NH3 and the hard alkali OH- for the metal ions. Although excessive ammonia is added, the hard acid metal ions mainly combine with the hard alkali OH- to form hydroxide precipitates with very small Ksp. Therefore, it is necessary to adopt the method of synthesizing the hard acid metal ammine in the liquid ammonia system of non-aqueous medium described later, that is, the substitution-coordination reaction in non-aqueous solution or the solid state reaction.
Example 2 Ligand Substitution Reaction in Nonaqueous Solvent
According to the "spectrochemical sequence", H2O is a ligand with moderate strength. If the ligand substitution reaction is to be carried out in aqueous solution, some ligands weaker than H2O are often unable to compete with them, and it is difficult to obtain the expected product. Therefore, this kind of reaction must be carried out in non-aqueous medium, so as to: (1) prevent the hydrolysis of some metal ions (such as Fe3+, Al3+, Cr3+, etc.)[112]; (2) to dissolve the ligand; (3) The solvent itself is a weak ligand and cannot compete with water; (4) The solvent itself is a ligand (such as a NH3);(5), and a weaker ligand than water is expected to participate in the coordination. In the synthesis of the complexes, the commonly used non-aqueous solvents are absolute ethanol, absolute methanol, acetone, dichloromethane, chloroform, tetrahydrofuran (THF), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and liquid ammonia. Examples will be given below.
Synthesis of (1)[Cr(en)3]Cl3
Using CrCl3·6H2O as metal source to react with en in aqueous solution, the target product can not be obtained, only hydroxide precipitation can be obtained.
Many similar reactions cannot use water as a solvent. If anhydrous Cr2(SO4)3 is used as a raw material, the reaction can be carried out according to the following reaction formula.
In Eq. 116, KI is added to provide a bulky counterion, favoring the precipitation of the product from the non-aqueous solvent. In order to obtain chlorine salt, two methods can be used: one is ion exchange method, but it is not suitable for large amount of product; The other is to mix and grind the newly precipitated AgCl with the [Cr(en)3]I3
[Cr(en)3]I3 + 3 AgCl → [Cr(en)3]Cl3 + 3 AgI
(2) synthesis of phenanthroline (phen) or bipyridine (bpy) complexes
The structures and properties of phenanthroline and the corresponding bipyridine complexes are very similar, so the preparation methods are also similar. As ligands, neither phen nor bpy is soluble in water. In general, therefore, there are several methods for the synthesis of these two types of complexes, one of which is to dissolve the ligand in a water-miscible organic solvent and then add the ligand solution to a concentrated solution of the metal salt.[Co(phen)3]Cl2 、[Co(bpy)3](ClO4)2 、[Mn(bpy)3](ClO4)2 、[Cu(bpy)2]Cl2 、[Cu(phen)2]Cl2, [Ag(phen)2]NO3, and [V(phen)3](ClO4)2 were all synthesized according to this method.
Solid state synthesis of 3[(CH3)4N]NiCl4.
The coordination reaction can be carried out in aqueous solution as shown in formula 118:
In early experiments, it was found that although pyridine reacted with Na3[RhCl6] in aqueous solution to form the complex RhCl3(py)3, the reaction remained in the step of forming [RhCl3(py)3], which is insoluble in water, and a long time of reflux was needed to replace Cl- by the fourth py. To overcome this difficulty, another solvent (ethanol) was added in an attempt to enlarge the solubility of [RhCl3(py)3] to speed up the reaction to form [RhX2(py)4]+. As expected, the target product was quantitatively obtained at room temperature by dissolving the insoluble substance when the reaction proceeded as expected, and no intermediate was observed. This experimental process is illustrated in Fig. 43.
However, the mechanism study in the water-ethanol solution of fig. 43 shows that ethanol acts as a reducing agent to reduce Rh (III) to Rh (I) species in the reaction, and the [Rh(py)4]+ formed by ethanol acts as a catalyst to promote internal double electron transfer. Thus, the addition of ethanol does not act to increase the solubility of [RhCl3(py)3] as originally thought, but rather acts as a reducing agent. The possible mechanism of the reaction can be represented as Fig. 44.
The mechanism shown in fig. 44 apparently requires only trace amounts of Rh (I) species to be present, and gas chromatography analysis shows that little ethanol is lost after the reaction. More importantly, traces of acetaldehyde (the product of ethanol oxidation) were found in the product. In order to prove the rationality of the mechanism, the Rh (Ⅰ) compound was deliberately added to the alcohol-free solution, and the [RhCl(CO)2]2 was found to be a very effective catalyst. The same method can be used to prepare other Rh (Ⅲ) complexes.
10 Summary and Prospect
In this paper, we take the above eight kinds of reactions as examples, based on the condensed state, and discuss their reactions in liquid water under general conditions (the reactions in liquid water under hydrothermal and supercritical conditions will be discussed separately).How does the composition (especially the existence and concentration of H+ and OH- in the reaction solution), structure and properties of liquid water and aqueous solution affect the chemical reaction?It includes reaction chemistry related to the existing state and chemical activity of reactants, the process and mechanism of chemical reaction, the composition and structure of intermediate and final products of reaction, and the results and laws. It is also worth mentioning that liquid water is not only a good solvent, but also has its own chemical activities, such as hydrolysis, acid-base, oxidation-reduction, coordination, etc., which make it more distinctive in condensed matter reaction chemistry. We hope that through the discussion of aqueous solution, based on the level of condensed (liquid) chemistry, we can put forward many arguments and discussions about the composition and structure of liquid state and the reaction chemistry in liquid state.It is helpful for the research, exploration and understanding of chemical reactions in other types of liquids (such as organic solvents, ionic liquids, molecular melts, etc.), and at the same time, it strengthens the exchange, discussion and criticism between them, and provides a useful basis for promoting the research and discipline construction of condensed matter chemistry with liquid as the main research object.
XuR R, YuJ H, YanW F. Progress in Chemistry, 2020, 3:1017.
[3]
(徐如人, 于吉红, 闫文付. 化学进展, 2020, 3:1017.).
[4]
LiW K, ZhouG D, MakT C W. Advanced Structural Inorganic Chemistry. Oxford: Oxford University Press, 2008, 623.
[5]
OhtakiH, RadnaiT. Chem. Rev., 1993, 93(3): 1157.
[6]
MaginiM, LicheriG, PaschinaG, PiccalugaG, PinnaG. X-Ray Diffraction of Ions in Aqueous Solutions:Hydration and Complex Formation. New York: CRC Press, 2018.
DengT L, WangS Q, GuoY F. Metastable Phase Equilibrium and Phase Diagram of Salt Lake Brine System in Qaidam Basin. Beijing: Science Press, 2017. 95-102.
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975, 319.
[54]
AtkinsP, OvertonT, RourkeJ, WellerM, ArmstrongF, HagermanM. Inorganic Chemistry. Fifth Edition. New York: Oxford University Press, 2010. 112.
[55]
CottonF A, WilkinsonG. Advanced Inorganic Chemistry. Fifth Edition. New York: John Wiley & Sons, 1988. 100.
[56]
ZundelG, MetzgerH. Z. Phys. Chem., 1968, 58: 225.
[57]
ZundelG. Hydrogen Bonds with Large Proton Polarizability and Proton Transfer Processes in Electrochemistry and Biology. In Advances in Chemical Physics, 1999. 1-217. DOI: 10.1002/9780470141700.ch1.
[58]
WickeE, EigenM, AckermannT. Zeitschrift Für Physikalische Chemie, 1954, 1(5/6): 340.
[59]
YehL I, OkumuraM, MyersJ D, PriceJ M, LeeY T. J. Chem. Phys., 1989, 91(12): 7319.
[60]
OkumuraM, YehL I, MyersJ D, LeeY T. J. Phys. Chem., 1990, 94(9): 3416.
[61]
ZavitsasA A. J. Phys. Chem. B, 2001, 105(32): 7805.
[62]
HultheG, StenhagenG, WennerströmO, OttossonC H. J. Chromatogr. A, 1997, 777(1): 155.
[63]
IyengarS S, PetersenM K, DayT J F, BurnhamC J, TeigeV E, VothG A. J. Chem. Phys., 2005, 123(8): 084309.
[64]
MarkovitchO, AgmonN. J. Phys. Chem. A, 2007, 111: 2253.
[65]
ChristeK O, CharpinP, SoulieE, BougonR, FawcettJ, RussellD R. Inorg. Chem., 1984, 23(23): 3756.
[66]
KearleyG J, PressmanH A, SladeR C T. Journal of the Chemical Society, Chemical Communications1986, 1801.
[67]
BorgiasB A, HardinG G, RaymondK N. Inorg. Chem., 1986, 25(7): 1057.
[68]
HenkeH, KuhsW F. Zeitschrift Für Kristallographie Cryst. Mater., 1987, 181(1/4): 113.
[69]
ArdonM, BinoA, JacksonW G. Polyhedron, 1987, 6(2): 181.
[70]
ArdonM, BinoA, MichelsenK. J. Am. Chem. Soc., 1987, 109(7): 1986.
LiuH Y, FangC H, FangY, ZhouY Q, ZhuF Y, GeH W, YangZ X, TangY L. Acta Phys. -Chim. Sin., 2014, 30: 1979.
[100]
IlerR K. The chemistry of silica. New York: Wiley-Interscience, 1979. 174.
[101]
XuR R, PangW Q, YuJ H, HuoQ S, ChenJ S. Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure. Singapore:John Wiley & Sons (Asia),2007. 278.
[102]
XuR R, PangW Q, YuJ H, HuoQ S, ChenJ S. Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure. Singapore:John Wiley & Sons (Asia),2007. 281.
[103]
XuR R, PangW Q, YuJ H, HuoQ S, ChenJ S. Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure. Singapore:John Wiley & Sons (Asia),2007. 282.
[104]
IlerR K. The Chemistry of Silica. New York: Wiley-Interscience. 1979. 234.
[105]
IlerR K. The Chemistry of Silica. New York: Wiley-Interscience. 1979. 235.
[106]
XuR R, PangW Q, HuoQ S. Molecular Sieves and Porous Materials Chemistry. The Second Edition. Beijing: Science Press,2015.265-266
McCormickA V, BellA T, RadkeC J. Application of29Si and27Al NMR to Determine the Distribution of Anions in Sodium Silicate and Sodium Alumino-Silicate Solutions, In Studies in Surface Science and Catalysis. Murakami Y, Iijima A, Ward J W, Eds. Elsevier,1986. 28:247-254.
[109]
CejkaJ, CormaA, ZonesS. Zeolites and Catalysis-Synthesis, Reactions and Applications; Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA, 2010. 13-20.
[110]
XuR R, PangW Q, HuoQ S. Molecular Sieves and Porous Materials Chemistry. The Second Edition. Beijing: Science Press,2015. 232-325
DentGlasser L, HarreyG. MR studies of aluminosilicates in solution In Proceedings of the Sixth International Zeolite Conference. Olson D, Bisio A, Eds. Butterworths, Stoneham, 1984.925-933.
[113]
ZhdanovS P. "Molecular Sieves", Proceedings of the 1st International Zeolite Conference. Barrer R M, Eds, London: Soc. Chem. Ind., 1968. 62.
[114]
RobsonH. Verified Syntheses of Zeolitic Materials. Amsterdam: Elsevier, 2001. 1.
[115]
BaerlocherC, MeierW M, OlsonD H. Atlas of Zeolite Framework Types. 6th Ed. Amsterdam: Elsevier, 2001. 120-121.
[116]
BaerlocherC, MeierW M, OlsonD H. Atlas of Zeolite Framework Types.6th Ed. Amsterdam: Elsevier, 2001: 194-195.
[117]
BaerlocherC, MeierW M, OlsonD H. Atlas of Zeolite Framework Types. 6th Ed. Amsterdam: Elsevier, 2001: 140-141.
[118]
FlanigenE M. A Review and New Perspectives in Zeolite Crystallization. In Molecular Sieves. American Chemical Society, 1973. 121:119-139.
[119]
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975. 518.
[120]
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975. 516-536.
[121]
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975. 523.
[122]
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975. 524.
[123]
WellsA F. Structural Inorganic Chemistry. 4th Edition. London: Oxford University Press, 1975. 526.
[124]
ShriverD, WellerM, OvertonT, RourkeJ, ArmstrongF. Inorganic Chemistry. 6th Ed. Great Britain: Oxford University Press, 2014. 165.
[125]
SongT Y, XuJ N, ChengG Z, WangL. Inorganic Chemistry. The Third Edition. Beijing: Higher Education Press, 2015.337.
[126]
(曹惠民,译. 无机化学.第3版. 北京: 化学工业出版社, 2015.337.)
[127]
SongT Y, XuJ N, ChengG Z, WangL. Inorganic Chemistry. The Third Edition. Beijing: Higher Education Press, 1988.1460-1474.
[128]
(曹惠民,译. 无机化学.第3版. 北京: 化学工业出版社, 1988.1460-1474.)
[129]
ShriverD, WellerM, OvertonT, RourkeJ, ArmstrongF. Inorganic Chemistry. 6th Ed. Great Britain: Oxford University Press, 2014. 169.
PDF
