1.2 MAIN DIRECTIONS IN THE DEVELOPMENT OF THE THEORY OF SOLUTIONS

Physical theory of solutions. The development of views on the nature of solutions since ancient times has been associated with the general course of development of science and production, as well as with philosophical ideas about the causes of chemical affinity between different substances. In the 17th and in the first half of the 18th century. The corpuscular theory of solutions has become widespread in the field of natural sciences and philosophy. In this theory, the dissolution process was considered as a mechanical process, when the corpuscles of the solvent enter the pores of the bodies and tear off the particles of the dissolved substance, which occupy the pores of the solvent forming a single solution. Such ideas initially satisfactorily explained the fact that a given solvent can dissolve not all substances, but only some.

At the beginning of the 19th century prerequisites are being created for the development of a physical theory of solutions, which was a generalization of a number of studies. The physical theory of solutions, which arose mainly on the basis of the works of J. Van't Hoff, S. Arrhenius and W. Ostwald, was based on an experimental study of the properties of dilute solutions (osmotic pressure, an increase in the boiling point, a decrease in the freezing point of a solution, a decrease in vapor pressure over a solution) , depending mainly on the concentration of the solute, and not on its nature. Osmosis is the spontaneous penetration of a solvent into a solution that is separated from it by a semi-permeable partition through which the solvent can enter, cannot, the solute passes.

Solution and solvent separated by a semi-permeable partition can be considered as two phases. The equilibrium of the solvent on both sides of the partition is expressed by the equality of its chemical potential in solution (to which additional pressure is applied) and the chemical potential of a pure solvent.

Quantitative laws (van't Hoff, Raoult) were interpreted in the continuation that in dilute solutions the molecules of the solute are similar to the molecules of an ideal gas. Deviations from these laws, observed for electrolyte solutions, were explained on the basis of the theory of electrolytic dissociation by S. Arrhenius.

The analogy between highly dilute solutions and gases seemed so convincing to many scientists that they began to consider the process of dissolution as a physical act. From the point of view of these scientists, the solvent is only a medium into which solute particles can diffuse. The simplicity of the representations of the physical theory of solutions and its successful application to explain many properties of solutions ensured the rapid success of this theory.

Chemical theory of solutions. DI. Mendeleev and his followers considered the process of solution formation as a kind of chemical process, which is characterized by the interaction between the particles of the components. DI. Mendeleev considered solutions as systems formed by particles of a solvent, a solute and unstable chemical compounds that form between them and are in a state of partial dissociation. DI. Mendeleev noted that the processes occurring in a solution are dynamic in nature and the need to use the entire amount of physical and chemical information about the properties of the particles that form the solution, emphasized that all components of the solution are equal and without taking into account the properties and states of each of them it is impossible to give a complete characterization systems as a whole. The scientist attached great importance to the study of the properties of solutions as a function of temperature, pressure, concentration; he was the first to express the idea of ​​the need to study the properties of solutions in mixed solvents. Developing the teachings of D.I. Mendeleev, supporters of the chemical view of the nature of solutions pointed out that the particles of the dissolved substance do not move in a vacuum, but in the space occupied by the particles of the solvent, with which they interact, forming complex compounds with different stability. The development of the theory of D.I. Mendeleev is the polyhedral theory of the formation of solutions, according to which elementary space groups-polyhedra are created in a liquid from homogeneous and heterogeneous molecules. However, the chemical theory cannot explain the mechanism of formation of ideal solutions, deviations in the properties of real solutions from the properties of ideal solutions.

The development of the chemical theory of solutions proceeded in several directions united by a single idea of ​​the interaction of a solvent with a solute. These studies concerned finding certain compounds in solution based on the study of property-composition diagrams, the study of vapor pressure over solutions, the distribution of substances between two solvents, and the study of the thermochemistry of solutions. Work on the determination of compounds in solutions was associated with great difficulties, since it was impossible to prove the existence of complex compounds (hydrates) in aqueous solutions by direct experiment, since they are in a state of dissociation, and attempts to isolate them from solutions in an undecomposed form ended in failure. Thermodynamic studies were of great importance for confirming the chemical theory of solutions. On many systems, it was shown that during the formation of a solution, cooling or heating of the system is observed, which was explained by the chemical interaction between the components. The chemical nature of the dissolution process was confirmed both by studies of the vapor pressure over the solution and by the study of the distribution of substances between two solvents.

By the beginning of the 20th century extensive experimental material has been accumulated showing that solutions are complex systems in which the phenomenon of association, dissociation, complex formation is observed, and in their study it is necessary to take into account all types of interaction between particles present and formed in a solution.

Due to the wide variety of solutions, both the physical and chemical theory of solutions are used to explain their nature and properties.

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Solutions

One of the components is necessarily a solvent, the remaining components are solutes.

A solvent is a substance that, in its pure form, has the same state as a solution. If there are several such components, then the solvent is the one whose content in the solution is greater.

Solutions are:

1. Liquid (NaCl solution in water, I 2 solution in alcohol).

2. Gaseous (mixtures of gases, for example: air - 21% O 2 + 78% N 2 + 1% other gases).

3. Solid (metal alloys, for example: Cu + N, Au + Ag).

Liquid solutions are the most common. They consist of a solvent (liquid) and solutes (gaseous, liquid, solid).

Liquid solutions

Such solutions can be aquatic and non-aqueous.

Aquatic

non-aqueous

For a long time, there were two points of view on the nature of dissolution: physical and chemical. According to the first, solutions were considered as mechanical mixtures, according to the second, as unstable chemical compounds of molecules of the solute and the solvent. The last point of view was expressed by D.I. Mendeleev in 1887 and is now universally recognized.

BASIC PROVISIONS OF THE CHEMICAL THEORY OF SOLUTIONS, created by Mendeleev, are reduced to the following:

1. The formation and existence of a solution is due to the interactions between all particles, both those that already existed and those formed during dissolution.

2. A solution is a dynamic system in which decaying compounds are in mobile equilibrium with decay products in accordance with the law of mass action.

When a substance is dissolved, two processes occur associated with changes in the energy of the “substance-solvent” system:

1) destruction of the structure of the dissolved substance (in this case, a certain energy is expended) - the reaction is endothermic.

2) the interaction of the solvent with the particles of the solute (heat is released) - the reaction is exothermic.

Depending on the ratio of these thermal effects, the process of substance dissolution can be exothermic (∆H< O) или эндотермическим (∆H >O).

The heat of solution ∆H is the amount of heat released or absorbed when 1 mol of a substance is dissolved.

The heat of dissolution for different substances is different. So, when potassium hydroxide or sulfuric acid is dissolved in water, the temperature rises significantly (∆H< O), а при растворении нитратов калия или аммония резко снижается (∆H >O).

The release or absorption of heat during dissolution is a sign of a chemical reaction. As a result of the interaction of a solute with a solvent, compounds are formed, which are called solvates (or hydrates if the solvent is water). Many compounds of this type are fragile, however, in some cases strong compounds are formed, which can be easily separated from the solution by crystallization.

In this case, crystalline substances containing water molecules fall out, they are called crystalline hydrates(for example: copper sulfate CuSO 4 * 5 H 2 O - crystalline hydrate); water, which is part of crystalline hydrates, is called crystallization water.

The concept of hydration (the connection of a substance with water) was put forward and developed by the Russian scientist I.A. Kablukov and V.A. Kistyakovsky. on the basis of these ideas, the chemical and physical points of view on solutions were united.

In this way, dissolution solutions– physical and chemical systems.

1.Solutions- homogeneous (homogeneous) systems of variable composition, which contain two or more components and products of their interaction.

2. Solutions consist of a solvent and a solute.

3. Solutions are:

A) Liquid (NaCl solution in water, I 2 solution in alcohol).

B) Gaseous (mixtures of gases, for example: air - 21% O 2 + 78% N 2 + 1% other gases).

C) Solid (metal alloys, for example: Cu + N, Au + Ag).

Liquid solutions
liquid + gaseous substance (solution O 2 in water)	liquid + liquid substance (solution H 2 SO 4 in water)	liquid + solid (solution of sugar in water)

Such solutions can be aquatic and non-aqueous.

5.Water solutions in which the solvent is water.

6. Non-aqueous- solutions in which solvents are other liquids (benzene, alcohol, ether, etc.)

7. MAIN PROVISIONS OF THE CHEMICAL THEORY OF SOLUTIONS:

1. The formation and existence of a solution is due to the interactions between all particles, both those that already existed and those formed during dissolution.

2. A solution is a dynamic system in which decaying compounds are in mobile equilibrium with decay products in according to the law of mass action.

8. When a substance is dissolved, two processes occur associated with changes in the energy of the “substance-solvent” system:

1.destruction of the structure of the dissolved substance (in this case, a certain energy is expended) - the reaction is endothermic.

2. interaction of the solvent with the particles of the dissolved substance (heat is released) - the reaction is exothermic.

9. The release or absorption of heat during dissolution is a sign of a chemical reaction.

10. As a result of the interaction of a solute with a solvent, compounds are formed that are called solvates (or hydrates if the solvent is water)

11. Crystalline substances containing water molecules in their composition are called crystalline hydrates(for example: copper sulfate CuSO 4 * 5 H 2 O - crystalline hydrate); water, which is part of crystalline hydrates, is called crystallization

12. Dissolution is not only a physical, but also a chemical process, and solutions– physical and chemical systems.

Types of solutions (to know).

Dissolution is a reversible process:

According to the ratio of the predominance of the number of particles passing into the solution and removed from the solution, solutions are distinguished rich, unsaturated And oversaturated.

On the other hand, according to the relative amounts of solute and solvent, solutions are divided into diluted concentrated

A solution in which a given substance at a given temperature no longer dissolves, i.e. the solution is in equilibrium with the solute is called rich unsaturated. IN oversaturated Solubility measure solubility or coefficient The solubility of a substance at a certain temperature is the number of grams of it that dissolves in 100 g of water.

By solubility in water, solids are conventionally divided into 3 groups:

1. Substances that are highly soluble in water (10 g of a substance in 100.0 water. For example, 200 g of sugar dissolves in 1 liter of water).

2. Substances that are slightly soluble in water (from 0.01 to 10 g of a substance in 100 g of water. For example: gypsum CaSO 4 dissolves 2.0 in 1 liter).

3. Substances that are practically insoluble in water (0.01 g in 100.0 water. For example, AgCl - 1.5 * 10 -3 g dissolves in 1 liter of water).

The solubility of a substance depends on the nature of the solvent, on the nature of the solute, temperature, pressure (for gases).

The solubility of gases decreases with increasing temperature and increases with increasing pressure.

The dependence of the solubility of solids on temperature is shown by the solubility curve.

The solubility of many solids increases with increasing temperature.

Solubility curves can be used to determine:

1. The coefficient of solubility of substances at different temperatures.

2. The mass of the solute that precipitates when the solution is cooled from t 1 0 C to t 2 0 C.

The process of isolating a substance by evaporating or cooling its saturated solution is called recrystallization. Recrystallization is used to purify substances.

Unfortunately, until now there is no theory that would allow us to combine the results of individual studies and derive general laws of solubility. This situation is largely due to the fact that the solubility of various substances depends very differently on temperature.

The only thing that can be guided to some extent is the old rule found in experience: like dissolves into like. Its meaning in the light of modern views on the structure of molecules is that if the solvent itself has non-polar or low-polar molecules (for example, benzene, ether), then it will dissolve well from a substance with non-polar or low-polar molecules, worse - substances with greater polarity and substances constructed according to the ionic type will practically not dissolve. On the contrary, a solvent with a strongly pronounced polar nature of the molecules (for example, water) will, as a rule, dissolve substances with molecules of polar and partly ionic types well, and poorly - substances with non-polar molecules.

1. Dissolution is a reversible process: solute + solvent ↔ substance in solution ± Q.

2. According to the ratio of the predominance of the number of particles passing into the solution and removed from the solution, solutions are distinguished rich, unsaturated And oversaturated.

3. According to the relative amounts of the solute and solvent, solutions are divided into diluted(contain little solute) and concentrated(contains a lot of solute).

4. A solution in which a given substance at a given temperature no longer dissolves is called rich, and a solution in which an additional amount of a given substance can still be dissolved - unsaturated. IN oversaturated solutions contain more substances than saturated solutions.

5.Solubility The property of a substance to dissolve in water and other solvents is called.

6. The solubility of a substance depends on the nature of the solvent, on the nature of the solute, temperature, pressure (for gases).

4. Methods for expressing the concentration of solutions: mass fraction

(know).

The quantitative composition of the solution is determined by its concentration.

Concentration is the amount of solute per unit volume.

There are two types of designations for the concentration of substances - analytical and technical.

Lecture 1

"THE CONCEPT OF "SOLUTION". CHEMICAL THEORY OF SOLUTIONS»

Solutions are important in human life and practical activities. Solutions are all the most important physiological fluids (blood, lymph, etc.). The body is a complex chemical system, and the vast majority of chemical reactions in the body occur in aqueous solutions. It is for this reason that the human body is 70% water, and severe dehydration occurs quickly and is a very dangerous condition.

Many technological processes, such as the production of soda or nitric acid, the isolation and purification of rare metals, the bleaching and dyeing of fabrics, proceed in solutions.

To understand the mechanism of many chemical reactions, it is necessary to study the processes occurring in solutions.

The concept of "solution". Types of solutions

Solution- solid, liquid or gaseous homogeneous system consisting of two or more components.

homogeneous system consists of one phase.

Phase- a part of the system, separated from its other parts by the interface, when passing through which the properties (density, thermal conductivity, electrical conductivity, hardness, etc.) change abruptly. The phase can be solid, liquid, gaseous.

The most important type of solutions are liquid solutions, but in a broad sense, solutions are also solid (brass alloy: copper, zinc; steel: iron, carbon) and gaseous (air: a mixture of nitrogen, oxygen, carbon dioxide and various impurities).

The solution contains at least two components, of which one is solvent, while others are solutes.

Solvent is a component of a solution that is in the same state of aggregation as the solution. The solvent in the solution by mass is always greater than the rest of the components. The solute is in solution in the form of atoms, molecules or ions.

Differ from solutions:

Suspension is a system consisting of fine solid particles suspended in a liquid (talc in water)

Emulsion- this is a system in which one liquid is fragmented in another liquid that does not dissolve it (i.e., small drops of a liquid that are in another liquid: for example, gasoline in water).

Spray can- gas with solid or liquid particles suspended in it (fog: air and liquid droplets)

Suspensions, emulsions and aerosols consist of several phases, they are not homogeneous and are dispersed systems . Suspensions, emulsions and aerosols are not solutions!

Chemical theory of solutions.

The solvent chemically interacts with the solute.

The chemical theory of solutions was created by D.I. Mendeleev at the end of the nineteenth century. based on the following experimental facts:

1) The dissolution of any substance is accompanied by the absorption or release of heat. That is, dissolution is an exothermic or endothermic reaction.

exothermic process is a process accompanied by heat release into the environment (Q>0).

Endothermic process is a process accompanied by the absorption of heat from the external environment (Q<0).

(example: dissolution of CuSO 4 - exothermic process, NH 4 Cl - endothermic). Explanation: in order for the solvent molecules to tear off the particles of the solute from each other, it is necessary to expend energy (this is the endothermic component of the dissolution process), when the particles of the solute interact with the solvent molecules, energy is released (exothermic process). As a result, the thermal effect of dissolution is determined by the stronger component. ( Example: when dissolving 1 mol of a substance in water, it took 250 kJ to break its molecules, and 450 kJ was released when the resulting ions interacted with solvent molecules. What is the total thermal effect of dissolution? Answer: 450-250=200 kJ, exothermic effect, because exothermic component is greater than endothermic ).

2) Mixing the components of a solution with a certain volume does not give the sum of the volumes ( example: 50 ml of ethyl alcohol + 50 ml of water when mixed gives 95 ml of solution)

Explanation: due to the interaction of the molecules of the solute and the solvent (attraction, chemical bonding, etc.), the volume is “saved”.

Attention! Weight solution is strictly equal to the sum of the masses of the solvent and solutes.

3) When dissolving some colorless substances, colored solutions are formed. ( example: CuSO 4 - colorless, gives a blue solution ).

Explanation: when dissolving some colorless salts, colored crystalline hydrates are formed.

Conclusion: Dissolution is a complex physical and chemical process in which there is an interaction (electrostatic, donor-acceptor, hydrogen bonding) between the particles of the solvent and the dissolved substances.

The process of interaction of a solvent with a solute is called solvation. The products of this interaction are solvates. For aqueous solutions, the terms hydration And hydrates.

Sometimes, when water is evaporated, the crystals of the solute leave a part of the water molecules in their crystal lattice. Such crystals are called crystalline hydrates. They are written as follows: CuSO 4 * 5H 2 O. That is, each molecule of copper sulfate CuSO 4 holds 5 water molecules around itself, embedding them into its crystal lattice.

The chemical, or solvate, theory of solutions was proposed in 1887 by D.I. Mendeleev, who found that real solution contains not only individual components, but also products of their interaction. Studies of aqueous solutions of sulfuric acid and ethyl alcohol, carried out by D.I. Mendeleev, formed the basis of the theory, the essence of which is that interactions occur between particles of a solute and solvent molecules, as a result of which unstable compounds of variable composition are formed, called solvates or hydrates if the solvent is water. The main role in the formation of solvates is played by unstable intermolecular forces, in particular, hydrogen bonding.

In this regard, the following interpretation of the concept of "solution" should be adopted:

A solution is a homogeneous system of variable composition, consisting of two or more components and products of their interaction.

From this definition it follows that solutions occupy an intermediate position between chemical compounds and mixtures. On the one hand, the solutions are homogeneous, which allows us to consider them as chemical compounds. On the other hand, there is no strict stoichiometric ratio between the components in solutions. In addition, solutions can be divided into component parts (for example, when NaCl solution is evaporated, salt can be isolated individually).

The main ways of expressing the concentration of solutions

The quantitative composition of the solution is most often estimated using the concept concentration, which is understood as the content of a solute (in certain units) per unit mass (volume) of a solution (solvent). The main ways of expressing the concentration of solutions are as follows:

1. Mass fraction of a substance  (x) is the ratio of the mass of the given component x contained in the system to the total mass of this system:

The unit of the amount of a substance is the mole, i.e., the amount of a substance that contains as many real or conditional particles as there are atoms in 0.012 kg of the C 12 isotope. When using a mole as a unit of quantity of a substance, one should know which particles are meant: molecules, atoms, electrons, or others. Molar mass M (x) is the ratio of mass to the amount of substance (g / mol):

3. Molar concentration equivalent of C(x) - this is the ratio of the amount of the equivalent of a substance n (x) to the volume of the solution V p-ra:

A chemical equivalent is a real or conditional particle of a substance that can replace, add or release 1 hydrogen ion in acid-base or ion-exchange reactions.

Just like a molecule, atom, or ion, the equivalent is dimensionless.

The mass of mole equivalents is called molar mass equivalent M(x). The value is called equivalence factor. It shows what proportion of a real particle of a substance corresponds to an equivalent. To correctly determine the equivalent of a substance, one must proceed from the specific reaction in which this substance participates, for example, in the reaction of the interaction of H 3 PO 4 with NaOH, one, two or three protons can be replaced:

1. H 3 PO 4 + NaOH  NaH 2 PO 4 + H 2 O;

2. H 3 PO 4 + 2NaOH  Na 2 HPO 4 + 2H 2 O;

3. H 3 PO 4 + 3NaOH  Na 3 PO 4 + 3H 2 O.

In accordance with the definition of the equivalent, one proton is replaced in the 1st reaction, therefore, the molar mass of the substance equivalent is equal to the molar mass, i.e. z  l and . In this case:

In the 2nd reaction, two protons are replaced, therefore, the molar mass of the equivalent will be half the molar mass of H 3 PO 4, i.e. e. z  2, and
. Here:

In the 3rd reaction, three protons are replaced and the molar mass of the equivalent will be one third of the molar mass of H 3 PO 4, i.e. z = 3, a
. Respectively:

In exchange reactions where protons do not directly participate, equivalents can be determined indirectly by introducing auxiliary reactions, the analysis of the results of which allows us to derive the rule that z for all reactions is equal to total charge exchangeable ions in a molecule of a substance involved in a particular chemical reaction.

1. AlCl 3 + 3AgNO 3 = Al(NO 3) 3 + 3AgCl.

For AlCl 3, 1 Al 3+ ion with a charge of +3 is exchanged, therefore, z \u003d 13 \u003d 3. Thus:

It can also be said that 3 chlorine ions with a charge of -1 are exchanged. Then z = 31 = 3 and

For AgNO 3 z = 11 = 1 (1 Ag + ion is exchanged with charge +1 or 1 NO 3 - ion is exchanged with charge -1).

2. Al 2 (SO 4) 3 + 3BaCl 2 = 3BaSO 4  + 2AlCl 3.

For Al 2 (SO 4) 3 z \u003d 23 \u003d 6 (2 Al 3+ ions with a charge of +3 or 3 SO 4 2 - ions with a charge of -2 are exchanged). Consequently,

So, the record C (H 2 SO 4) \u003d 0.02 mol / l means that there is a solution, 1 liter of which contains 0.02 mol equivalent of H 2 SO 4, and the molar mass of the equivalent of H 2 SO 4 is at the same time molar mass H 2 SO 4, i.e. 1 liter of solution contains
H2SO4.

With the equivalence factor the molar concentration of the equivalent is equal to the molar concentration of the solution.

4. Title T(x) is the ratio of the mass of the substance to the volume of the solution (in ml):

6. Mole fraction N(x) is the ratio of the amount of a substance of a given component contained in the system to the total amount of substances in the system:

Expressed in fractions of a unit or in % .

7. Solubility coefficient substances R(x) called the maximum mass of a substance, expressed in g, that can be dissolved in 100 g of a solvent.

As a result of studying this topic, you will learn: Why do solutions occupy an intermediate position between mixtures and chemical compounds. What is the difference between an unsaturated solution and a dilute solution and a saturated solution from a concentrated one. What rules should be followed when compiling ionic equations. Why, when some salts are dissolved in water, the reaction of the medium changes (from neutral to acidic or alkaline). As a result of studying this topic, you will learn: Write equations for the ion exchange reaction. Compose full and reduced ionic equations for the hydrolysis of salts. Predict the reaction of the environment in salt solutions. Solve problems to determine the concentration of solutions. Study questions:

9.1. Solutions and their classification

Solutions are homogeneous systems in which one substance is distributed in the environment of another (other) substances.

Solutions consist of a solvent and a solute(s). These concepts are conditional. If one of the components of a solution of substances is a liquid, and the others are gases or solids, then the solvent is usually considered a liquid. In other cases, the solvent is considered to be the component that is larger.

Gaseous, liquid and solid solutions

depending from the state of aggregation solvent distinguish gaseous, liquid and solid solutions. The gaseous solution is, for example, air and other mixtures of gases. Sea water is the most common liquid solution of various salts and gases in water. Many metal alloys belong to solid solutions.

True and colloidal solutions

According to the degree of dispersion distinguish true and colloidal solutions(colloidal systems). In the formation of true solutions, the solute is in the solvent in the form of atoms, molecules, or ions. The particle size in such solutions is 10–7 - 10–8 cm. Colloidal solutions are heterogeneous systems in which particles of one substance (dispersed phase) are evenly distributed in another (dispersion medium). The size of particles in dispersed systems ranges from 10–7 cm to 10–3 and more cm. It should be noted that here and below, we will consider true solutions everywhere.

Unsaturated, saturated and supersaturated solutions

The dissolution process is associated with diffusion, i.e., with the spontaneous distribution of particles of one substance between particles of another. Thus, the process of dissolution of solids with an ionic structure in liquids can be represented as follows: under the influence of a solvent, the crystal lattice of a solid is destroyed, and ions are distributed evenly throughout the volume of the solvent. The solution will remain unsaturated as long as some more substance can pass into it.

A solution in which a substance no longer dissolves at a given temperature, i.e. a solution that is in equilibrium with the solid phase of the solute is called rich. The solubility of a given substance is equal to its concentration in a saturated solution. Under strictly defined conditions (temperature, solvent), solubility is a constant value.

If the solubility of a substance increases with increasing temperature, then by cooling a solution saturated at a higher temperature, one can obtain supersaturated solution, i.e. such a solution, the concentration of a substance in which is higher than the concentration of a saturated solution (at a given temperature and pressure). Supersaturated solutions are very unstable. A slight shaking of the vessel or the introduction of crystals of a substance in solution into the solution causes the excess of the solute to crystallize, and the solution becomes saturated.

Diluted and concentrated solutions

Do not confuse unsaturated and saturated solutions with dilute and concentrated ones. The concepts of dilute and concentrated solutions are relative and it is impossible to draw a clear line between them. They determine the ratio between the amounts of solute and solvent. In general, dilute solutions are solutions containing small amounts of solute compared to the amount of solvent, concentrated solutions are those with a large amount of solute.

For example, if at 20 o C dissolve 25 g of NaCl in 100 g of water, then the resulting solution will be concentrated, but unsaturated, since the solubility of sodium chloride at 20 o C is 36 g in 100 g of water. The maximum mass of AgI that dissolves at 20 o C in 100 g of H 2 O is 1.3 10 -7 g. The AgI solution obtained under these conditions will be saturated, but very dilute.

9.2. Physical and chemical theory of solutions; thermal phenomena during dissolution

Physical theory solutions was proposed by W. Ostwald (Germany) and S. Arrhenius (Sweden). According to this theory, the particles of the solvent and the solute (molecules, ions) are evenly distributed throughout the volume of the solution due to diffusion processes. There is no chemical interaction between the solvent and the solute.

chemical theory was proposed by D.I. Mendeleev. According to D.I. Mendeleev, between the molecules of the solute and the solvent, a chemical interaction occurs with the formation of unstable, converting into each other compounds of the solute with the solvent - solvates.

Russian scientists I.A. Kablukov and V.A. Kistyakovsky combined the ideas of Ostwald, Arrhenius and Mendeleev, thus laying the foundation for the modern theory of solutions. According to modern theory, not only particles of a solute and a solvent can exist in a solution, but also products of the physicochemical interaction of a solute with a solvent - solvates. solvates are unstable compounds of variable composition. If the solvent is water, they are called hydrates. Solvates (hydrates) are formed due to ion-dipole, donor-acceptor interactions, formation of hydrogen bonds, etc. For example, when NaCl is dissolved in water, an ion-dipole interaction occurs between Na + , Cl - ions and solvent molecules. The formation of ammonia hydrates when it is dissolved in water occurs due to the formation of hydrogen bonds.

Hydrated water is sometimes so strongly associated with the solute that it is released with it from the solution. Crystalline substances containing water molecules are called crystalline hydrates, and the water that is part of such crystals is called crystallization. Examples of crystalline hydrates are copper sulfate CuSO 4 5H 2 O, potassium alum KAl (SO 4) 2 12H 2 O.

Thermal effects during dissolution

As a result of a change in the structure of substances during their transition from an individual state to a solution, as well as as a result of ongoing interactions, the properties of the system change. This is indicated, in particular, by the thermal effects of dissolution. During dissolution, two processes occur: the destruction of the structure of the solute and the interaction of the molecules of the solute with the molecules of the solvent. The interaction of a solute with a solvent is called solvation. Energy is expended on the destruction of the structure of the dissolved substance, and the interaction of the particles of the dissolved substance with the particles of the solvent (solvation) is an exothermic process (goes with the release of heat). Thus, the dissolution process can be exothermic or endothermic, depending on the ratio of these thermal effects. For example, when dissolving sulfuric acid, a strong heating of the solution is observed, i.e. release of heat, and when potassium nitrate is dissolved, a strong cooling of the solution (endothermic process).

9.3. Solubility and its dependence on the nature of substances

Solubility is the most studied property of solutions. The solubility of substances in various solvents varies widely. In table. 9.1 shows the solubility of some substances in water, and in table. 9.2 - solubility of potassium iodide in various solvents.

Table 9.1

Solubility of certain substances in water at 20 o C

Substance		Substance	Solubility, g per 100 g H 2 O

Table 9.2

Solubility of potassium iodide in various solvents at 20 o C

Solubility depends on the nature of the solute and solvent, as well as on external conditions (temperature, pressure). In currently used reference tables, it is proposed to subdivide substances into highly soluble, slightly soluble and insoluble substances. This division is not entirely correct, since there are no absolutely insoluble substances. Even silver and gold are soluble in water, but their solubility is extremely low. Therefore, in this tutorial, we will use only two categories of substances: highly soluble And sparingly soluble. Finally, the concepts of “easily” and “hardly” soluble are inapplicable to the interpretation of solubility, since these terms characterize the kinetics of the dissolution process, and not its thermodynamics.

Dependence of solubility on the nature of the solute and solvent

At present, there is no theory by which it would be possible not only to calculate, but even to predict solubility. This is due to the absence of a general theory of solutions.

Solubility of solids in liquids depends on the type of bond in their crystal lattices. For example, substances with atomic crystal lattices (carbon, diamond, etc.) are slightly soluble in water. Substances with an ionic crystal lattice, as a rule, are highly soluble in water.

The rule, established from centuries of experience in the study of solubility, says: "like dissolves well in like." Substances with an ionic or polar type of bond dissolve well in polar solvents. For example, salts, acids, alcohols are highly soluble in water. At the same time, non-polar substances, as a rule, dissolve well in non-polar solvents.

Inorganic salts are characterized by different solubility in water.

Thus, most alkali metal and ammonium salts are highly soluble in water. Highly soluble nitrates, nitrites and halides (except for silver, mercury, lead and thallium halides) and sulfates (except for sulfates of alkaline earth metals, silver and lead). Transition metals are characterized by a low solubility of their sulfides, phosphates, carbonates, and some other salts.

The solubility of gases in liquids also depends on their nature. For example, in 100 volumes of water at 20 o C dissolves 2 volumes of hydrogen, 3 volumes of oxygen. Under the same conditions, 700 volumes of ammonia dissolve in 1 volume of H 2 O. Such a high solubility of NH 3 can be explained by its chemical interaction with water.

The influence of temperature on the solubility of gases, solids and liquids

When gases are dissolved in water, heat is released due to the hydration of the dissolved gas molecules. Therefore, in accordance with Le Chatelier's principle, as the temperature rises, the solubility of gases decreases.

Temperature affects the solubility of solids in water in various ways. In most cases, the solubility of solids increases with increasing temperature. For example, the solubility of sodium nitrate NaNO 3 and potassium nitrate KNO 3 increases when heated (the dissolution process proceeds with the absorption of heat). The solubility of NaCl increases slightly with increasing temperature, which is due to the almost zero thermal effect of the dissolution of table salt. The solubility of slaked lime in water decreases with increasing temperature, since the enthalpy of hydration prevails over the value of Δ H of the destruction of the crystal lattice of this compound, i.e. the dissolution process of Ca(OH) 2 is exothermic.

In most cases, the mutual solubility of liquids also increases with increasing temperature.

Effect of pressure on the solubility of gases, solids and liquids

The solubility of solid and liquid substances in liquids is practically not affected by pressure, since the volume change during dissolution is small. When gaseous substances are dissolved in a liquid, the volume of the system decreases, therefore, an increase in pressure leads to an increase in the solubility of gases. In general, the dependence of the solubility of gases on pressure obeys W. Henry's law(England, 1803): the solubility of a gas at constant temperature is directly proportional to its pressure over the liquid.

Henry's law is valid only at low pressures for gases whose solubility is relatively low and provided there is no chemical interaction between the molecules of the dissolved gas and the solvent.

Influence of foreign substances on solubility

In the presence of other substances (salts, acids and alkalis) in water, the solubility of gases decreases. The solubility of gaseous chlorine in a saturated aqueous solution of table salt is 10 times less. than pure water.

The effect of decreasing solubility in the presence of salts is called salting out. The decrease in solubility is due to the hydration of salts, which causes a decrease in the number of free water molecules. Water molecules associated with electrolyte ions are no longer a solvent for other substances.

9.4. Solution concentration

There are various ways to numerically express the composition of solutions: mass fraction of a dissolved substance, molarity, titer, etc.

Mass fraction is the ratio of the mass of the solute m to the mass of the entire solution. For a binary solution consisting of a solute and a solvent:

where ω is the mass fraction of the solute, m is the mass of the solute, and M is the mass of the solvent. The mass fraction is expressed in fractions of a unit or as a percentage. For example, ω = 0.5 or ω = 50%.

It should be remembered that only the mass is an additive function (the mass of the whole is equal to the sum of the masses of the components). The volume of the solution does not obey this rule.

Molar concentration or molarity is the amount of solute in 1 liter of solution:

where C is the molar concentration of solute X, mol/l; n is the amount of the dissolved substance, mol; V is the volume of the solution, l.

Molar concentration is indicated by a number and the letter “M”, for example: 3M KOH. If 1 liter of a solution contains 0.1 mol of a substance, then it is called decimolar, 0.01 mol - centomolar, 0.001 mol - millimolar.

Titer is the number of grams of solute contained in 1 ml of solution, i.e.

where T is the titer of the dissolved substance, g/ml; m is the mass of the dissolved substance, g; V is the volume of the solution, ml.

Mole fraction of solute- a dimensionless quantity equal to the ratio of the amount of solute n to the total amount of solute n and solvent n ":

,

where N is the mole fraction of the solute, n is the amount of the solute, mol; n" is the amount of the solvent substance, mol.

The mole percentage is the corresponding fraction multiplied by 100%.

9.5. Electrolytic dissociation

Substances whose molecules in solutions or melts completely or partially decompose into ions are called electrolytes. Solutions and melts of electrolytes conduct electric current.

Substances whose molecules in solutions or melts do not decompose into ions and do not conduct electric current are called non-electrolytes.

Electrolytes include most inorganic acids, bases and almost all salts, non-electrolytes include many organic compounds, such as alcohols, esters, carbohydrates, etc.

In 1887, the Swedish scientist S. Arrhenius put forward the hypothesis of electrolytic dissociation, according to which, when electrolytes are dissolved in water, they decompose into positively and negatively charged ions.

Dissociation is a reversible process: in parallel with dissociation, the reverse process of ion joining (association) proceeds. Therefore, when writing the equations for the reaction of the dissociation of electrolytes, especially in concentrated solutions, the sign of reversibility is put. For example, the dissociation of potassium chloride in a concentrated solution should be written as:

KS1 K + + С1 – .

Let us consider the mechanism of electrolytic dissociation. Substances with an ionic type of bond dissociate most easily in polar solvents. When they are dissolved, for example, in water, polar H 2 O molecules are attracted by their positive poles to anions, and by their negative poles to cations. As a result, the bond between the ions weakens, and the electrolyte decomposes into hydrated ions, i.e. ions associated with water molecules. Electrolytes formed by molecules with a covalent polar bond (HC1, HBr, H 2 S) dissociate in a similar way.

Thus, hydration (solvation) of ions is the main cause of dissociation. It is now generally accepted that most of the ions in an aqueous solution are hydrated. For example, the hydrogen ion H + forms a hydrate of the composition H3O +, which is called the hydronium ion. In addition to H 3 O +, the solution also contains H 5 O 2 + ions (H 3 O + H 2 O), H 7 O 3 + (H 3 O + 2H 2 O) and H 9 O 4 + (H 3 O + 3H 2 O). When compiling equations for dissociation processes and writing reaction equations in ionic form, to simplify writing, the hydroxonium ion H 2 O + is usually replaced by an unhydrated ion H +. However, it should be remembered that this substitution is conditional, since the proton cannot exist in aqueous solutions, since the reaction proceeds almost instantly:

H + + H 2 O \u003d H 3 O +.

Since the exact number of water molecules associated with hydrated ions has not been established, the symbols for non-hydrated ions are used when writing the equations for the dissociation reaction:

CH3COOH CH3COO - + H + .

9.6. Degree of dissociation; associated and non-associated electrolytes

The quantitative characteristic of the dissociation of the electrolyte into ions in solution is the degree of dissociation. The degree of dissociation α is the ratio of the number of molecules that have decayed into ions N "to the total number of dissolved molecules N:

The degree of dissociation is expressed as a percentage or fractions of a unit. If α = 0, then there is no dissociation, and if α = 1, then the electrolyte completely decomposes into ions. According to modern concepts of the theory of solutions, electrolytes are divided into two groups: associated (weak) and non-associated (strong).

For non-associated (strong) electrolytes in dilute solutions, α = 1 (100%), i.e. in solutions they exist exclusively as hydrated ions.

Associated electrolytes can be roughly divided into three groups:

weak electrolytes exist in solutions mainly in the form of undissociated molecules; the degree of their dissociation is low;

ion associates are formed in solutions as a result of electrostatic interaction of ions; as noted above, association takes place in concentrated solutions of well-dissociating electrolytes; examples of associates are ion pairs(K + Cl -, CaC1 +), ionic tees(K 2 Cl +, KCl 2 -) and ionic quadrupoles(K 2 Cl 2 , KCl 3 2– , K 3 Cl 2 +);

ionic and molecular complexes, (for example, 2+ , 3–) which slightly dissociate in water.

The nature of the dissociation of the electrolyte depends on the nature of the solute and solvent, the concentration of the solution, and the temperature. An illustration of this provision can serve as the behavior of sodium chloride in various solvents, Table. 9.3.

Table 9.3

Properties of sodium chloride in water and in benzene at various concentrations and at a temperature of 25 o C

Strong electrolytes in aqueous solutions include most salts, alkalis, a number of mineral acids (HC1, HBr, HNO 3, H 2 SO 4, HC1O 4, etc.). Almost all organic acids belong to weak electrolytes, some inorganic acids, for example, H 2 S, HCN, H 2 CO 3, HClO and water.

Dissociation of strong and weak electrolytes

The dissociation equations for strong electrolytes in dilute aqueous solutions can be represented as follows:

HCl \u003d H + + Cl -,

Ba (OH) 2 \u003d Ba 2+ + 2OH -,

K 2 Cr 2 O 7 \u003d 2K + + Cr 2 O 7 2–.

Between the right and left parts of the reaction equation for the dissociation of a strong electrolyte, you can also put a sign of reversibility, but then it is indicated that a 1. For example:

NaOH Na + + OH - .

The process of dissociation of associated electrolytes is reversible, therefore, it is necessary to put the sign of reversibility into the equations of their dissociation:

HCN H + + CN – .

NH 3 H 2 O NH 4 + + OH -.

The dissociation of associated polybasic acids occurs in steps:

H 3 PO 4 H + + HPO 4 -,

H 2 PO 4 H + + HPO 4 2–,

HPO 4 2– H + + RO 4 3–,

The dissociation of acid salts formed by weak acids and basic salts formed by strong acids in dilute solutions occurs as follows. The first stage is characterized by a degree of dissociation close to unity:

NaHCO 3 \u003d Na + + HCO 3 -,

Cu(OH)Cl = Cu(OH) + + Cl - .

The degree of dissociation for the second stage is much less than unity:

HCO 3 H + + CO 3 2–,

Cu(OH) + Cu 2+ + OH -.

Obviously, with an increase in the solution concentration, the degree of dissociation of the associated electrolyte decreases.

9.7. Ion exchange reactions in solutions

According to the theory of electrolytic dissociation, all reactions in aqueous electrolyte solutions occur not between molecules, but between ions. To reflect the essence of such reactions, the so-called ionic equations are used. When compiling ionic equations, one should be guided by the following rules:

Slightly soluble and slightly dissociated substances, as well as gases, are written in molecular form.

Strong electrolytes, almost completely dissociated in aqueous solution, are recorded as ions.

The sum of the electric charges on the right and left sides of the ionic equation must be equal.

Let's consider these positions on specific examples.

We write two equations for neutralization reactions in molecular form:

KOH + HCl \u003d KCl + H 2 O, (9.1)

2NaOH + H 2 SO 4 = Na 2 SO 4 + 2H 2 O. (9.2)

In ionic form, equations (9.1) and (9.2) have the following form:

K + + OH - + H + + Cl - = K + + Cl - + H 2 O, (9.3)

2Na + + 2OH – + 2H + + SO 4 2– = 2Na + + SO 4 2– + 2H 2 O. (9.4)

Having reduced the same ions in both parts of equations (9.3) and (9.4), we convert them into one reduced ionic equation for the interaction of an alkali with an acid:

H + + OH - \u003d H 2 O.

Thus, the essence of the neutralization reaction is reduced to the interaction of H + and OH - ions, as a result of which water is formed.

Reactions between ions in aqueous electrolyte solutions proceed almost to the end if a precipitate, gas, or a weak electrolyte (for example, H 2 O) is formed in the reaction.

Consider now the reaction between solutions of potassium chloride and sodium nitrate:

KCl + NaNO 3 KNO 3 + NaCl. (9.5)

Since the resulting substances are highly soluble in water and are not removed from the reaction sphere, the reaction is reversible. The ionic reaction equation (9.5) is written as follows:

K + + Cl – + Na + + NO 3 – K + + NO 3 – + Na + + Cl – . (9.6)

From the point of view of the theory of electrolytic dissociation, this reaction does not occur, since all soluble substances in the solution are present exclusively in the form of ions, equation (9.6). But if you mix hot saturated solutions of KCl and NaNO 3, then NaCl will precipitate. This is due to the fact that at a temperature of 30 o C and above, the lowest solubility among the considered salts is observed in sodium chloride. Thus, in practice, it should be taken into account that processes that are reversible under certain conditions (in the case of dilute solutions) become irreversible under some other conditions (hot saturated solutions).

A special case of the exchange reaction in solutions is hydrolysis.

9.8. Salt hydrolysis

Experience shows that not only acids and bases, but also solutions of some salts, have an alkaline or acid reaction. A change in the reaction of the environment occurs as a result hydrolysis but a solute. Hydrolysis is the exchange interaction of a solute (for example, salt) with water.

The electrolytic dissociation of salts and water is the cause of hydrolysis. Hydrolysis occurs when the ions formed during the dissociation of the salt are able to exert a strong polarizing effect on water molecules (cations) or form hydrogen bonds with them (anions), which leads to the formation of slightly dissociated electrolytes.

Salt hydrolysis equations are usually written in ionic and molecular forms, while it is necessary to take into account the rules for writing ionic equations for exchange reactions.

Before proceeding to the consideration of the equations of hydrolysis reactions, it should be noted that salts formed by a strong base and a strong acid(for example, NaNO 3, BaCl 2, Na 2 SO 4), when dissolved in water, they do not undergo hydrolysis. Ions of such salts do not form weak electrolytes with H 2 O, and solutions of these salts have a neutral reaction.

Various cases of hydrolysis of salts

1. Salts formed by a strong base and a weak acid, for example, CH 3 COONa, Na 2 CO 3 , Na 2 S, KCN are hydrolyzed by the anion. As an example, consider the hydrolysis of CH 3 COONa, leading to the formation of low-dissociating acetic acid:

CH3COO - + NON CH 3 COOH + OH -,

CH3COOHa + HOH CH 3 COOH + NaOH.

Since an excess of hydroxide ions appears in the solution, the solution becomes alkaline.

The hydrolysis of salts of polybasic acids proceeds stepwise, and in this case acid salts are formed, more precisely, anions of acid salts. For example, the hydrolysis of Na 2 CO 3 can be expressed by the equations:

1 step:

CO 3 2– + HOH HCO 3 – + OH –,

Na 2 CO 3 + HOH NaHCO 3 + NaOH.

2 step

HCO 3 - + HOH H 2 CO 3 + OH -,

NaHCO 3 + HOH H 2 CO 3 + NaOH.

The OH- ions formed as a result of hydrolysis in the first stage largely suppress the second stage of hydrolysis; as a result, hydrolysis in the second stage proceeds to a small extent.

2. Salts formed from a weak base and a strong acid, for example, NH 4 Cl, FeCl 3, Al 2 (SO 4) 3 are hydrolyzed by the cation. An example is the process

NH 4 + + HOH NH 4 OH + H +,

NH 4 Cl + HOH NH 4 OH + HCl.

Hydrolysis is due to the formation of a weak electrolyte - NH 4 OH (NH 3 H 2 O). As a result, the balance of electrolytic dissociation of water is shifted, and an excess of H + ions appears in the solution. Thus, a solution of NH 4 Cl will show an acidic reaction.

During the hydrolysis of salts formed by polyacid bases, basic salts are formed, more precisely, cations of basic salts. Consider, as an example, the hydrolysis of iron (II) chloride:

1 step

Fe 2+ + HOH FeOH + + H + ,

FeCl 2 + HOH FeOHCl + HCl.

2 step

FeOH + + HOH Fe (OH) 2 + H +,

FeOHCl + HOH Fe(OH) 2 + HCl.

Hydrolysis in the second stage proceeds insignificantly in comparison with hydrolysis in the first stage, and the content of hydrolysis products in the solution in the second stage is very small.

3. Salts formed from a weak base and a weak acid, for example, CH 3 COONH 4, (NH 4) 2 CO 3, HCOONH 4, are hydrolyzed both by the cation and by the anion. For example, when CH 3 COONH 4 is dissolved in water, low-dissociating acid and base are formed:

CH 3 COO - + NH 4 + + HOH CH 3 COOH + NH 4 OH,

CH3COONH 4 + HOH CH 3 COOH + NH 4 OH.

In this case, the reaction of the solution depends on the strength of the weak acids and bases formed as a result of hydrolysis. Since in this example CH 3 COOH and NH 4 OH are approximately equal in strength, the salt solution will be neutral.

During the hydrolysis of HCOONH 4, the reaction of the solution will be slightly acidic, since formic acid is stronger than acetic acid.

The hydrolysis of a number of salts formed by very weak bases and weak acids, for example, aluminum sulfide, proceeds irreversibly:

Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 + 3H 2 S.

4. A number of exchange reactions in solutions are accompanied by hydrolysis and proceed irreversibly.

A) When solutions of salts of divalent metals (except calcium, strontium, barium and iron) interact with aqueous solutions of alkali metal carbonates, basic carbonates precipitate as a result of partial hydrolysis:

2MgSO 4 + 2Na 2 CO 3 + H 2 O \u003d Mg 2 (OH) 2 CO 3 + CO 2 + 2Na 2 SO 4,

3 Pb (NO 3) 2 + 3Na 2 CO 3 + H 2 O \u003d Pb 3 (OH) 2 (CO 3) 2 + CO 2 + 6NaNO 3.

B) When aqueous solutions of trivalent aluminum, chromium and iron are mixed with aqueous solutions of carbonates and sulfides of alkali metals, carbonates and sulfides of trivalent metals are not formed - their irreversible hydrolysis proceeds and hydroxides precipitate:

2AlCl 3 + 3K 2 CO 3 + 3H 2 O \u003d 2Al (OH) 3 + 3CO 2 + 6KCl,

2Cr(NO 3) 3 + 3Na 2 S + 6H 2 O = 2Cr(OH) 3 + 3H 2 S + 6NaNO 3 .