Solutionsare called thermodynamically stable homogeneous systems of variable composition, consisting of two or more components. The components of solutions are a solvent (component 1) and a solute or substances in any amount (component 2). The division into solvent and substance is relative, usually the solvent is the component that is larger. If the solution is isolated from the environment, it will remain in this state for an arbitrarily long time, the system is stable. The solvent-substance ratio can vary over a wide range, and the solution remains the same solution, we are talking about a system of variable composition.

The solute particles in solution have different sizes. Depending on the size of dissolved particles, solutions are divided into true solutions and dispersed heterogeneous systems. In true solutions, solutes in the form of individual molecules or ions are evenly distributed throughout the volume of the system among the same solvent particles. The sizes of ordinary molecules and ions do not exceed 1 nm (10 -9 m), therefore, true solutions are homogeneous systems, i.e. homogeneous at the molecular level. There is no interfacial surface in them.

Dispersed heterogeneous systems have a highly developed interfacial surface, since dissolved particles are much larger than molecules. Such systems consist of a continuous solvent phase - a dispersion medium and fragmented particles of a dissolved substance - a dispersed phase, located in this medium. In colloidal solutions, dissolved particles can have sizes in the range of 1-100 nm. Systems that include crushed particles with a diameter of more than 100 nm belong to finely and coarsely dispersed heterogeneous systems. A prerequisite for obtaining dispersed systems is the mutual insolubility of the dispersion medium and the dispersed phase. Dispersed heterogeneous systems are thermodynamically unstable, because processes spontaneously occur in them, leading to the enlargement of the particles of the dissolved substance. Therefore, over time, such systems are divided into the original components (stratify, disperse, dry out).

Many gas, liquid and solid systems fall into the group of true solutions. An example of a gas solution is ordinary air - a mixture of gases O 2, N 2, CO 2, etc., if there were no dust and liquid water in it. Liquid solutions are obtained by dissolving gases (CO 2), liquid substances (C 2 H 5 OH) or solids (NaCl). Examples of solid solutions are a solution of gaseous hydrogen in solid palladium, amalgams (a solution of liquid mercury in solid metals), and alloys. The most common liquid solutions in which the solvent is water. In what follows, only aqueous solutions will be discussed.

For example, we dissolve a teaspoon of sugar in water. Add another spoon, stir, etc. Starting from a certain concentration, sugar ceased to dissolve and its excess is at the bottom of the glass. A solution in equilibrium with a solute is called saturated. Until this concentration was reached, the solution was unsaturated, homogeneous. The concentration of a saturated solution is called solubility substance and is usually expressed in grams of this substance per 100 g of water. The solubility of gases always decreases with heating. Why?

(X) "(X) H 2 O

The transition from left to right from a disordered state of a gas to an aqueous solution is accompanied by a decrease in the entropy DS<0, поэтому необходимо выделение тепла DН<0, в противном случае DG>0 and the process will be disabled. According to Le Chatelier's principle, the equilibrium shifts to the left as the temperature rises.

In the case of a solid dissolved in water, the situation is different:

[X] « (X) H 2 O , DS>0

and there will be no strict restriction on the sign of the thermal effect, the process of dissolution of a solid can be both exothermic and endothermic. For example, when NaCl is dissolved, polar water molecules begin to destroy the crystal lattice of table salt, “pulling out” sodium cations and chloride anions from it and moving into the aqueous phase. The thermal effect is called the enthalpy of the lattice, it is always an endothermic value: DH resh > 0. Sodium and chlorine ions, once in water, interact with it:

Na + + mH 2 O "Na + mH 2 O, CI ─ + nH 2 O" CI ─ nH 2 O.

This is an exothermic reaction. hydration- reactions of interaction with water. The reaction products are called hydrates. Thus, the sign of the thermal effect of the dissolution of a solid depends on the ratio of the enthalpies of the lattice and hydration, which are opposite in sign: DH dissolution = DH sol + DH hydr.

The hydration reaction was discovered by DIMendeleev, the author of the chemical theory of solutions. He was the first to note that the dissolution of substances is accompanied by thermal and volumetric changes, and these are signs of chemical reactions, for example, the dilution of ethyl alcohol in water is accompanied by a large release of heat and a decrease in volume:

50 ml of alcohol + 50 ml of water = 96 ml of solution (!).

The study of the ethanol hydration process was the rationale for the composition of Russian vodka.

The number of water molecules during hydration (m, n) is called the hydration number. The ion is surrounded by water molecules due to the forces of electrostatic attraction:

Fig.3.1 Hydrated sodium ion.

The hydration number m=6, it is determined only by the size of the ion. In this hydrated form, all ions are found in water. This is a hydration shell or "coat" of water molecules. Such compounds with water are fragile, they exist only in a state of solution. Stronger hydrates form some salts, water is part of the salt crystal lattice and can be removed only by heating. For example, anhydrous copper(II) sulfate adds five moles of water to form blue copper sulfate:

CuSO 4 + 5H 2 O « CuSO 4 5H 2 O.

Ferrous vitriol FeSO 4 7H 2 O, Glauber's salt Na 2 SO 4 12H 2 O, alum, etc. are known. Sodium oxide hydrate (NaOH) is a very strong hydrate and can be distilled without decomposition at 1400°C.

The solute-solvent (water) ratio is called concentration solution. Depending on the problem being solved, a variety of ways of expressing this ratio are used:

1. Percent concentration.

Mass of substance (g) per 100 g of solution. For example, in 95 g of water we dissolve 5 g of sodium chloride. The mass of the solution m= (95 + 5) g, we are talking about 5% NaCI.

1. Mass of substance (g) per 1 liter of solution (g/l).
2. Mass of a substance (g) per 1 liter of water (1 kg of water). This is how the salinity of the world's oceans is usually expressed ( ppm, OOO).
3. Molarity(M). The mass of a substance in moles per 1 liter of solution (mol / l). For example, we have a solution containing 98g/l H 2 SO 4 . The molar mass of sulfuric acid is just 98 g/mol. 98g / l H 2 SO 4 \u003d 1M H 2 SO 4.
4. Molality (m). The mass of a substance in moles per 1 liter (kg) of water.

There is a group of solution properties that do not depend on the nature of the solute, but are determined only by its concentration. Such properties are called common or collective.

1. Lowering the vapor pressure of the solvent over the solution compared to the pure solvent.

Imagine a system consisting of two identical glasses. Water is poured into the first glass, and the same amount of a solution in water of any non-volatile substance is poured into the second. The glasses are placed in a thermostat, isolated from the environment. Water molecules from the surface layer can leave its surface and move into the air, creating at equilibrium the pressure of saturated water vapor (P about H2O), that is, water evaporates. In the case of a solution, part of the water surface is occupied by foreign molecules or ions of the dissolved substance, the water vapor pressure (P H2O) will thereby be reduced by a certain value ΔP, depending on the amount of these foreign particles: ΔP = P o H2O - P H2O. In an isolated system, a pressure gradient arises, water from the first glass will pass through steam into the second glass until an equilibrium state is established. Important practical conclusions follow from this circumstance. Consider the state diagram of water (Р–Т).

Fig.3.2 Diagram of the state of water.

a is the water evaporation pressure; b – water vapor pressure over the solution.

In Fig. 3.2, the exponent (a) characterizes the equilibrium of water evaporation (equilibrium well liquid - G az), at negative temperatures – sublimation equilibrium ( T – G), and, finally, the third line corresponds to the equilibrium of water melting. The reference points are 0 o C - the melting point of water and 100 o C - the normal boiling point of water, while the water vapor pressure is equal to the external normal pressure of 1 atm.

The water vapor pressure over the solution is reduced, line (b) is slightly lower than (a), while the melting point Tm shifts to negative temperatures by ΔTm, and the boiling point Tbp shifts upward by ΔTbp. The effects are proportional to the molality of the solution, known in chemistry as Raoult's laws:

ΔT pl \u003d K H 2 O m; ΔT bale \u003d E H 2 O m.

The constants K H 2 O \u003d 1.86 o, E H 2 O \u003d 0.52 o are called, respectively cryoscopic And ebullioscopic permanent water.

For example, at m = 1 mol / kg of water, the freezing point will be ─ 1.86 o C, and the boiling point is 100.52 o C. So in technology, liquids (antifreezes) that do not freeze in the cold are obtained. The higher the molar concentration, the stronger the freezing point lowering effect. To achieve the maximum effect, a good solubility of the substance in water and a low molar mass are required, ( m get more). Most often, a solution in water of dihydric alcohol ethylene glycol CH 2 OH - CH 2 OH (antifreeze) is used.

2. Osmosis.

Osmosis is the one-way diffusion of water molecules through a semi-permeable membrane. The property of semi-permeability, that is, the ability to pass only water molecules, and not substances dissolved in it, is possessed by many materials - all tissues of a living organism, some polymers, ceramics, cermets. The phenomenon was studied for the first time by Pfeffer in Germany. His device (osmometer) consisted of a glass of water, into which he immersed a tube with a solution, at the end of which a membrane of bull skin was stretched.

Fig. 3.3 Pfeffer osmometer.

Diffusion of water molecules is directed from the beaker to the tube with the solution, towards equalization of concentrations, achievement of equilibrium. The so-called osmotic pressure (P osm) acts from the bottom up, the level of the solution in the tube rises until P osm becomes equal to the gravity of the solution column. Pfeffer only had time to add water to the glass.

The resulting pressure, in accordance with the Van't Hoff osmotic law, is numerically equal to the pressure of the solute if it were in the state of an ideal gas at temperature T and in the volume of the glass V:

R osm \u003d C R T, where C is the molarity, mol / l.

The expression coincides with the Mendeleev-Clapeyron ideal gas equation of state. For example, at a solution concentration of C \u003d 1 mol / l, the osmotic pressure at a standard temperature will be:

R osm \u003d 1 0.082 298 \u003d 24.5 atm (!).

The entire interstitial water exchange in a living organism proceeds according to the osmotic mechanism. The presence of a large excess of ordinary table salt in the intestines will lead to an immediate flow of water into the intestines from all other tissues, there will be a loss of water by the cells, that is, their mass death, which is called dehydration. Diffusion of water from the root system of plants up to the leaves, swelling of wood when immersed in water or in humid air, etc. - example ow a lot.

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

General information

Solutions - components.

"solvent" And "solute" polar non-polar

hydrophilic(attracting water) and hydrophobic diphilic

Theories of solutions

Physical theory of solutions.

ideal

Chemical theory of solutions.

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).

Communication between different ways

Acids and bases

Despite the fact that the concepts of "acid" and "base" are widely used to describe chemical processes, there is no single approach to the classification of substances in terms of classifying them as acids or bases. Current theories ( ionic theory S. Arrhenius, protolytic theory I. Bronsted and T. Lowry And electronic theory G. Lewis) have certain limitations and are therefore applicable only in particular cases. Let's take a closer look at each of these theories.

Arrhenius theory.

In the ionic theory of Arrhenius, the concepts of "acid" and "base" are closely related to the process of electrolytic dissociation:

An acid is an electrolyte that dissociates in solutions to form H + ions;

The base is an electrolyte that dissociates in solutions to form OH - ions;

Ampholyte (amphoteric electrolyte) is an electrolyte that dissociates in solutions to form both H + ions and OH - ions.

For example:

ON ⇄ H + + A - nH + + MeO n n - ⇄Me (OH) n ⇄Me n + + nOH -

In accordance with the ionic theory, both neutral molecules and ions can be acids, for example:

HF⇄H++F-

H 2 PO 4 - ⇄ H + + HPO 4 2 -

NH 4 + ⇄H + + NH 3

Similar examples can be given for the grounds:

KOH K + + OH -

- ⇄Al(OH) 3 + OH -

+ ⇄Fe 2+ + OH -

Ampholytes include hydroxides of zinc, aluminum, chromium and some others, as well as amino acids, proteins, nucleic acids.

In general, the acid-base interaction in solution is reduced to a neutralization reaction:

H + + OH - H 2 O

However, a number of experimental data show the limitations of the ionic theory. So, ammonia, organic amines, metal oxides such as Na 2 O, CaO, anions of weak acids, etc. in the absence of water, they exhibit the properties of typical bases, although they do not contain hydroxide ions.

On the other hand, many oxides (SO 2 , SO 3 , P 2 O 5 , etc.), halides, acid halides, without hydrogen ions in their composition, even in the absence of water, exhibit acidic properties, i.e. bases are neutralized.

In addition, the behavior of an electrolyte in an aqueous solution and in a non-aqueous medium can be opposite.

So, CH 3 COOH in water is a weak acid:

CH 3 COOH⇄CH 3 COO - + H +,

and in liquid hydrogen fluoride it exhibits the properties of a base:

HF + CH 3 COOH⇄CH 3 COOH 2 + +F -

Studies of these types of reactions, and especially those occurring in non-aqueous solvents, have led to more general theories of acids and bases.

Theory of Bronsted and Lowry.

A further development of the theory of acids and bases was the protolytic (proton) theory proposed by I. Bronsted and T. Lowry. According to this theory:

An acid is any substance whose molecules (or ions) are capable of donating a proton, i.e. be a proton donor;

A base is any substance whose molecules (or ions) are capable of attaching a proton, i.e. be a proton acceptor;

Thus, the concept of the basis is significantly expanded, which is confirmed by the following reactions:

OH - + H + H 2 O

NH 3 + H + NH 4 +

H 2 N-NH 3 + + H + H 3 N + -NH 3 +

According to the theory of I. Bronsted and T. Lowry, an acid and a base form a conjugated pair and are connected by equilibrium:

ACID ⇄ PROTON + BASE

Since the proton transfer reaction (protolytic reaction) is reversible, and a proton is also transferred in the reverse process, the reaction products are acid and base in relation to each other. This can be written as an equilibrium process:

ON + B ⇄ VN + + A -,

where HA is an acid, B is a base, BH + is an acid conjugated with base B, A - is a base conjugated with acid HA.

Examples.

1) in reaction:

HCl + OH - ⇄Cl - + H 2 O,

HCl and H 2 O are acids, Cl - and OH - are the corresponding conjugate bases;

2) in reaction:

HSO 4 - + H 2 O⇄SO 4 2 - + H 3 O +,

HSO 4 - and H 3 O + - acids, SO 4 2 - and H 2 O - bases;

3) in reaction:

NH 4 + + NH 2 - ⇄ 2NH 3,

NH 4 + is an acid, NH 2 - is a base, and NH 3 acts as both an acid (one molecule) and a base (another molecule), i.e. shows signs of amphotericity - the ability to exhibit the properties of an acid and a base.

Water also has this ability:

2H 2 O ⇄ H 3 O + + OH -

Here, one H 2 O molecule adds a proton (base), forming a conjugate acid - a hydroxonium ion H 3 O +, the other gives a proton (acid), forming a conjugate base OH -. This process is called autoprotolysis.

It can be seen from the above examples that, in contrast to the ideas of Arrhenius, in the theory of Brönsted and Lowry, the reactions of acids with bases do not lead to mutual neutralization, but are accompanied by the formation of new acids and bases.

It should also be noted that the protolytic theory considers the concepts of "acid" and "base" not as a property, but as a function that the compound in question performs in the protolytic reaction. The same compound can react as an acid under certain conditions and as a base under others. So, in an aqueous solution of CH 3 COOH exhibits the properties of an acid, and in 100% H 2 SO 4 - a base.

However, despite its merits, the protolytic theory, like the Arrhenius theory, is not applicable to substances that do not contain hydrogen atoms, but, at the same time, exhibit the function of an acid: boron, aluminum, silicon, and tin halides.

Lewis theory.

A different approach to the classification of substances in terms of classifying them as acids and bases was the electronic theory of Lewis. Within the electronic theory:

an acid is a particle (molecule or ion) capable of attaching an electron pair (electron acceptor);

A base is a particle (molecule or ion) capable of donating an electron pair (electron donor).

According to Lewis, an acid and a base interact with each other to form a donor-acceptor bond. As a result of the addition of a pair of electrons, an electron-deficient atom has a complete electronic configuration - an octet of electrons. For example:

The reaction between neutral molecules can be represented in a similar way:

The neutralization reaction in terms of the Lewis theory is considered as the addition of an electron pair of a hydroxide ion to a hydrogen ion, which provides a free orbital to accommodate this pair:

Thus, the proton itself, which easily attaches an electron pair, from the point of view of the Lewis theory, performs the function of an acid. In this regard, Bronsted acids can be considered as reaction products between Lewis acids and bases. So, HCl is the product of neutralization of the acid H + with the base Cl -, and the H 3 O + ion is formed as a result of the neutralization of the acid H + with the base H 2 O.

Reactions between Lewis acids and bases are also illustrated by the following examples:

Lewis bases also include halide ions, ammonia, aliphatic and aromatic amines, oxygen-containing organic compounds of the type R 2 CO, (where R is an organic radical).

Lewis acids include halides of boron, aluminum, silicon, tin and other elements.

Obviously, in the theory of Lewis, the concept of "acid" includes a wider range of chemical compounds. This is explained by the fact that, according to Lewis, the assignment of a substance to the class of acids is due solely to the structure of its molecule, which determines the electron-acceptor properties, and is not necessarily associated with the presence of hydrogen atoms. Lewis acids that do not contain hydrogen atoms are called aprotic.

SOLUTIONS

General information

Solutions - These are homogeneous systems of variable composition, consisting of two or more substances, called components. According to the state of aggregation, solutions can be gaseous (air), liquid (blood, lymph) and solid (alloys). In medicine, liquid solutions, which play an exceptional role in the life of living organisms, are of the greatest importance. The processes of assimilation of food and excretion of waste products from the body are associated with the formation of solutions. A large number of drugs are administered in the form of solutions.

For the qualitative and quantitative description of liquid solutions, the terms "solvent" And "solute", although in some cases such a division is rather conditional. So, medical alcohol (96% solution of ethanol in water) should rather be considered as a solution of water in alcohol. All solvents are divided into inorganic and organic. The most important inorganic solvent (and in the case of biological systems, the only one) is water. This is due to such properties of water as polarity, low viscosity, the tendency of molecules to associate, and relatively high boiling and melting points. Organic solvents are divided into polar(alcohols, aldehydes, ketones, acids) and non-polar(hexane, benzene, carbon tetrachloride).

The process of dissolution equally depends on the nature of the solvent and on the properties of the solute. Obviously, the ability to form solutions is expressed in different substances in different ways. Some substances can be mixed with each other in any quantities (water and ethanol), others - in limited quantities (water and phenol). However, it should be remembered: absolutely insoluble substances do not exist!

The propensity of a substance to dissolve in a particular solvent can be determined using a simple rule of thumb: like dissolves into like. Indeed, substances with an ionic (salts, alkalis) or polar (alcohols, aldehydes) type of bond are highly soluble in polar solvents, for example, in water. Conversely, the solubility of oxygen in benzene is an order of magnitude higher than in water, since the O 2 and C 6 H 6 molecules are nonpolar.

The degree of affinity of a compound for a certain type of solvent can be assessed by analyzing the nature and quantitative ratio of its constituent functional groups, among which are hydrophilic(attracting water) and hydrophobic(repel water). Hydrophilic include polar groups, such as hydroxyl (-OH), carboxyl (-COOH), thiol (-SH), amino (-NH 2). Non-polar groups are considered hydrophobic: hydrocarbon radicals of the aliphatic (-CH 3, -C 2 H 5) and aromatic (-C 6 H 5) series. Compounds containing both hydrophilic and hydrophobic groups are called diphilic. Such compounds include amino acids, proteins, nucleic acids.

Theories of solutions

Currently, two main theories of solutions are known: physical and chemical.

Physical theory of solutions.

The physical theory of solutions was proposed by S. Arrhenius (1883) and J. G. van't Hoff (1885). In this theory, the solvent is considered as a chemically inert medium in which particles (molecules, ions) of the solute are uniformly distributed. In this case, it is assumed that there is no intermolecular interaction both between the particles of the solute and between the molecules of the solvent and the particles of the solute. However, later it turned out that the conditions of this model are satisfied by the behavior of only a small group of solutions, which were named ideal. In particular, gas mixtures and very dilute solutions of non-electrolytes can be considered ideal solutions.

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.

Additional material on the topic: “Physical theory of solutions. Chemical theory of solutions. Theory of solutions I. A. Kablukova. Unified Theory of Solutions. All practical human activity since the most ancient times is associated with the use of water and aqueous solutions. A variety of solutions were used in the manufacture of building materials, paints, glass, and ceramics. Clay products, unsolved recipes for colored glazes covering the walls of the underground tombs of the pharaohs, the art of embalming, which reached remarkable development in ancient Egypt - all these are again solutions, and quite complex in composition and very skillfully prepared then, by the first naturalists. centuries, in order to obtain a solution, the process of dissolution must go through, which is accompanied, like any chemical reaction, either by the release of heat (an exothermic process) or by its absorption (an endothermic process). This can be verified by simple experiments. If a thermometer is lowered into a glass half-filled with water and concentrated sulfuric acid is added, then the mercury in the thermometer will rise sharply. It is difficult to even take such a glass in your hands - it is so hot. If you begin to dissolve ammonium nitrate or ammonium thiocyanate, pouring a little water into the glass where these salts are located, then the glass will cool down so much that it can even freeze to the laboratory table. What kind of compounds appear in solutions? D. I. Mendeleev, who most profoundly developed the chemical theory of solutions, proposed calling them solvates (from the Latin word solvere - to dissolve). When dissolved in water, the process of formation of such compounds is called hydration, and the resulting products are called hydrates. Therefore, the chemical theory of solutions was called the solvate or hydrate theory of solutions. Based on this theory, phenomena such as, for example, the transformation of a mixture of calcium sulfate with water into a solid mass became clear. This process is used when applying plaster bandages for fractures. Calcium sulfate and water form strong compounds from which water is very difficult to isolate. The nature of the change in the color of salts precipitated from solutions under various conditions has also become clear. For example, CuSO4 salt is known to be white. However, a blue solution of this substance is formed in water, and when precipitated from its salt, blue crystals are obtained. By calcining at 250°C they can be made white again. The steam collected in this case turns out to be ordinary water. So blue or

blue crystals are made up of salt and water molecules, which gives the crystals their color. Blue salt looks and feels completely dry. Such compounds of a substance with water, having the form of crystals, are called crystalline hydrates. Their composition can be determined by weighing crystals per salt molecule, for example:  10H2O CuSO4  5H2O H2SO4  H2O MgSO4 However, the chemical theory did not allow quantitative prediction of changes in the properties of solutions depending on the concentration of the solute, did not explain how solvated molecules can be built. The authors of the chemical theory of solutions transferred the relations well known for gases to matter in the dissolved state. True, the matter was limited only to highly dilute solutions. “I established,” Van’t Hoff wrote, “for weak solutions, laws similar to the laws of Boyle-Mariotte and Gay-Lussac for gases ...” Using the equations of regularities that are widely known for gases, it was possible to make fairly accurate calculations. The van't Hoff theory was justified for dilute solutions of many substances. However, for solutions of inorganic salts, such as NaCl, KNO3, the results of experiment and calculation differed by almost two times, and for MgCl2 or Ca(NO3)2 even more. Moreover, the more dilute the solution in water, the more deviations from the calculated value were observed. The same thing happened with solutions of acids and bases. S. Arrhenius suggested that substances whose solutions conduct an electric current decompose into separate charged particles - ions, and ions behave like "free" particles and do not interact with the environment. In the theory of S. Arrhenius, it was also noted that when dissolved, electrolytes do not completely decompose into ions: only a certain part of the substance is in solution in the form of ions. To simplify the physical picture, the author of the theory suggested that, due to the low concentration of the substance, there is no electrostatic interaction between the formed ions. Opponents of the theory of electrolytic dissociation immediately saw its main drawback: it did not indicate the reason for the dissociation of electrolytes. It is not surprising that the theory of S. Arrhenius was sharply criticized by many scientists. At the height of the struggle between supporters of physical and chemical theories in the laboratory of one of the founders of physical chemistry - W. Ostwald

- a young Russian chemist I. A. Kablukov appeared, sent from Moscow University to familiarize himself with the methods of a new field of chemistry. He quickly became friends with S. Arrhenius and the Russian chemist V. A. Kistyakovsky. Having repeated the study of the dependence of the electrical conductivity of electrolyte solutions in water on their concentration, I. A. Kablukov confirmed the correctness of the conclusions of S. Arrhenius. This means that, indeed, in aqueous solutions, electrolyte molecules dissociate into ions. But is the solvent involved? I. A. Kablukov compares the behavior of electrolytes in various solvents and concludes that hydration is the cause of the dissociation of electrolytes in water. It is this process that leads to the weakening of bonds in molecules and their decay into ions. Supporters of the chemical theory, while asserting the idea of ​​the interaction of a solute and a solvent, did not allow the thought of the decay of matter into separate charged ions. I. A. Kablukov found that the chemical interaction of a solvent with electrolyte molecules leads to their decomposition into ions. Water, forming a compound with electrolyte molecules, "pulls" them apart, divides them into ions. Moreover, it forms compounds with these ions. As a result, the concept of ion hydration was put forward. (It must be said that the idea of ​​the interaction of ions with water, almost simultaneously with I. A. Kablukov, was expressed by another young Russian chemist V. A. Kistyakovsky.) I. A. Kablukov consistently developed this idea and defended in a dispute with supporters of physical and chemical theory. He showed that the main provisions of the theory of chemical interaction of substances and the physical theory of van't Hoff, mutually complementing each other, are able to explain almost all the facts related to the dissolution of substances and their behavior in solutions. So, if sulfuric acid is mixed with ether, then such a solution does not conduct current: the interaction of these substances is small. In this case, the Van't Hoff formulas turned out to be valid. It turns out, as it were, a special case of the general theory. With a strong interaction of a solute with a solvent (for example, sulfuric acid or copper sulfate CuSO4 with water), decomposition into ions occurs. However, it is impossible to look at ions only as "free" gas-like particles. Kablukov believed that water, decomposing the molecules of a dissolved body, enters with ions into unstable compounds that are in a state of dissociation. According to S. Arrhenius, ions move freely, like atoms. Ions exist in solution surrounded by molecules

water. Each ion corresponds to a certain number of molecules included in its "retinue". So, copper sulfate has the formula CuSO4  5H2O. Of these 2-. four water molecules surround the copper ion and only one - the SO4 anion. Negative ions are generally poorly hydrated. The interpretation of the ionization of dissolved substances proposed by I. A. Kablukov has become generally accepted. “The solvent, acting on the dissolved body, changes its physical and chemical properties,” wrote I. A. Kablukov, “and all the properties of the solution depend on the magnitude of the interaction between the dissolved body and the solvent.” The concept of hydration of ions by I. A. Kablukov and V. A. Kistyakovsky made it possible to correctly explain the decomposition of a substance into individual charged ions, that is, electrolytic dissociation: electrolyte molecules (acids, bases and salts) in aqueous solutions decompose into ions under the action of water molecules. A small number of solvents have the same dissociating effect, but water stands apart, as it were. Its strong dissociating effect is due to the fact that the water molecule is polar. In a water molecule, the centers of positive and negative charges do not coincide, and it is like a rod with excess charges at the ends. Electrolytes in molecules also have positive and negative charges unevenly distributed. Let us consider the process of dissociation of molecules of any electrolyte, using the concepts of the unified theory of solutions (Mendeleev - Van't Hoff - Arrhenius - Kablukov). Let us choose, for example, hydrogen chloride (HCl), that substance, the electrical conductivity of solutions of which in various solvents has been studied extensively by I. A. Kablukov. The molecule of this substance is polar. Hydrogen and chlorine atoms have some excess charge: the first of them is positive, and the second is negative. Solvent molecules, if they also have a charge shift (+ and -), are oriented around the HCl molecule in a very definite way. Opposite poles of these molecules are attracted. Now it all depends on how strong the chemical interaction is between the ions that make up HCl and the solvent. If it is small, then the substance will mainly be in the form of whole molecules and will practically not conduct electric current. These are solutions of HCl in ether and benzene. In water and in other more or less polar solvents (for example, methyl alcohol), the interaction of the solvent with the ions included in hydrogen chloride is great. For example, the hydrogen cation H+ simply cannot exist alone: ​​this cation (proton) binds so tightly to the water molecule that it exists only in the form of a hydronium ion:

H+ + H2O  H3O+ Molecules of polar solvents are attracted to ions more than the cation and anion bind to each other in the molecule of the substance. As a result, the electrolyte dissociates: HCl ⇄ H+ + Cl–, or, more precisely, ions exist in solution in complex with solvent molecules, like, in particular, the Cu2+ ion with 4H2O; Fe2+ ​​c 6H2O, etc. This determines the change in the properties of the solution compared to the characteristics of the solvent and the solute, taken separately. I. A. Kablukov carried out a number of fundamental studies of various solutions. The results of his work made it possible to expand the boundaries of the theory of solutions and extend it to all aqueous and non-aqueous media, including mixed media consisting of several solvents. This gave a powerful impetus to the further development of the theory of ions in solution, the ability of solutions to transfer electric charges, etc.