Physical and chemical theory of solutions. Solvate (hydration) theory of dissolution General theory of solutions and solvents
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 was associated with the general course of development of science and production, as well as with philosophical ideas about the reasons for the chemical affinity between various substances. At 17 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 solvent particles 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. Initially, such ideas satisfactorily explained the fact that a given solvent can dissolve not all substances, but only some.
At the beginning of the 19th century. preconditions are created for the development of the 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 the 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 the vapor pressure above the 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, which is distant from it by a semi-permeable partition through which the solvent can enter, but 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 the solution (to which additional pressure is applied) and the chemical potential of the 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. The deviation from these laws, observed for electrolyte solutions, was explained on the basis of the theory of electrolytic dissociation by S. Arrhenius.
The analogy between highly dilute solutions and gases seemed to many scientists so convincing that they began to consider the dissolution process as a physical act. From the point of view of these scientists, a solvent is only a medium into which particles of a solute can diffuse. The simplicity of the physical theory of solutions and its successful application to explain many of the properties of solutions ensured the rapid success of this theory.
Chemical theory of solutions. DI. Mendeleev and his followers considered the process of formation of a solution 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 are formed between them and are in a state of partial dissociation. DI. Mendeleev noted that the processes occurring in a solution are of a dynamic nature and on the need to use the entire sum of physical and chemical information about the properties of the particles that form the solution, he 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 system 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 doctrine of D.I. Mendeleev, supporters of the chemical view of the nature of solutions pointed out that the particles of a solute do not move in a void, but in a space occupied by solvent particles, with which they interact, forming complex compounds of 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-polyhedrons are created in a liquid from homogeneous and dissimilar 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 dissolved substance. These studies concerned the finding of 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. In many systems, it was shown that during the formation of a solution, cooling or heating of the system is observed, which was explained by chemical interactions between the components. The chemical nature of the dissolution process was confirmed by both the study of the vapor pressure over the solution and the study of the distribution of substances between two solvents.
By the beginning of the 20th century. extensive experimental material has accumulated, showing that solutions are complex systems in which the phenomenon of association, dissociation, complexation is observed, and when studying them, it is necessary to take into account all types of interactions between particles present and formed in a solution.
Due to the wide variety of solutions, the concepts of both the physical and chemical theory of solutions are used to explain their nature and properties.
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The solution is homogeneous system containing at least two substances. There are solutions of solid, liquid and gaseous substances in liquid solvents, as well as homogeneous mixtures (solutions) of solid, liquid and gaseous substances. As a rule, a substance taken in excess and in the same state of aggregation as the solution itself is considered to be a solvent, and a component taken in a deficiency is considered to be a dissolved substance.
Depending on the state of aggregation of the solvent, gaseous, liquid and solid solutions are distinguished.
Gaseous solution- this is primarily air, as well as other mixtures of gases.
TO liquid solutions include homogeneous mixtures of gases, liquids and solids with liquids.
Solid solutions are represented by alloys, as well as glasses.
In practice, liquid solutions (mixtures of liquids, where the solvent is a liquid) are of great importance. The most common solvent among inorganic substances is water. Among organic substances, methanol, ethanol, diethyl ether, acetone, benzene, carbon tetrachloride and others are used as solvents.
Under the action of chaotically moving particles of the solvent, the particles (ions or molecules) of the dissolved substance pass into the solution, forming, due to the random movement of particles, a qualitatively new homogeneous ( homogeneous) system. Solubility in various solvents - characteristic property of a substance. Some substances can be mixed with each other in any ratio (water and alcohol), others have limited solubility (sodium chloride and water).
Consider the dissolution of a solid in a liquid. Within the framework of the molecular kinetic theory, when solid sodium chloride is introduced into a solvent (for example, into water), the Na + and C1 “ions, which are on the surface, interact with the solvent (with molecules and other particles of the solvent), can detach and pass into solution. After removing the surface layer, the process extends to the next layers of solid. This is how the particles gradually move from the crystal to the solution. The destruction of ionic crystals of NaCl in water consisting of polar molecules is shown in Figure 6.1.
Rice. 6.1. Destruction of the crystal lattice of NaCl in water. a- attack of solvent molecules; b- ions in solution
Particles that have passed into the solution are distributed due to diffusion throughout the volume of the solvent. At the same time, as the concentration increases, particles (ions, molecules) in continuous motion, upon collision with a solid surface of a still undissolved solid, can linger on it, i.e., dissolution is always accompanied by reverse process - crystallization. A moment may come when at the same time the same number of particles (ions, molecules) are released from the solution as they pass into the solution, i.e. equilibrium.
A solution in which a given substance no longer dissolves at a given temperature, that is, a solution in equilibrium with the substance being dissolved is called saturated, and a solution in which a certain amount of this substance can still be further dissolved is called unsaturated.
A saturated solution contains the maximum possible (for given conditions) amount of a solute. The concentration of a substance in a saturated solution is a constant value under given conditions (temperature, solvent), it characterizes solubility of a substance; see § 6.4 for details.
A solution in which the solute content is greater than in a saturated solution under these conditions is called supersaturated. This unstable, nonequilibrium systems, which spontaneously pass into an equilibrium state, and when an excess of a solute is released in solid form, the solution becomes saturated.
Saturated and unsaturated solutions should not be confused with diluted and concentrated. Diluted solutions - solutions with a low solute content; concentrated solutions - solutions with a high solute content. It should be emphasized that the concepts of dilute and concentrated solutions are relative and are based on a qualitative assessment of the ratio of the amounts of a solute and a solvent in a solution (sometimes a solution is called strong and weak in the same sense). We can say that these definitions arose from practical necessity. So, they say that a solution of sulfuric acid H 2 S0 4 is concentrated (strong) or diluted (weak), but at what concentration the solution of sulfuric acid should be considered concentrated, and at what concentration it is not determined exactly.
When comparing the solubility of various substances, it can be seen that in the case of poorly soluble substances, saturated solutions are dilute, in the case of highly soluble substances, their unsaturated solutions can be rather concentrated.
For example, at 20 ° C, 0.00013 g of calcium carbonate CaCO 3 dissolves in 100 g of water. A solution of CaCO 3 under these conditions is saturated, but very dilute (its concentration is very low). But here's an example. A solution of 30 g of sodium chloride in 100 g of water at 20 ° C, unsaturated, but concentrated (the solubility of NaCl at 20 ° C is 35.8 g in 100 g of water).
In conclusion, we note that here and below (except for § 6.8) we will talk about true solutions. The particles that make up such solutions are so small that they cannot be seen; these are atoms, molecules or ions, their diameter usually does not exceed 5 nm (5 10 ~ 9 m).
And the last thing about the classification of solutions. Depending on whether electrically neutral or charged particles are present in the solution, solutions can be molecular (this solutions of non-electrolytes) and ionic (electrolyte solutions). A characteristic property of electrolyte solutions is electrical conductivity (they conduct an electric current).
It is shown above that the reaction of pure water is neutral (pH = 7). Aqueous solutions of acids and bases are, respectively, acidic (pH< 7) и щелочную (рН >7) reaction. Practice, however, shows that not only acids and bases, but also salts can have an alkaline or acidic reaction - the reason for this is the hydrolysis of salts. The interaction of salts with water, as a result of which an acid (or acid salt) and a base (or basic salt) are formed, is called salt hydrolysis. Let us consider the hydrolysis of salts of the following main types: 1. Salts of a strong base and a strong acid (for example, KBr, NaNO3) do not hydrolyze when dissolved in water, and the salt solution has a neutral reaction….
It is well known that some substances in a dissolved or molten state conduct an electric current, while others do not conduct a current under the same conditions. This can be observed with a simple instrument. It consists of carbon rods (electrodes) connected by wires to an electrical network. An electric light is included in the circuit, which indicates the presence or absence of current in the circuit. If the electrodes are immersed in a sugar solution, the light will not light up. But it will light up brightly if they are dipped in a sodium chloride solution. Substances that decompose into ions in solutions or melts and therefore conduct electric current are called electrolytes. Substances that under the same conditions do not decompose into ions and do not conduct electric current are called non-electrolytes. Electrolytes include acids, bases and almost all salts, non-electrolytes - most organic compounds, ...
To explain the features of aqueous solutions of electrolytes, the Swedish scientist S. Arrhenius in 1887 proposed the theory of electrolytic dissociation. Later it was developed by many scientists on the basis of the theory of the structure of atoms and chemical bonds. The modern content of this theory can be reduced to the following three provisions: 1. Electrolytes, when dissolved in water, disintegrate (dissociate) into ions - positive and negative. Ions are in more stable electronic states than atoms. They can consist of one atom - these are simple ions (Na +, Mg2 +, Al3 +, etc.) - or from several atoms - these are complex ions (NO3—, SO2-4, POZ-4, etc.). 2. Under the action of an electric current, ions acquire a directional movement: positively charged ions move to the cathode, negatively charged to the anode. Therefore, the former are called cations, the latter anions. The directional movement of ions occurs as a result of their attraction by oppositely charged electrodes. 3. Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) proceeds. Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is put. For instance,…
The question of the mechanism of electrolytic dissociation is essential. Substances with ionic bonds dissociate most easily. As you know, these substances are composed of ions. When they dissolve, the water dipoles are oriented around the positive and negative ions. Forces of mutual attraction arise between ions and water dipoles. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. At…
With the help of the theory of electrolytic dissociation, they define and describe the properties of acids, bases and salts. Acids are called electrolytes, during the dissociation of which only hydrogen cations Н3РО4 Н + + Н2РО-4 (first stage) Н2РО-4 Н + + НРO2-4 (second stage) НРО2-4 Н + PОЗ-4 (third stage) are formed Dissociation of a polybasic acid proceeds mainly through the first stage, to a lesser extent through the second, and only to a small extent through the third. Therefore, in an aqueous solution of, for example, phosphoric acid, along with Н3РО4 molecules, there are ions (in successively decreasing amounts) Н2РО2-4, НРО2-4 and РО3-4. Bases are called electrolytes, the dissociation of which only hydroxide ions are formed as anions. For example: KOH K + + OH—; ...
Since electrolytic dissociation is a reversible process, molecules are also present in electrolyte solutions along with their ions. Therefore, electrolyte solutions are characterized by the degree of dissociation (denoted by the Greek letter alpha α). The degree of dissociation is the ratio of the number of N 'molecules disintegrated into ions to the total number of dissolved N molecules: The degree of dissociation of the electrolyte is determined empirically and is expressed in fractions of a unit or in percent. If α = 0, then there is no dissociation, and if α = 1 or 100%, then the electrolyte completely decomposes into ions. If α = 20%, then this means that out of 100 molecules of a given electrolyte, 20 decayed into ions. Different electrolytes have different degrees of dissociation. Experience shows that it depends on the concentration of the electrolyte and on the temperature. With a decrease in the concentration of electrolyte, ...
According to the theory of electrolytic dissociation, all reactions in aqueous solutions of electrolytes are reactions between ions. They are called ionic reactions, and the equations of these reactions are called ionic equations. They are simpler than the equations of reactions written in molecular form and are more general in nature. When drawing up the ionic equations of reactions, one should be guided by the fact that substances that are poorly dissociated, slightly soluble (precipitated) and gaseous are recorded in molecular form. The ↓ sign, standing at the formula of a substance, means that this substance leaves the reaction sphere in the form of a precipitate, the sign means that the substance is removed from the reaction sphere in the form of a gas. Strong electrolytes, as completely dissociated, are recorded as ions. The sum of the electric charges on the left side of the equation must be equal to the sum of the electric charges on the right side. To reinforce these provisions, consider two examples. Example 1. Write the equations of reactions between solutions of iron (III) chloride and sodium hydroxide in molecular and ionic forms. Let's break the solution of the problem into four stages. one….
КH2O = 1.10-4 This constant for water is called the ionic product of water, which depends only on temperature. During the dissociation of water for each H + ion, one OH– ion is formed, therefore, in pure water, the concentrations of these ions are the same: [H +] = [OH–]. Using the value of the ionic product of water, we find: = [OH—] = mol / l. These are the concentrations of Н + and ОН– ions ...
Exists 2 solution theories: physical and chemical.
Physical theory of solutions.
It was discovered by Jacob G. Van't Hoff and Swate A. Arrhenius.
The essence of the theory of solutions: a solvent is a chemical indifferent medium in which particles of a solute are evenly distributed. The theory does not imply the presence of intermolecular bonds between the solvent and the solute.
Only ideal solutions are suitable for this theory, where the components of the solvent do not affect the soluble compound in any way. An example is gas solutions, where gases that do not react with each other are mixed with each other in unlimited quantities. All physical data (boiling and melting points, pressure, heat capacity) are calculated based on the properties of all compounds included in the composition.
By Dalton's law: the total pressure of the gas mixture is equal to the sum of the partial pressures of its components:
Ptotal= Р 1 + Р 2 + Р 3 + ...
Chemical theory of solutions.
Chemical(solvated) solution theory described by D.I. Mendeleev. The bottom line is as follows: the particles of the solvent and the solute react with each other, as a result of which unstable compounds of variable composition are obtained - hydrates ( solvates). The main bonds here are hydrogen.
The substance can disintegrate into layers (dissolve) in the case of a polar solvent (water). A striking example is the dissolution of table salt.
The reaction between the components of the mixture can also occur:
H 2 O + Cl 2 = HCl + HOCl,
During the dissolution process, the composition and volume of the reaction mixture change, because 2 processes take place: the destruction of the structure of the solute and the chemical reaction between the particles. Both processes proceed with a change in energy.
Thermal effects can be exothermic and endothermic (with the release and absorption of energy).
Compounds with solvent particles are called hydrates.
Crystalline substances, which include hydrates, are called crystal hydrates and have different colors. For example, crystalline copper sulfate hydrate: CuSO 4 ·5H 2 O... The solution of crystalline hydrate is blue. If we consider crystalline cobalt hydrate CoCl 2 6H 2 O, then it has a pink color, CoCl 2 · 4H 2 O- Red, CoCl 2 · 2H 2 O- blue-violet, CoCl 2 · H 2 O- dark blue, and anhydrous cobalt chloride solution - pale blue.
