Physical and chemical theories of solution formation. Hydrate theory of solutions D.I.
Topic 7. Solutions. Disperse systems
Lectures 15-17 (6 hours)
The purpose of the lectures: to study the main provisions of the solvate (hydrate) theory of dissolution; general properties of solutions (laws of Raoult, van't Hoff, osmotic pressure, Arrhenius equation); types of liquid solutions, define solubility; consider the properties of weak electrolytes (solubility constant, Ostwald's dilution law, ionic product of water, pH of the medium, solubility product); properties of strong electrolytes (Debye-Hückel theory, ionic strength of the solution); give a classification of dispersed systems; consider the stability of colloidal solutions, coagulation, peptization, obtaining colloidal-dispersed systems and properties of colloidal-dispersed systems (molecular-kinetic, optical and electro-kinetic).
Issues under study:
7.1. Solvate (hydrate) theory of dissolution.
7.2. General properties of solutions.
7.3. Types of liquid solutions. Solubility.
7.4. Properties of weak electrolytes.
7.5. Properties of strong electrolytes.
7.6. Classification of dispersed systems.
7.7. Obtaining colloid-dispersed systems.
7.8. Stability of colloidal solutions. Coagulation. Peptization.
7.9. Properties of colloid-dispersed systems.
Solutions homogeneous systems are called, consisting of two or more substances, the composition of which can vary within a fairly wide range allowed by solubility. Any solution consists of several components: a solvent ( BUT) and a solute of one or more ( AT).
Component- this is a part of a thermodynamic system that is homogeneous in chemical properties, which can be isolated from it and exist in a free form for an arbitrarily long time.
Solvent is a component whose concentration is higher than the concentration of other components in the solution. It retains its phase state during the formation of solutions.
Any solution is characterized by such properties as density, boiling point, freezing point, viscosity, surface tension, solvent pressure over the solution, osmotic pressure, etc. These properties change smoothly with changes in pressure, temperature, composition (concentration). The concentration of a solution indicates the amount of a substance that is contained in a certain weight of a solution or solvent, or in a certain volume of solution. In chemistry, various methods are used to express the concentration of solutions:
Mass fraction of solute (percentage concentration (w)) shows the number of grams of a solute ( m in) in 100 g of solution ( m p), expressed in %:
Molar concentration (C) shows the number of moles of a solute (n) in 1 dm³ of a solution (V):
Expressed in mol / dm³, for example, C (1 / 1H 2 SO 4) \u003d 0.1 mol / dm³.
Molar equivalent concentration is the number of mole equivalents of a solute in 1 dm³ of solution (V):
Expressed in mol/dm³. For example, C (1 / 2H 2 SO 4) \u003d 0.1 mol / dm³; C (1/5 KMnO 4) \u003d 0.02 mol / dm³.
The concepts of equivalent, equivalence factor (for example, f equiv (HCl) \u003d 1/1; f equiv (H 2 SO 4) \u003d 1/2; f equiv (Na 2 CO 3) \u003d 1/2; f equiv (KMnO 4) = 1/5) and the molar mass equivalent (for example, for sodium carbonate: M(1/2 Na 2 CO 3) = f eq M(Na 2 CO 3) = 1/2 M(Na 2 CO 3)) were considered in the introduction (paragraph 2).
Molality (C m) shows the number of moles (n) of a solute in 1000 g of solvent (m p-la):
Expressed in mol/kg of solvent, for example C m (NaCl) = 0.05 mol/kg.
Mole fraction is the ratio of the number of moles of a substance to the sum of the numbers of moles in a solution:
where N A and N B are the mole fraction of the solvent and the solute, respectively. The mole fraction multiplied by 100% is the mole percentage, so
N A + N B = 1. (7.6)
In practical work, it is important to be able to quickly move from one concentration unit to another, so it is important to remember that
m r-ra = V r-ra ρ, (7.7)
where m r-ra is the mass of the solution, g; V p-ra - the volume of the solution, cm 3; ρ is the density of the solution, g/cm3.
The dissolution process is a complex physical and chemical process in which the interaction between particles (molecules or ions) of various chemical nature is most clearly manifested.
The processes of dissolution of many substances in different states of aggregation are greatly influenced by the polarity of the molecules of the solvent and the solute. It should be noted that like dissolves like. Polar solvents (water, glycerin) dissolve polar molecules (KCl, NH 4 Cl, etc.); non-polar solvents (toluene, gasoline, etc.) dissolve non-polar molecules (hydrocarbons, fats, etc.).
Modern dissolution theory based on the physical theory of Van't Hoff and S. Arrhenius and the chemical theory of D. I. Mendeleev. According to this theory, the dissolution process consists of three stages:
1) mechanical destruction of bonds between particles of a dissolved substance, for example, the destruction of the crystal lattice of salt (this is a physical phenomenon);
2) education solvates (hydrates), i.e., unstable compounds of solute particles with solvent molecules (this is a chemical phenomenon);
3) a spontaneous process of diffusion of solvated (hydrated) ions throughout the volume of the solvent (this is a physical process). In solution, any charged particle (ion or polar molecule) is surrounded by solvation shell , which consists of solvent molecules oriented in an appropriate way. If the solvent is water, then the term hydration shell , and the phenomenon itself is called hydration .
The process of formation of solutions is accompanied by a thermal effect, which can be both endothermic and exothermic. The first stage of dissolution always takes place with the absorption of heat, and the second can take place both with the absorption and release of heat. Therefore, the total thermal effect of dissolution depends on the thermal effect of the formation of solvates (hydrates). The combination of molecules or ions of a solute with molecules of a solvent is carried out mainly due to hydrogen bonding, or due to the electrostatic interaction of polar molecules of substances. The composition of solvates (hydrates) varies depending on the temperature and concentration of the solute. With their increase, the number of solvent molecules included in the solvate (hydrate) decreases. Thus, solutions occupy an intermediate position between mechanical mixtures and chemical compounds.
The theory of solutions does not yet make it possible in any case to predict the properties of solutions from the properties of their components. This is explained by the extremely large variety and complexity of interactions between solvent molecules and solute particles. The structure of solutions, as a rule, is much more complicated than the structure of its individual components.
According to the state of aggregation, all solutions are divided into three groups: solutions of gases in gases or gas mixtures; liquid solutions; solid solutions (metal alloys). In what follows, only liquid solutions will be considered.
It is shown above that the reaction of pure water is neutral (pH = 7). Aqueous solutions of acids and bases have, 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, which results in the formation of an acid (or acid salt) and a base (or basic salt), is called salt hydrolysis. 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 electric current, while others do not conduct 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 bulb 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 lamp does not light up. But it will light up brightly if they are lowered into a solution of sodium chloride. Substances that decompose into ions in solutions or melts and therefore conduct electricity are called electrolytes. Substances that do not decompose into ions under the same conditions 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 bonding. The current content of this theory can be reduced to the following three propositions: 1. When dissolved in water, electrolytes decompose (dissociate) into positive and negative ions. Ions are in more stable electronic states than atoms. They can consist of one atom - these are simple ions (Na +, Mg2 +, Al3 +, etc.) - or of several atoms - these are complex ions (NO3-, SO2-4, ROZ-4, etc.). 2. Under the action of an electric current, the ions acquire a directed movement: positively charged ions move towards the cathode, negatively charged ones move towards the anode. Therefore, the former are called cations, the latter anions. The directed 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 sign of reversibility is put. For example,…
The question of the mechanism of electrolytic dissociation is essential. Substances with an ionic bond dissociate most easily. As you know, these substances are composed of ions. When they dissolve, the dipoles of water orient themselves around the positive and negative ions. Forces of mutual attraction arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. At…
Using the theory of electrolytic dissociation, definitions are given and the properties of acids, bases and salts are described. Electrolytes are called acids, during the dissociation of which only hydrogen cations are formed as cations H3PO4 H+ + H2PO-4 (first stage) H2PO-4 H+ + HPO2-4 (second stage) HPO2-4 H+ P3-4 (third stage) The 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 H3PO4 molecules, there are ions (in successively decreasing amounts) H2PO2-4, HPO2-4 and PO3-4. Bases are called electrolytes, during the dissociation of which only hydroxide ions are formed as anions. For example: KOH K+ + OH—;…
Since electrolytic dissociation is a reversible process, electrolyte solutions contain molecules 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 molecules N 'decayed into ions to the total number of dissolved molecules N: 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 this electrolyte, 20 decomposed 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 decreasing electrolyte concentration, ...
According to the theory of electrolytic dissociation, all reactions in aqueous electrolyte solutions are reactions between ions. They are called ionic reactions, and the equations of these reactions are called ionic equations. They are simpler than reaction equations written in molecular form and are more general. When compiling ionic reaction equations, one should be guided by the fact that poorly dissociated, slightly soluble (precipitating) and gaseous substances are written 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, being 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 consolidate these provisions, consider two examples. Example 1. Write the reaction equations 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….
KH2O = 1.10-4 This constant for water is called the ionic product of water, which depends only on temperature. During the dissociation of water, one OH– ion is formed for each H+ ion, 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: \u003d [OH -] \u003d mol / l. These are the concentrations of H+ and OH- ions…
The solution is a 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 deficiency is considered a solute.
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 represented by alloys, as well as glasses.
In practice, liquid solutions (mixtures of liquids, where the solvent is a liquid) are of great importance. Of the inorganic substances, the most common solvent is water. From organic substances, methanol, ethanol, diethyl ether, acetone, benzene, carbon tetrachloride and others are used as solvents.
Under the action of randomly 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 the particles, a qualitatively new homogeneous ( homogeneous) system. Solubility in different 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 located on the surface, interacting with the solvent (with molecules and other particles of the solvent), can come off and go into solution. After removing the surface layer, the process extends to the next layers of the solid. So gradually the particles pass from the crystal into 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
The particles that have passed into the solution are distributed due to diffusion throughout the entire volume of the solvent. At the same time, as the concentration increases, particles (ions, molecules) that are in continuous motion, when colliding with a solid surface of a solid that has not yet dissolved, can linger on it, i.e., dissolution is always accompanied by reverse process - crystallization. A moment may come when at the same time as many 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, i.e., a solution that is in equilibrium with the solute, 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 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 that of a saturated solution under given conditions is said to be supersaturated. This unstable, non-equilibrium systems, which spontaneously pass into an equilibrium state, and when an excess of a dissolved substance is released in a solid form, the solution becomes saturated.
Saturated and unsaturated solutions should not be confused with diluted and concentrated. Dilute solutions - solutions with a small content of a solute; concentrated solutions - solutions with a high content of solute. It must 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 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 a sulfuric acid solution should be considered concentrated, and at what dilute, it is not exactly defined.
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 quite concentrated.
For example, at 20 ° C, 0.00013 g of calcium carbonate CaCO 3 dissolves in 100 g of water. The CaCO 3 solution under these conditions is saturated, but very dilute (its concentration is very low). But here's an example. A solution of 30 g of table salt in 100 g of water at 20 ° C is 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 § 6.8) we will deal with 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 non-electrolyte solutions) and ionic (solutions of electrolytes). A characteristic property of electrolyte solutions is electrical conductivity (they conduct electric current).
Physical theory of solutions(Vant - Hoff, Arrhenius - scientists who contributed to the development) considered the solvent as an inert medium and equated solutions to simple mechanical mixtures.
Disadvantages of the theory: a) did not explain the energy effect of the solvent; b) did not explain the change in volume during the dissolution process; c) did not explain the color change during dissolution
Chemical theory of solutions(DI Mendeleev) Solutions were considered as chemical compounds. However, in solutions there is no strict relationship between the substance and the solvent, i.e. solutions do not obey the law of constancy of composition. In addition, many properties of its individual components can be found in the properties of solutions, which is not observed in the case of a chemical compound.
Physico-chemical theory of solutions(Kablukov) From this point of view, solutions occupy an intermediate position between mechanical mixtures and chemical compounds.
The dissolution process is closely related to diffusion, under the influence of the solvent from the surface of the solid, molecules or ions gradually come off and in the diffusion solution are distributed over the entire volume of the solvent. A new layer of molecules is then removed from the surface, and so on. The ions that have passed in solution remain associated with water molecules and form ion hydrates. In the general case of any solvent, these compounds are called ion solvates. At the same time, the reverse process of isolation of molecules from the solution occurs. The reverse process is the greater, the higher the concentration of the solution at dynamic equilibrium, how many molecules are dissolved, the same amount is released from the solution.
5. Hydrolysis of salt solutions. Degree of hydrolysis and factors influencing it. Typical cases of hydrolysis (show with examples).
The interaction of salt ions with water leading to the formation of a weak electrolyte is called salt hydrolysis. There are several types of hydrolysis:
Salt of a strong base and a weak acid.(CH 3 COONa, NaCO 3 , KCN, Na 2 S)
In an aqueous solution, the salt first dissociates into cations and anions.
dissociation:
The anion of a weak acid interacts with water, creating an alkaline environment (anion hydrolysis):
hydrolysis:
The dissociation constant of the hydrolysis equation:
because \u003d const, then K D \u003d K G (hydrolysis constant)
because Quads = , then = Quads/
Those. the weaker the acid, the lower its Kd, the more the salt will be hydrolyzed.
Hydrolysis of salts of polybasic acids proceeds stepwise:
1 step:
2 step:
In solutions of ordinary concentration, the hydrolysis of this salt proceeds only in the first step with the formation of an acid salt. In highly dilute solutions, hydrolysis partially proceeds in 2 steps with the formation of free carbonic acid. Hydrolysis in the 2nd stage is not significant, because high concentration of OH ions.
Salt of a weak base and a strong acid (nh4no3, ZnCl2, Al2(so4)3)
dissociation:
A weak base cation interacts with water, creating an acidic environment:
hydrolysis:
molecular hydrolysis equation:
The weaker the base, the more the salt will hydrolyze.
Salts of polyacid bases hydrolyze stepwise:
1 step:
2 step:
Under normal conditions, the hydrolysis of this salt proceeds only in the first stage.
Salt of a weak acid and a weak base (CH 3 COONH 4 , Al 2 S 3 , (NH 4 ) 2 CO 3 )
In this case, both the cation and the anion of the salt undergo hydrolysis (hydrolysis by cation and anion)
Dissociation:
Hydrolysis:
The hydrolysis medium is determined by the salt ion in which the degree of hydrolysis is higher (acidic, alkaline, neutral)
Salts of a strong base and a strong acid (NaOH, CaCl 2 , NaNO 3 )
These salts do not hydrolyze when dissolved in water, their solutions are neutral.
Complete (combined) hydrolysis
It proceeds by draining 2 solutions of different salts, and one of the salts is hydrolyzed by the cation, and the other by the anion, with the formation of a weak acid and a weak base.
Degree of hydrolysis
Under degree of hydrolysis refers to the ratio of the part of the salt undergoing hydrolysis to the total concentration of its ions in solution.
The degree of salt hydrolysis is the higher, the weaker the acid or base that forms it.
h is related to K G by an equation similar to the Ostwald dilution law
Most often, the hydrolyzed part of the salt is very small and the concentration of hydrolysis products is not significant, then h<1, а 1-h≈1
those. when a salt solution is diluted, the degree of its hydrolysis increases.
In addition to diluting the solution, hydrolysis can be enhanced by heating the solution, as well as by adding special reagents.
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 the 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.
Thus, 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. AT 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 character 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. AT 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.
