What is the essence of the chemical theory of solutions. Chemical theory of solutions
Lecture 1
"THE CONCEPT OF "SOLUTION". CHEMICAL THEORY OF SOLUTIONS»
Solutions are important in human life and practical activities. Solutions are all the most important physiological fluids (blood, lymph, etc.). The body is a complex chemical system, and the vast majority of chemical reactions in the body occur in aqueous solutions. It is for this reason that the human body is 70% water, and severe dehydration occurs quickly and is a very dangerous condition.
Many technological processes, such as the production of soda or nitric acid, the isolation and purification of rare metals, the bleaching and dyeing of fabrics, proceed in solutions.
To understand the mechanism of many chemical reactions, it is necessary to study the processes occurring in solutions.
The concept of "solution". Types of solutions
Solution- solid, liquid or gaseous homogeneous system consisting of two or more components.
homogeneous system consists of one phase.
Phase- a part of the system separated from its other parts by the interface, when passing through which the properties (density, thermal conductivity, electrical conductivity, hardness, etc.) change abruptly. The phase can be solid, liquid, gaseous.
The most important type of solutions are liquid solutions, but in a broad sense, solutions are also solid (brass alloy: copper, zinc; steel: iron, carbon) and gaseous (air: a mixture of nitrogen, oxygen, carbon dioxide and various impurities).
The solution contains at least two components, of which one is solvent, while others are solutes.
Solvent is a component of a solution that is in the same state of aggregation as the solution. The solvent in the solution by mass is always greater than the rest of the components. The solute is in solution in the form of atoms, molecules or ions.
Differ from solutions:
Suspension is a system consisting of fine solid particles suspended in a liquid (talc in water)
Emulsion- this is a system in which one liquid is fragmented in another liquid that does not dissolve it (i.e., small drops of a liquid that are in another liquid: for example, gasoline in water).
Spray can- gas with solid or liquid particles suspended in it (fog: air and liquid droplets)
Suspensions, emulsions and aerosols consist of several phases, they are not homogeneous and are dispersed systems . Suspensions, emulsions and aerosols are not solutions!
Chemical theory of solutions.
The solvent chemically interacts with the solute.
The chemical theory of solutions was created by D.I. Mendeleev at the end of the nineteenth century. based on the following experimental facts:
1) The dissolution of any substance is accompanied by the absorption or release of heat. That is, dissolution is an exothermic or endothermic reaction.
exothermic process is a process accompanied by heat release into the environment (Q>0).
Endothermic process is a process accompanied by the absorption of heat from the external environment (Q<0).
(example: dissolution of CuSO 4 - exothermic process, NH 4 Cl - endothermic). Explanation: in order for the solvent molecules to tear off the particles of the solute from each other, it is necessary to expend energy (this is the endothermic component of the dissolution process), when the particles of the solute interact with the solvent molecules, energy is released (exothermic process). As a result, the thermal effect of dissolution is determined by the stronger component. ( Example: when dissolving 1 mol of a substance in water, it took 250 kJ to break its molecules, and 450 kJ was released when the resulting ions interacted with solvent molecules. What is the total thermal effect of dissolution? Answer: 450-250=200 kJ, exothermic effect, because exothermic component is greater than endothermic ).
2) Mixing the components of a solution with a certain volume does not give the sum of the volumes ( example: 50 ml of ethyl alcohol + 50 ml of water when mixed gives 95 ml of solution)
Explanation: due to the interaction of the molecules of the solute and the solvent (attraction, chemical bonding, etc.), the volume is “saved”.
Attention! Weight solution is strictly equal to the sum of the masses of the solvent and solutes.
3) When dissolving some colorless substances, colored solutions are formed. ( example: CuSO 4 - colorless, gives a blue solution ).
Explanation: when dissolving some colorless salts, colored crystalline hydrates are formed.
Conclusion: Dissolution is a complex physical and chemical process in which interaction (electrostatic, donor-acceptor, hydrogen bonding) occurs between the particles of the solvent and the solutes.
The process of interaction of a solvent with a solute is called solvation. The products of this interaction are solvates. For aqueous solutions, the terms hydration and hydrates.
Sometimes, when water is evaporated, the crystals of the solute leave a part of the water molecules in their crystal lattice. Such crystals are called crystalline hydrates. They are written as follows: CuSO 4 * 5H 2 O. That is, each molecule of copper sulfate CuSO 4 holds 5 water molecules around itself, embedding them into its crystal lattice.
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…
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9.1. Solutions and their classification
Solutions are homogeneous systems in which one substance is distributed in the environment of another (other) substances.
Solutions consist of a solvent and a solute(s). These concepts are conditional. If one of the components of a solution of substances is a liquid, and the others are gases or solids, then the solvent is usually considered a liquid. In other cases, the solvent is considered to be the component that is larger.
Gaseous, liquid and solid solutions
depending from the state of aggregation solvent distinguish gaseous, liquid and solid solutions. The gaseous solution is, for example, air and other mixtures of gases. Sea water is the most common liquid solution of various salts and gases in water. Many metal alloys belong to solid solutions.
True and colloidal solutions
According to the degree of dispersion distinguish true and colloidal solutions(colloidal systems). In the formation of true solutions, the solute is in the solvent in the form of atoms, molecules, or ions. The particle size in such solutions is 10–7 - 10–8 cm. Colloidal solutions are heterogeneous systems in which particles of one substance (dispersed phase) are evenly distributed in another (dispersion medium). The particle size in dispersed systems ranges from 10–7 cm to 10–3 cm and more. It should be noted that here and below, we will consider true solutions everywhere.
Unsaturated, saturated and supersaturated solutions
The dissolution process is associated with diffusion, i.e., with the spontaneous distribution of particles of one substance between particles of another. Thus, the process of dissolution of solids with an ionic structure in liquids can be represented as follows: under the influence of a solvent, the crystal lattice of a solid is destroyed, and ions are distributed evenly throughout the volume of the solvent. The solution will remain unsaturated as long as some more substance can pass into it.
A solution in which a substance no longer dissolves at a given temperature, i.e. a solution that is in equilibrium with the solid phase of the solute is called rich. The solubility of a given substance is equal to its concentration in a saturated solution. Under strictly defined conditions (temperature, solvent), solubility is a constant value.
If the solubility of a substance increases with increasing temperature, then by cooling a solution saturated at a higher temperature, one can obtain supersaturated solution, i.e. such a solution, the concentration of a substance in which is higher than the concentration of a saturated solution (at a given temperature and pressure). Supersaturated solutions are very unstable. A slight shaking of the vessel or the introduction of crystals of a substance in solution into the solution causes the excess of the solute to crystallize, and the solution becomes saturated.
Diluted and concentrated solutions
Do not confuse unsaturated and saturated solutions with dilute and concentrated ones. The concepts of dilute and concentrated solutions are relative and it is impossible to draw a clear line between them. They determine the ratio between the amounts of solute and solvent. In general, dilute solutions are solutions containing small amounts of solute compared to the amount of solvent, concentrated solutions are those with a large amount of solute.
For example, if at 20 o C dissolve 25 g of NaCl in 100 g of water, then the resulting solution will be concentrated, but unsaturated, since the solubility of sodium chloride at 20 o C is 36 g in 100 g of water. The maximum mass of AgI that dissolves at 20 o C in 100 g of H 2 O is 1.3 10 -7 g. The AgI solution obtained under these conditions will be saturated, but very dilute.
9.2. Physical and chemical theory of solutions; thermal phenomena during dissolution
Physical theory solutions was proposed by W. Ostwald (Germany) and S. Arrhenius (Sweden). According to this theory, the particles of the solvent and the solute (molecules, ions) are evenly distributed throughout the volume of the solution due to diffusion processes. There is no chemical interaction between the solvent and the solute.
chemical theory was proposed by D.I. Mendeleev. According to D.I. Mendeleev, between the molecules of the solute and the solvent, a chemical interaction occurs with the formation of unstable, converting into each other compounds of the solute with the solvent - solvates.
Russian scientists I.A. Kablukov and V.A. Kistyakovsky combined the ideas of Ostwald, Arrhenius and Mendeleev, thus laying the foundation for the modern theory of solutions. According to modern theory, not only particles of a solute and a solvent can exist in a solution, but also products of the physicochemical interaction of a solute with a solvent - solvates. solvates are unstable compounds of variable composition. If the solvent is water, they are called hydrates. Solvates (hydrates) are formed due to ion-dipole, donor-acceptor interactions, formation of hydrogen bonds, etc. For example, when NaCl is dissolved in water, an ion-dipole interaction occurs between Na + , Cl - ions and solvent molecules. The formation of ammonia hydrates when it is dissolved in water occurs due to the formation of hydrogen bonds.
Hydrated water is sometimes so strongly associated with the solute that it is released with it from the solution. Crystalline substances containing water molecules are called crystalline hydrates, and the water that is part of such crystals is called crystallization. Examples of crystalline hydrates are copper sulfate CuSO 4 5H 2 O, potassium alum KAl (SO 4) 2 12H 2 O.
Thermal effects during dissolution
As a result of a change in the structure of substances during their transition from an individual state to a solution, as well as as a result of ongoing interactions, the properties of the system change. This is indicated, in particular, by the thermal effects of dissolution. During dissolution, two processes occur: the destruction of the structure of the solute and the interaction of the molecules of the solute with the molecules of the solvent. The interaction of a solute with a solvent is called solvation. Energy is expended on the destruction of the structure of the dissolved substance, and the interaction of the particles of the dissolved substance with the particles of the solvent (solvation) is an exothermic process (goes with the release of heat). Thus, the dissolution process can be exothermic or endothermic, depending on the ratio of these thermal effects. For example, when dissolving sulfuric acid, a strong heating of the solution is observed, i.e. release of heat, and when potassium nitrate is dissolved, a strong cooling of the solution (endothermic process).
9.3. Solubility and its dependence on the nature of substances
Solubility is the most studied property of solutions. The solubility of substances in various solvents varies widely. In table. 9.1 shows the solubility of some substances in water, and in table. 9.2 - solubility of potassium iodide in various solvents.
Table 9.1
Solubility of certain substances in water at 20 o C
Substance |
Substance |
Solubility, g per 100 g H 2 O |
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Table 9.2
Solubility of potassium iodide in various solvents at 20 o C
Solubility depends on the nature of the solute and solvent, as well as on external conditions (temperature, pressure). In currently used reference tables, it is proposed to subdivide substances into highly soluble, slightly soluble and insoluble substances. This division is not entirely correct, since there are no absolutely insoluble substances. Even silver and gold are soluble in water, but their solubility is extremely low. Therefore, in this tutorial, we will use only two categories of substances: highly soluble and sparingly soluble. Finally, the concepts “easily” and “hardly” soluble are inapplicable for the interpretation of solubility, since these terms characterize the kinetics of the dissolution process, and not its thermodynamics.
Dependence of solubility on the nature of the solute and solvent
At present, there is no theory by which it would be possible not only to calculate, but even to predict solubility. This is due to the absence of a general theory of solutions.
Solubility of solids in liquids depends on the type of bond in their crystal lattices. For example, substances with atomic crystal lattices (carbon, diamond, etc.) are slightly soluble in water. Substances with an ionic crystal lattice, as a rule, are highly soluble in water.
The rule, established from centuries of experience in the study of solubility, says: "like dissolves well in like." Substances with an ionic or polar type of bond dissolve well in polar solvents. For example, salts, acids, alcohols are highly soluble in water. At the same time, non-polar substances, as a rule, dissolve well in non-polar solvents.
Inorganic salts are characterized by different solubility in water.
Thus, most alkali metal and ammonium salts are highly soluble in water. Highly soluble nitrates, nitrites and halides (except for silver, mercury, lead and thallium halides) and sulfates (except for sulfates of alkaline earth metals, silver and lead). Transition metals are characterized by a low solubility of their sulfides, phosphates, carbonates, and some other salts.
The solubility of gases in liquids also depends on their nature. For example, in 100 volumes of water at 20 o C dissolves 2 volumes of hydrogen, 3 volumes of oxygen. Under the same conditions, 700 volumes of ammonia dissolve in 1 volume of H 2 O. Such a high solubility of NH 3 can be explained by its chemical interaction with water.
The influence of temperature on the solubility of gases, solids and liquids
When gases are dissolved in water, heat is released due to the hydration of the dissolved gas molecules. Therefore, in accordance with Le Chatelier's principle, as the temperature rises, the solubility of gases decreases.
Temperature affects the solubility of solids in water in various ways. In most cases, the solubility of solids increases with increasing temperature. For example, the solubility of sodium nitrate NaNO 3 and potassium nitrate KNO 3 increases when heated (the dissolution process proceeds with the absorption of heat). The solubility of NaCl increases slightly with increasing temperature, which is due to the almost zero thermal effect of the dissolution of table salt. The solubility of slaked lime in water decreases with increasing temperature, since the enthalpy of hydration prevails over the value of Δ H of destruction of the crystal lattice of this compound, i.e. the dissolution process of Ca(OH) 2 is exothermic.
In most cases, the mutual solubility of liquids also increases with increasing temperature.
Effect of pressure on the solubility of gases, solids and liquids
The solubility of solid and liquid substances in liquids is practically not affected by pressure, since the change in volume during dissolution is small. When gaseous substances are dissolved in a liquid, the volume of the system decreases, therefore, an increase in pressure leads to an increase in the solubility of gases. In general, the dependence of the solubility of gases on pressure obeys W. Henry's law(England, 1803): the solubility of a gas at constant temperature is directly proportional to its pressure over the liquid.
Henry's law is valid only at low pressures for gases whose solubility is relatively low and provided there is no chemical interaction between the molecules of the dissolved gas and the solvent.
Influence of foreign substances on solubility
In the presence of other substances (salts, acids and alkalis) in water, the solubility of gases decreases. The solubility of gaseous chlorine in a saturated aqueous solution of table salt is 10 times less. than pure water.
The effect of decreasing solubility in the presence of salts is called salting out. The decrease in solubility is due to the hydration of salts, which causes a decrease in the number of free water molecules. Water molecules associated with electrolyte ions are no longer a solvent for other substances.
9.4. Solution concentration
There are various ways to numerically express the composition of solutions: mass fraction of a dissolved substance, molarity, titer, etc.
Mass fraction is the ratio of the mass of the solute m to the mass of the entire solution. For a binary solution consisting of a solute and a solvent:
where ω is the mass fraction of the solute, m is the mass of the solute, and M is the mass of the solvent. The mass fraction is expressed in fractions of a unit or as a percentage. For example, ω = 0.5 or ω = 50%.
It should be remembered that only the mass is an additive function (the mass of the whole is equal to the sum of the masses of the components). The volume of the solution does not obey this rule.
Molar concentration or molarity is the amount of solute in 1 liter of solution:
where C is the molar concentration of solute X, mol/l; n is the amount of the dissolved substance, mol; V is the volume of the solution, l.
Molar concentration is indicated by a number and the letter “M”, for example: 3M KOH. If 1 liter of a solution contains 0.1 mol of a substance, then it is called decimolar, 0.01 mol - centomolar, 0.001 mol - millimolar.
Titer is the number of grams of solute contained in 1 ml of solution, i.e.
where T is the titer of the dissolved substance, g/ml; m is the mass of the dissolved substance, g; V is the volume of the solution, ml.
Mole fraction of solute- a dimensionless quantity equal to the ratio of the amount of solute n to the total amount of solute n and solvent n ":
,
where N is the mole fraction of the solute, n is the amount of the solute, mol; n" is the amount of the solvent substance, mol.
The mole percentage is the corresponding fraction multiplied by 100%.
9.5. Electrolytic dissociation
Substances whose molecules in solutions or melts completely or partially decompose into ions are called electrolytes. Solutions and melts of electrolytes conduct electric current.
Substances whose molecules in solutions or melts do not decompose into ions and do not conduct electric current are called non-electrolytes.
Electrolytes include most inorganic acids, bases and almost all salts, non-electrolytes include many organic compounds, such as alcohols, esters, carbohydrates, etc.
In 1887, the Swedish scientist S. Arrhenius put forward the hypothesis of electrolytic dissociation, according to which, when electrolytes are dissolved in water, they decompose into positively and negatively charged ions.
Dissociation is a reversible process: in parallel with dissociation, the reverse process of ion joining (association) proceeds. Therefore, when writing the equations for the reaction of the dissociation of electrolytes, especially in concentrated solutions, the sign of reversibility is put. For example, the dissociation of potassium chloride in a concentrated solution should be written as:
KS1 K + + С1 – .
Let us consider the mechanism of electrolytic dissociation. Substances with an ionic type of bond dissociate most easily in polar solvents. When they are dissolved, for example, in water, polar H 2 O molecules are attracted by their positive poles to anions, and by their negative poles to cations. As a result, the bond between the ions weakens, and the electrolyte decomposes into hydrated ions, i.e. ions associated with water molecules. Electrolytes formed by molecules with a covalent polar bond (HC1, HBr, H 2 S) dissociate in a similar way.
Thus, hydration (solvation) of ions is the main cause of dissociation. It is now generally accepted that most of the ions in an aqueous solution are hydrated. For example, the hydrogen ion H + forms a hydrate of the composition H3O +, which is called the hydronium ion. In addition to H 3 O +, the solution also contains H 5 O 2 + ions (H 3 O + H 2 O), H 7 O 3 + (H 3 O + 2H 2 O) and H 9 O 4 + (H 3 O + 3H 2 O). When compiling equations for dissociation processes and writing reaction equations in ionic form, to simplify writing, the hydroxonium ion H 2 O + is usually replaced by an unhydrated ion H +. However, it should be remembered that this substitution is conditional, since the proton cannot exist in aqueous solutions, since the reaction proceeds almost instantly:
H + + H 2 O \u003d H 3 O +.
Since the exact number of water molecules associated with hydrated ions has not been established, the symbols for non-hydrated ions are used when writing the equations for the dissociation reaction:
CH3COOH CH3COO - + H + .
9.6. Degree of dissociation; associated and non-associated electrolytes
The quantitative characteristic of the dissociation of the electrolyte into ions in solution is the degree of dissociation. The degree of dissociation α is the ratio of the number of molecules that have decayed into ions N "to the total number of dissolved molecules N:
The degree of dissociation is expressed as a percentage or fractions of a unit. If α = 0, then there is no dissociation, and if α = 1, then the electrolyte completely decomposes into ions. According to modern concepts of the theory of solutions, electrolytes are divided into two groups: associated (weak) and non-associated (strong).
For non-associated (strong) electrolytes in dilute solutions, α = 1 (100%), i.e. in solutions they exist exclusively as hydrated ions.
Associated electrolytes can be roughly divided into three groups:
weak electrolytes exist in solutions mainly in the form of undissociated molecules; the degree of their dissociation is low;
ion associates are formed in solutions as a result of electrostatic interaction of ions; as noted above, association takes place in concentrated solutions of well-dissociating electrolytes; examples of associates are ion pairs(K + Cl -, CaC1 +), ionic tees(K 2 Cl +, KCl 2 -) and ionic quadrupoles(K 2 Cl 2 , KCl 3 2– , K 3 Cl 2 +);
ionic and molecular complexes, (for example, 2+ , 3–) which slightly dissociate in water.
The nature of the dissociation of the electrolyte depends on the nature of the solute and solvent, the concentration of the solution, and the temperature. An illustration of this provision can serve as the behavior of sodium chloride in various solvents, Table. 9.3.
Table 9.3
Properties of sodium chloride in water and in benzene at various concentrations and at a temperature of 25 o C
Strong electrolytes in aqueous solutions include most salts, alkalis, a number of mineral acids (HC1, HBr, HNO 3, H 2 SO 4, HC1O 4, etc.). Almost all organic acids belong to weak electrolytes, some inorganic acids, for example, H 2 S, HCN, H 2 CO 3, HClO and water.
Dissociation of strong and weak electrolytes
The dissociation equations for strong electrolytes in dilute aqueous solutions can be represented as follows:
HCl \u003d H + + Cl -,
Ba (OH) 2 \u003d Ba 2+ + 2OH -,
K 2 Cr 2 O 7 \u003d 2K + + Cr 2 O 7 2–.
Between the right and left parts of the reaction equation for the dissociation of a strong electrolyte, you can also put a sign of reversibility, but then it is indicated that a 1. For example:
NaOH Na + + OH - .
The process of dissociation of associated electrolytes is reversible, therefore, it is necessary to put the sign of reversibility into the equations of their dissociation:
HCN H + + CN – .
NH 3 H 2 O NH 4 + + OH -.
The dissociation of associated polybasic acids occurs in steps:
H 3 PO 4 H + + HPO 4 -,
H 2 PO 4 H + + HPO 4 2–,
HPO 4 2– H + + RO 4 3–,
The dissociation of acid salts formed by weak acids and basic salts formed by strong acids in dilute solutions proceeds as follows. The first stage is characterized by a degree of dissociation close to unity:
NaHCO 3 \u003d Na + + HCO 3 -,
Cu(OH)Cl = Cu(OH) + + Cl - .
The degree of dissociation for the second stage is much less than unity:
HCO 3 H + + CO 3 2–,
Cu(OH) + Cu 2+ + OH -.
Obviously, with an increase in the solution concentration, the degree of dissociation of the associated electrolyte decreases.
9.7. Ion exchange reactions in solutions
According to the theory of electrolytic dissociation, all reactions in aqueous electrolyte solutions occur not between molecules, but between ions. To reflect the essence of such reactions, the so-called ionic equations are used. When compiling ionic equations, one should be guided by the following rules:
Slightly soluble and slightly dissociated substances, as well as gases, are written in molecular form.
Strong electrolytes, almost completely dissociated in aqueous solution, are recorded as ions.
The sum of the electric charges on the right and left sides of the ionic equation must be equal.
Let's consider these positions on concrete examples.
We write two equations for neutralization reactions in molecular form:
KOH + HCl \u003d KCl + H 2 O, (9.1)
2NaOH + H 2 SO 4 = Na 2 SO 4 + 2H 2 O. (9.2)
In ionic form, equations (9.1) and (9.2) have the following form:
K + + OH - + H + + Cl - = K + + Cl - + H 2 O, (9.3)
2Na + + 2OH – + 2H + + SO 4 2– = 2Na + + SO 4 2– + 2H 2 O. (9.4)
Having reduced the same ions in both parts of equations (9.3) and (9.4), we transform them into one reduced ionic equation for the interaction of an alkali with an acid:
H + + OH - \u003d H 2 O.
Thus, the essence of the neutralization reaction is reduced to the interaction of H + and OH - ions, as a result of which water is formed.
Reactions between ions in aqueous electrolyte solutions proceed almost to the end if a precipitate, gas, or a weak electrolyte (for example, H 2 O) is formed in the reaction.
Consider now the reaction between solutions of potassium chloride and sodium nitrate:
KCl + NaNO 3 KNO 3 + NaCl. (9.5)
Since the resulting substances are highly soluble in water and are not removed from the reaction sphere, the reaction is reversible. The ionic reaction equation (9.5) is written as follows:
K + + Cl – + Na + + NO 3 – K + + NO 3 – + Na + + Cl – . (9.6)
From the point of view of the theory of electrolytic dissociation, this reaction does not occur, since all soluble substances in the solution are present exclusively in the form of ions, equation (9.6). But if you mix hot saturated solutions of KCl and NaNO 3, then NaCl will precipitate. This is due to the fact that at a temperature of 30 o C and above, the lowest solubility among the considered salts is observed in sodium chloride. Thus, in practice it should be taken into account that processes that are reversible under certain conditions (in the case of dilute solutions) become irreversible under some other conditions (hot saturated solutions).
A special case of the exchange reaction in solutions is hydrolysis.
9.8. Salt hydrolysis
Experience shows that not only acids and bases, but also solutions of some salts, have an alkaline or acid reaction. A change in the reaction of the environment occurs as a result hydrolysis but a solute. Hydrolysis is the exchange interaction of a solute (for example, salt) with water.
The electrolytic dissociation of salts and water is the cause of hydrolysis. Hydrolysis occurs when the ions formed during the dissociation of the salt are able to exert a strong polarizing effect on water molecules (cations) or form hydrogen bonds with them (anions), which leads to the formation of slightly dissociated electrolytes.
Salt hydrolysis equations are usually written in ionic and molecular forms, while it is necessary to take into account the rules for writing ionic equations for exchange reactions.
Before proceeding to the consideration of the equations of hydrolysis reactions, it should be noted that salts formed by a strong base and a strong acid(for example, NaNO 3, BaCl 2, Na 2 SO 4), when dissolved in water, they do not undergo hydrolysis. Ions of such salts do not form weak electrolytes with H 2 O, and solutions of these salts have a neutral reaction.
Various cases of hydrolysis of salts
1. Salts formed by a strong base and a weak acid, for example, CH 3 COONa, Na 2 CO 3 , Na 2 S, KCN are hydrolyzed by the anion. As an example, consider the hydrolysis of CH 3 COONa, leading to the formation of low-dissociating acetic acid:
CH3COO - + NON CH 3 COOH + OH -,
CH3COOHa + HOH CH 3 COOH + NaOH.
Since an excess of hydroxide ions appears in the solution, the solution becomes alkaline.
The hydrolysis of salts of polybasic acids proceeds stepwise, and in this case acid salts are formed, more precisely, anions of acid salts. For example, the hydrolysis of Na 2 CO 3 can be expressed by the equations:
1 step:
CO 3 2– + HOH HCO 3 – + OH –,
Na 2 CO 3 + HOH NaHCO 3 + NaOH.
2 step
HCO 3 - + HOH H 2 CO 3 + OH -,
NaHCO 3 + HOH H 2 CO 3 + NaOH.
The OH- ions formed as a result of hydrolysis in the first stage largely suppress the second stage of hydrolysis; as a result, hydrolysis in the second stage proceeds to a small extent.
2. Salts formed from a weak base and a strong acid, for example, NH 4 Cl, FeCl 3, Al 2 (SO 4) 3 are hydrolyzed by the cation. An example is the process
NH 4 + + HOH NH 4 OH + H +,
NH 4 Cl + HOH NH 4 OH + HCl.
Hydrolysis is due to the formation of a weak electrolyte - NH 4 OH (NH 3 H 2 O). As a result, the balance of electrolytic dissociation of water is shifted, and an excess of H + ions appears in the solution. Thus, a solution of NH 4 Cl will show an acidic reaction.
During the hydrolysis of salts formed by polyacid bases, basic salts are formed, more precisely, cations of basic salts. Consider, as an example, the hydrolysis of iron (II) chloride:
1 step
Fe 2+ + HOH FeOH + + H + ,
FeCl 2 + HOH FeOHCl + HCl.
2 step
FeOH + + HOH Fe (OH) 2 + H +,
FeOHCl + HOH Fe(OH) 2 + HCl.
Hydrolysis in the second stage proceeds insignificantly in comparison with hydrolysis in the first stage, and the content of hydrolysis products in the solution in the second stage is very small.
3. Salts formed from a weak base and a weak acid, for example, CH 3 COONH 4, (NH 4) 2 CO 3, HCOONH 4, are hydrolyzed both by the cation and by the anion. For example, when CH 3 COONH 4 is dissolved in water, low-dissociating acid and base are formed:
CH 3 COO - + NH 4 + + HOH CH 3 COOH + NH 4 OH,
CH3COONH 4 + HOH CH 3 COOH + NH 4 OH.
In this case, the reaction of the solution depends on the strength of the weak acids and bases formed as a result of hydrolysis. Since in this example CH 3 COOH and NH 4 OH are approximately equal in strength, the salt solution will be neutral.
During the hydrolysis of HCOONH 4, the reaction of the solution will be slightly acidic, since formic acid is stronger than acetic acid.
The hydrolysis of a number of salts formed by very weak bases and weak acids, for example, aluminum sulfide, proceeds irreversibly:
Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 + 3H 2 S.
4. A number of exchange reactions in solutions are accompanied by hydrolysis and proceed irreversibly.
A) When solutions of salts of divalent metals (except calcium, strontium, barium and iron) interact with aqueous solutions of alkali metal carbonates, basic carbonates precipitate as a result of partial hydrolysis:
2MgSO 4 + 2Na 2 CO 3 + H 2 O \u003d Mg 2 (OH) 2 CO 3 + CO 2 + 2Na 2 SO 4,
3 Pb (NO 3) 2 + 3Na 2 CO 3 + H 2 O \u003d Pb 3 (OH) 2 (CO 3) 2 + CO 2 + 6NaNO 3.
B) When aqueous solutions of trivalent aluminum, chromium and iron are mixed with aqueous solutions of carbonates and sulfides of alkali metals, carbonates and sulfides of trivalent metals are not formed - their irreversible hydrolysis proceeds and hydroxides precipitate:
2AlCl 3 + 3K 2 CO 3 + 3H 2 O \u003d 2Al (OH) 3 + 3CO 2 + 6KCl,
2Cr(NO 3) 3 + 3Na 2 S + 6H 2 O = 2Cr(OH) 3 + 3H 2 S + 6NaNO 3 .
Solution is a homogeneous mixture of variable composition, consisting of a solute, a solvent, and products of their interaction.
A solution in which a given substance no longer dissolves at a certain temperature is called rich, and the solution in which this substance can still dissolve, - unsaturated.
Crystal hydrates
If the solvent is water, then the products of the addition of water molecules to the particles of the solute are called hydrates, and the process of their formation is hydration.
Hydrates are very unstable compounds, and when water is evaporated from a solution, they are easily destroyed. However, some hydrates can hold water even in a solid crystalline state.
Such substances are called crystalline hydrates. Most natural minerals are crystalline hydrates. Many substances are obtained in pure form in the form of crystalline hydrates.
chemical theory was proposed by D.I. Mendeleev. According to D.I. Mendeleev, between the molecules of the solute and the solvent, a chemical interaction occurs with the formation of unstable, converting into each other compounds of the solute with the solvent - solvates.
solvates are unstable compounds of variable composition. If the solvent is water, they are called hydrates. Solvates (hydrates) are formed due to ion-dipole, donor-acceptor interactions, formation of hydrogen bonds, etc.
9. Concentration of solutions. Solubility, saturated and unsaturated solutions.
Concentration is the relative amount of a solute in a solution.
Molar concentration ( FROM ) is the ratio of the amount of solute v (in moles) to the volume of the solution V in liters.
The unit of molar concentration is mol/L. Knowing the number of moles of a substance in 1 liter of a solution, it is easy to measure the required number of moles for the reaction using a suitable measuring vessel.
Mass fraction of solute is the ratio of the mass of the solute m 1 to the total mass of the solution m, expressed as a percentage.
Normality solution means the number of gram equivalents of a given substance in one liter of solution or the number of milligram equivalents in one milliliter of solution. Gram - the equivalent of a substance is the number of grams of a substance, numerically equal to its equivalent.
Solubility- the ability of a substance to form homogeneous systems with other substances - solutions in which the substance is in the form of individual atoms, ions, molecules or particles.
Solubility is expressed by the concentration of a solute in its saturated solution, either as a percentage, or in weight or volume units, related to 100 g or 100 cm³ of the solvent.
unsaturated solution- a solution in which the concentration of a solute is less than in a saturated solution, and in which, under given conditions, some more of it can be dissolved.
saturated solution A solution in which the solute has reached its maximum concentration under given conditions and is no longer soluble. The precipitate of a given substance is in equilibrium with the substance in solution.
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).
