The equation of electrolytic dissociation of water. Dissociation of water and pH. Hydrogen indicator - pH

Pure water does not conduct electric current well, but still has measurable electrical conductivity, which is explained by the partial dissociation of H 2 O molecules into hydrogen ions and hydroxide ions:

H 2 O H + + OH -

By the magnitude of the electrical conductivity of pure water, you can calculate the concentration of H + and OH ions in it. At 25 ° C, it is equal to 10 -7 mol / l.

The dissociation constant H 2 O is calculated as follows:

Let's rewrite this equation:

It should be emphasized that this formula contains the equilibrium concentrations of H 2 O molecules, H + and OH - ions, which were established at the moment of equilibrium in the H 2 O dissociation reaction.

But, since the degree of H 2 O dissociation is very small, we can assume that the concentration of undissociated H 2 O molecules at the moment of equilibrium is practically equal to the total initial concentration of water, i.e. 55.56 mol / dm 3 (1 dm 3 H 2 O contains 1000 g H 2 O or 1000: 18 ≈ 55.56 (moles). In dilute aqueous solutions, we can assume that the concentration of H 2 O will be the same. Therefore, replacing in equation (42) the product of two constants with a new constant (or KW ), will have:

The resulting equation shows that for water and dilute aqueous solutions at a constant temperature, the product of the molar concentrations of hydrogen ions and hydroxide ions is a constant value. It's called differently ion product of water .

In pure water at 25°C.
So for the specified temperature:

As the temperature increases, the value increases. At 100 ° C, it reaches 5.5 ∙ 10 -13 (Fig. 34).

Rice. 34. Dependence of the dissociation constant of water K w
from temperature t(°C)

Solutions in which the concentrations of H + and OH ions are the same are called neutral solutions. AT sour solutions contain more hydrogen ions, and alkaline– hydroxide ions. But whatever the reaction of the medium in solution, the product of the molar concentrations of H + and OH ions will remain constant.

If, for example, a certain amount of acid is added to pure H 2 O and the concentration of H + ions increases to 10 -4 mol / dm 3, then the concentration of OH - ions, respectively, will decrease so that the product remains equal to 10 -14. Therefore, in this solution, the concentration of hydroxide ions will be equal to 10 -14: 10 -4 \u003d 10 -10 mol / dm 3. This example shows that if the concentration of hydrogen ions in an aqueous solution is known, then the concentration of hydroxide ions is also determined. Therefore, the reaction of a solution can be quantitatively characterized by the concentration of H + ions:

neutral solution ®

sour solution ®

alkaline solution ®

In practice, to quantitatively characterize the acidity or alkalinity of a solution, it is not the molar concentration of H + ions in it that is used, but its negative decimal logarithm. This value is called pH indicator and is denoted by pH :

pH = –lg

For example, if , then pH = 2; if , then pH = 10. In a neutral solution, pH = 7. In acidic solutions, pH< 7 (и тем меньше, чем «кислее» раствор, т.е., чем больше в нём концентрация ионов Н +). В щёлочных растворах рН >7 (and the more, the more “alkaline” the solution, i.e., the lower the concentration of H + ions in it).

There are various methods for measuring the pH of a solution. It is very convenient to approximately evaluate the reaction of a solution using special reagents called acid-base indicators . The color of these substances in solution changes depending on the concentration of H + ions in it. The characteristics of some of the most common indicators are presented in Table 12.

Table 12 The most important acid-base indicators

Water- weak amphoteric electrolyte.

The water ionization equation, taking into account the hydration of hydrogen ions H +, is:

Without taking into account the hydration of H + ions, the water dissociation equation has the form:

As can be seen from the second equation, the concentrations of hydrogen ions H + and hydroxide ions OH - in water are the same. At 25 o C [H +] \u003d [OH -] \u003d 10 -7 mol / l.

The product of the concentrations of hydrogen ions and hydroxide ions is called ion product of water(K H 2 O).

K H 2 O = ∙

K H 2 O is a constant value, and at a temperature of 25 ° C

K H 2 O \u003d 10 -7 ∙10 -7 \u003d 10 -14

In dilute aqueous solutions of electrolytes, as in water, the product of the concentrations of hydrogen ions H + and hydroxide ions OH - - is a constant value at a given temperature. The ionic product of water makes it possible for any aqueous solution to calculate the concentration of hydroxide ions OH - if the concentration of hydrogen ions H + is known, and vice versa.

The environment of any aqueous solution can be characterized by the concentration of hydrogen ions H + or hydroxide ions OH - .

There are three types of media in aqueous solutions: neutral, alkaline, and acidic.

Neutral environment- this is an environment in which the concentration of hydrogen ions is equal to the concentration of hydroxide ions:

[H + ] = = 10 -7 mol/l

acid environment is an environment in which the concentration of hydrogen ions is greater than the concentration of hydroxide ions:

[H +] > [OH -], > 10 -7 mol / l

Alkaline environment- this is an environment in which the concentration of hydrogen ions is less than the concentration of hydroxide ions:

< , < 10 -7 моль/л

To characterize solution media, it is convenient to use the so-called pH value (pH).

pH pH is called the negative decimal logarithm of the concentration of hydrogen ions: pH = -lg.

For example, if \u003d 10 -3 mol / l, then pH \u003d 3, the solution medium is acidic; if [H + ] = 10 -12 mol / l, then pH = 12, the solution medium is alkaline:

The lower the pH is 7, the more acidic the solution. The higher the pH is 7, the higher the alkalinity of the solution.

The relationship between the concentration of H + ions, the pH value and the medium of the solution is shown in the following diagram:

There are various methods for measuring pH. Qualitatively, the nature of the environment of aqueous solutions of electrolytes is determined using indicators.

indicators substances are called that reversibly change their color depending on the medium of the solutions, i.e., the pH of the solution.

In practice, indicators are used litmus, methyl orange (methyl orange) and phenolphthalein. They change their color in a small pH range: litmus - in the pH range from 5.0 to 8.0; methyl orange - from 3.1 to 4.4; and phenolphthalein - from 8.2 to 10.0.

The change in the color of the indicators is shown in the diagram:

The shaded areas show the interval for changing the color of the indicator.

In addition to the above indicators, a universal indicator is also used, which can be used to approximately determine pH in a wide range from 0 to 14.

The pH value has great importance in chemical and biological processes, since, depending on the nature of the medium, these processes can proceed at different speeds and in different directions.

Therefore, the determination of the pH of solutions is very important in medicine, science, technology, agriculture. A change in the pH of the blood or gastric juice is a diagnostic test in medicine. Deviations of pH from normal values ​​even by 0.01 units indicate pathological processes in the body. The constancy of the concentrations of hydrogen ions H + is one of the important constants of the internal environment of living organisms.

So, with normal acidity, gastric juice has a pH of 1.7; the pH of human blood is 7.4; saliva - 6.9. Each enzyme functions at a certain pH value: blood catalase at pH 7 gastric pepsin at pH 1.5-2; etc.

The textbook is intended for students of non-chemical specialties of higher educational institutions. It can serve as a manual for people who independently study the basics of chemistry, and for students of chemical technical schools and senior secondary schools.

The legendary textbook, translated into many languages ​​​​of Europe, Asia, Africa and released with a total circulation of over 5 million copies.

When making the file, the site http://alnam.ru/book_chem.php was used

Book:

<<< Назад

Forward >>>

Pure water conducts electricity very poorly, but still has a measurable electrical conductivity, which is explained by the small dissociation of water into hydrogen ions and hydroxide ions:

The electrical conductivity of pure water can be used to calculate the concentration of hydrogen ions and hydroxide ions in water. At 25°C it is equal to 10 -7 mol/l.

Let's write an expression for the dissociation constant of water:

Let's rewrite this equation as follows:

Since the degree of dissociation of water is very small, the concentration of undissociated H 2 O molecules in water is practically equal to the total concentration of water, i.e. 55.55 mol / l (1 liter contains 1000 g of water, i.e. 1000: 18.02 = 55.55 mol). In dilute aqueous solutions, the concentration of water can be considered the same. Therefore, replacing the product in the last equation with a new constant K H 2 O, we will have:

The resulting equation shows that for water and dilute aqueous solutions at a constant temperature, the product of a concentrate of hydrogen ions and hydroxide ions is a constant value. This constant value is called the ionic product of water. Numerical value it is easy to obtain by substituting the concentrations of hydrogen ions and hydroxide ions into the last equation. In pure water at 25°C ==1·10 -7 mol/l. So for the specified temperature:

Solutions in which the concentrations of hydrogen ions and hydroxide ions are the same are called neutral solutions. At 25°C, as already mentioned, in neutral solutions, the concentration of both hydrogen ions and hydroxide ions is 10 -7 mol/l. In acidic solutions, the concentration of hydrogen ions is higher, in alkaline solutions, the concentration of hydroxide ions. But whatever the reaction of the solution, the product of the concentrations of hydrogen ions and hydroxide ions remains constant.

If, for example, enough acid is added to pure water so that the concentration of hydrogen ions rises to 10 -3 mol / l, then the concentration of hydroxide ions will decrease so that the product remains equal to 10 -14. Therefore, in this solution, the concentration of hydroxide ions will be:

10 -14 /10 -3 \u003d 10 -11 mol / l

On the contrary, if you add alkali to water and thus increase the concentration of hydroxide ions, for example, to 10 -5 mol / l, then the concentration of hydrogen ions will be:

10 -14 /10 -5 \u003d 10 -9 mol / l

These examples show that if the concentration of hydrogen ions in an aqueous solution is known, then the concentration of hydroxide ions is also determined. Therefore, both the degree of acidity and the degree of alkalinity of a solution can be quantitatively characterized by the concentration of hydrogen ions:

The acidity or alkalinity of a solution can be expressed in another, more convenient way: instead of the concentration of hydrogen ions, its decimal logarithm is indicated, taken with the opposite sign. The latter value is called the pH value and is denoted by pH:

For example, if =10 -5 mol/l, then pH=5; if \u003d 10 -9 mol / l, then pH \u003d 9, etc. From this it is clear that in a neutral solution (= 10 -7 mol / l) pH \u003d 7. In acidic pH solutions<7 и тем меньше, чем кислее раствор. Наоборот, в щелочных растворах pH>7 and the more, the greater the alkalinity of the solution.

There are various methods for measuring pH. Approximately, the reaction of a solution can be determined using special reagents called indicators, the color of which changes depending on the concentration of hydrogen ions. The most common indicators are methyl orange, methyl red, phenolphthalein. In table. 17 the characteristic of some indicators is given.

For many processes, the pH value plays important role. So, the pH of the blood of humans and animals has a strictly constant value. Plants can grow normally only when the pH values ​​of the soil solution lie within a certain range characteristic of a given plant species. The properties of natural waters, in particular their corrosivity, are highly dependent on their pH.

Table 17. Key indicators

<<< Назад

Forward >>>

Chemically pure water has, although negligible, but measurable electrical conductivity, since water dissociates into ions to a small extent. So at room temperature, only about one in 10 8 water molecules is in a dissociated form. Process electrolytic dissociation water is possible due to the sufficiently high polarity O-H connections and the presence of a system of hydrogen bonds between water molecules. The water dissociation equation is written as follows:

2H 2 O ↔ H 3 O + + OH -,

where H 3 O + is the hydrogen hydronium cation.

The water dissociation equation can be written in a simpler form:

H 2 O ↔ H + + OH -.

The presence of hydrogen and hydroxide ions in water gives it the specific properties of ampholyte, i.e. ability to perform the functions of a weak acid and a weak base. The dissociation constant of water at a temperature of 22 0 С:

where and are the equilibrium concentrations in g-ion/l, respectively, of hydrogen cations and hydroxo-anions, and is the equilibrium concentration of water in mol/l. Given that the degree of dissociation of water is extremely small, the equilibrium concentration of undissociated water molecules can be equated to the total amount of water in 1 liter of it:

Now expression (1) can be written in the following form:

hence = (1.8 10 -16) 55.56 = 10 -14 g-ion 2 / l 2.

The product of the concentrations of hydrogen ions and hydroxo ions is a constant not only for water, but also for aqueous solutions of salts, acids and alkalis. This value is called ion product of water or water constant. Therefore: K H2O \u003d \u003d 10 -14 g-ion 2 / l 2.

For neutral media = = 10 -7 g-ion/l. In acidic environments > , and in alkaline< . При этом в любых средах произведение концентраций ионов водорода и гидроксо-ионов при данной температуре остается постоянным и равным 10 -14 г-ион 2 /л 2 . Таким образом, пользуясь ионным произведением воды, любую реакцию среды (нейтральную, кислую или щелочную) можно количественно выразить при помощи концентрации водородных ионов.

1.2. Hydrogen indicator - pH

To quantitatively characterize the reaction of a medium, it is usually not the concentration of hydrogen ions that is given, but some conditional indicator, denoted by pH and called hydrogen index. It is the negative decimal logarithm of the concentration of hydrogen cations pH \u003d - lg.

For a neutral environment pH = -lg 10 -7 = 7;

for acid - pH< 7;

for alkaline - pH > 7.

The notion hydroxyl index pOH = - lg [OH -].

pH + pOH = 14.

The determination of pH is of great importance in engineering and, in particular, in the construction business. Typically, pH is measured using indicators- substances that can change their color depending on the concentration of hydrogen ions. Indicators are weak acids and bases, the molecules and ions of which are colored in different colors (Table 1).

Table 1

However, the indicators do not precise definition pH values, therefore modern pH measurements are made using electrochemical methods, the accuracy of which is ± 0.01 pH units.

An important feature of liquid water is its ability to spontaneously dissociate according to the reaction:

H 2 O (l) "H + (aq) + OH - (aq)

This process is also called self-ionization or autoprotolysis. The resulting H + protons and OH - anions are surrounded by a certain number of polar water molecules, i.e. hydrated: H + ×nH 2 O; OH - ×mH 2 O. Primary hydration can be represented by a number of aqua complexes: H 3 O + ; H 5 O 2 +; H 7 O 3 +; H 9 O 4 + , among which ions H 9 O 4 + (H + ×4H 2 O) predominate. The lifetime of all these ions in water is very short, because protons are constantly migrating away from the same molecules

water to others. Usually, for simplicity, only the cation of the composition H 3 O + (H + ×H 2 O), called the hydronium ion, is used in the equations.

The process of water dissociation, taking into account the hydration of the proton and the formation of the hydroxonium ion, can be written: 2H 2 O « H 3 O + + OH -

Water is a weak electrolyte, the degree of dissociation of which is

Since à C is equal to (H 2 O) "C ref (H 2 O) or [H 2 O] is equal to ≈ [H 2 O] ref

is the number of moles in one liter of water. C ref (H 2 O) in a dilute solution remains constant. This circumstance allows us to include C equals (H 2 O) in the equilibrium constant.

Thus, the product of two constants gives a new constant, which is called ion product of water. At a temperature of 298 K.

¾- The constancy of the ionic product of water means that in any aqueous solution: acidic, neutral or alkaline - there are always both types of ions (H + and OH -)

¾- In pure water, the concentrations of hydrogen and hydroxide ions are equal and under normal conditions are:

K w 1/2 \u003d 10 -7 mol / l.

¾- When acids are added, the concentration of [H + ] increases, i.e. the equilibrium shifts to the left, and the concentration of [OH - ] decreases, but K w remains equal to 10 -14.

In an acidic environment > 10 -7 mol/l, and< 10 -7 моль/л

In an alkaline environment< 10 -7 моль/л, а >10 -7 mol/l

In practice, for convenience, we use pH value (pH) and the hydroxyl index (pOH) of the medium.

This is the decimal logarithm of the concentrations (activities), respectively, of hydrogen ions or hydroxide ions in solution taken with the opposite sign: pH = - lg, pOH = - lg

In aqueous solutions, pH + pOH = 14.

Table number 14.

K w depends on temperature (since water dissociation is an endothermic process)

K w (25 o C) \u003d 10 -14 Þ pH \u003d 7

K w (50 o C) \u003d 5.47 × 10 -14 Þ pH \u003d 6.63

pH measurement is used extremely widely. In biology and medicine, the pH value of biological fluids is used to determine pathologies. For example, normal serum pH is 7.4±0.05; saliva - 6.35..6.85; gastric juice - 0.9..1.1; tears - 7.4±0.1. In agriculture, pH characterizes the acidity of soils, the ecological state of natural waters, etc.

Acid-base indicators are chemical compounds that change color depending on the pH of the environment in which they are located. You have probably paid attention to how the color of tea changes when you put lemon in it - this is an example of the action of an acid-base indicator.

Indicators are usually weak organic acids or bases and can exist in solution in two tautomeric forms:

HInd « H + + Ind - , where HInd is the acid form (this is the form that predominates in acidic solutions); Ind is the main form (predominant in alkaline solutions).

The behavior of the indicator is similar to the behavior of a weak electrolyte in the presence of a stronger one with the same ion. The more consequently the equilibrium shifts towards the existence of the acid form HInd and vice versa (Le Chatelier's principle).

Experience clearly shows the possibility of using some indicators:

Table No. 15

Special devices - pH meters allow you to measure pH with an accuracy of 0.01 in the range from 0 to 14. The definition is based on measuring the EMF of a galvanic cell, one of the electrodes of which is, for example, glass.

The most accurate concentration of hydrogen ions can be determined by acid-base titration. Titration is the process of gradually adding small portions of a solution of a known concentration (titrant) to the solution to be titrated, the concentration of which we want to determine.

buffer solutions- These are systems whose pH changes relatively little when diluted or added to them with small amounts of acids or alkalis. Most often they are solutions containing:

a) a) Weak acid and its salt (CH 3 COOH + CH 3 COOHa) - acetate buffer

c) Weak base and its salt (NH 4 OH + NH 4 Cl) - ammonium-ammonium buffer

c) Two acid salts with different K d (Na 2 HPO 4 + NaH 2 PO 4) - phosphate buffer

Let us consider the regulatory mechanism of buffer solutions using an acetate buffer solution as an example.

CH 3 COOH «CH 3 COO - + H +,

CH 3 COONa « CH 3 COO - + Na +

1. 1) if you add a small amount of alkali to the buffer mixture:

CH 3 COOH + NaOH " CH 3 COONa + H 2 O,

NaOH is neutralized with acetic acid to form a weaker electrolyte H 2 O. An excess of sodium acetate shifts the equilibrium towards the resulting acid.

2. 2) if you add a small amount of acid:

CH 3 COONa + HCl « CH 3 COOH + NaCl

Hydrogen cations H + bind ions CH3COO -

Let's find the concentration of hydrogen ions in the buffer acetate solution:

The equilibrium concentration of acetic acid wound C ref, to (since weak electrolyte), and [СH 3 COO - ] = C salt (since salt is a strong electrolyte), then . Henderson-Hasselbach equation:

Thus, the pH of buffer systems is determined by the ratio of salt and acid concentrations. When diluted, this ratio does not change and the pH of the buffer does not change when diluted; this distinguishes buffer systems from a pure electrolyte solution, for which the Ostwald dilution law is valid.

There are two characteristics of buffer systems:

1.buffer force. Absolute value buffer force depends on

total concentration of buffer system components, i.e. the greater the concentration of the buffer system, the more alkali (acid) is required for the same change in pH.

2.Buffer tank (B). The buffer capacity is the limit at which the buffering action occurs. The buffer mixture keeps the pH constant only if the amount of strong acid or base added to the solution does not exceed a certain limit value - B. The buffer capacity is determined by the number of g / eq of a strong acid (base) that must be added to one liter of the buffer mixture in order to change pH value per unit, i.e. . Conclusion: Properties of buffer systems:

1. 1. little dependent on dilution.

2. 2. The addition of strong acids (bases) makes little difference within the buffer capacity of B.

3. 3. The buffer capacity depends on the buffer strength (on the concentration of the components).

4. 4. The buffer exhibits the maximum effect when the acid and salt are present in the solution in equivalent quantities:

With salt \u003d C to-you; = K d, k; pH \u003d pK d, k (pH is determined by the value of K d).

Hydrolysis is the chemical interaction of water with salts.. The hydrolysis of salts is reduced to the process of proton transfer. As a result of its flow, a certain excess of hydrogen or hydroxyl ions appears, imparting acidic or alkaline properties to the solution. Thus, hydrolysis is the reverse of the neutralization process.

Salt hydrolysis includes 2 stages:

a) Electrolytic dissociation of salt with the formation of hydrated ions:. KCl à K + + Cl - K + + xH 2 O à K + × xH 2 O

acceptor - cations with vacant orbitals)

Cl - + yH 2 O "Cl - × yH 2 O (hydrogen bond)

c) Anion hydrolysis. Cl - + HOH à HCl + OH -

c) Hydrolysis at the cation. K + + HOH à KOH +

All salts formed with the participation of weak

electrolytes:

1. Salt formed by an anion of weak acids and a cation of strong bases

CH 3 COONa + HOH «CH 3 COOH + NaOH

CH 3 COO - + HOH "CH 3 COOH + OH - , pH> 7

Anions of weak acids perform the function of bases in relation to water - a proton donor, which leads to an increase in the concentration of OH - , i.e. alkalization of the environment.

The depth of hydrolysis is determined by: the degree of hydrolysis a g:

is the concentration of hydrolyzed salt

is the concentration of the initial salt

a g is small, for example, for a 0.1 mol solution of CH 3 COONa at 298 K, it is 10 -4.

During hydrolysis, an equilibrium is established in the system, characterized by К р

Therefore, the smaller the dissociation constant, the larger the hydrolysis constant. The degree of hydrolysis with the hydrolysis constant is related by the equation:

With increasing dilution, i.e. decrease in C 0 , the degree of hydrolysis increases.

2. 2. Salt formed by the cation of weak bases and the anion of strong acids

NH 4 Cl + HOH ↔ NH 4 OH +

NH 4 + + HOH ↔ NH 4 OH + H + , pH< 7

The protolytic equilibrium is shifted to the left, the weak base cation NH 4 + performs the function of an acid with respect to water, which leads to acidification of the medium. The hydrolysis constant is determined by the equation:

The equilibrium concentration of hydrogen ions can be calculated: [H + ] equals = a g × C 0 (initial salt concentration), where

The acidity of the environment depends on the initial concentration of salts of this type.

3. 3. Salt formed by an anion of weak acids and a cation of weak bases. Hydrolyzes both cation and anion

NH 4 CN + HOH à NH 4 OH + HCN

To determine the pH of the solution medium, compare K D, k and K D, basic

K D,k > K D,basic medium slightly acidic

K D, k< К Д,осн à среда слабо щелочная

K D,k \u003d K D,base à neutral medium

Consequently, the degree of hydrolysis of this type of salt does not depend on their concentration in solution.

because and [OH - ] are determined by K D, k and K D, base, then

The pH of the solution is also independent of the salt concentrations in the solution.

Salts formed by a multiply charged anion and a singly charged cation (ammonium sulfides, carbonates, phosphates) are almost completely hydrolyzed by the first stage, i.e. are in solution in the form of a mixture of a weak base NH 4 OH and its salt NH 4 HS, i.e. in the form of ammonium buffer.

For salts formed by a multiply charged cation and a singly charged anion (acetates, Al, Mg, Fe, Cu formates), hydrolysis is enhanced upon heating and leads to the formation of basic salts.

Hydrolysis of nitrates, hypochlorites, hypobromites Al, Mg, Fe, Cu proceeds completely and irreversibly, i.e. salts are not isolated from solutions.

Salts: ZnS, AlPO 4 , FeCO 3 and others are sparingly soluble in water, however, some of their ions take part in the hydrolysis process, this leads to some increase in their solubility.

Chromium and aluminum sulfides hydrolyze completely and irreversibly with the formation of the corresponding hydroxides.

4. 4. Salts formed by the anion of strong acids and strong bases do not undergo hydrolysis.

Most often, hydrolysis is a harmful phenomenon that causes various complications. So in synthesis inorganic substances from aqueous solutions in the resulting substance, impurities appear - products of its hydrolysis. Some compounds cannot be synthesized at all due to irreversible hydrolysis.

- if hydrolysis proceeds along the anion, then an excess of alkali is added to the solution

- if hydrolysis proceeds through the cation, then an excess of acid is added to the solution

So, the first qualitative theory of electrolyte solutions was expressed by Arrhenius (1883 - 1887). According to this theory:

1. 1. Electrolyte molecules dissociate into opposite ions

2. 2. Between the processes of dissociation and recombination, a dynamic equilibrium is established, which is characterized by K D. This equilibrium obeys the law of mass action. The fraction of disintegrated molecules is characterized by the degree of dissociation a. Ostwald's law connects to D and a.

3. 3. An electrolyte solution (according to Arrhenius) is a mixture of electrolyte molecules, its ions and solvent molecules, between which there is no interaction.

Conclusion: the Arrhenius theory made it possible to explain many properties of solutions of weak electrolytes at low concentrations.

However, the Arrhenius theory was only of a physical nature, i.e. did not consider the following questions:

Why do substances break up into ions in solution?

What happens to ions in solutions?

Further development the theory of Arrhenius received in the works of Ostwald, Pisarzhevsky, Kablukov, Nernst, etc. For example, the importance of hydration was first pointed out by Kablukov (1891), initiating the development of the theory of electrolytes in the direction indicated by Mendeleev (i.e., he was the first to succeed in combining Mendeleev's solvate theory with the physical theory of Arrhenius). Solvation is the process of electrolyte interaction

solvent molecules to form complex compounds of solvates. If the solvent is water, then the process of interaction of the electrolyte with water molecules is called hydration, and aqua complexes are called crystalline hydrates.

Consider an example of the dissociation of electrolytes in the crystalline state. This process can be presented in two stages:

1. 1.destruction of the crystal lattice of a substance DH 0 kr\u003e 0, the process of formation of molecules (endothermic)

2. 2. formation of solvated molecules, DH 0 solv< 0, процесс экзотермический

The resulting heat of dissolution is equal to the sum of the heats of the two stages DH 0 sol = DH 0 cr + DH 0 solv and can be both negative and positive. For example, the energy of the crystal lattice KCl = 170 kcal/mol.

The heat of hydration of ions K + = 81 kcal/mol, Cl - = 84 kcal/mol, and the resulting energy is 165 kcal/mol.

The heat of hydration partially covers the energy required for the release of ions from the crystal. The remaining 170 - 165 = 5 kcal / mol can be covered due to the energy of thermal motion, and the dissolution is accompanied by the absorption of heat from environment. Hydrates or solvates facilitate the endothermic dissociation process, making recombination more difficult.

And here is a situation where only one of the two named stages is present:

1. dissolution of gases - there is no first stage of destruction of the crystal lattice, exothermic solvation remains, therefore, the dissolution of gases, as a rule, is exothermic.

2. when dissolving crystalline hydrates, there is no solvation stage, only endothermic destruction of the crystal lattice remains. For example, a crystalline hydrate solution: CuSO 4 × 5H 2 O (t) à CuSO 4 × 5H 2 O (p)

DH solution = DH cr = + 11.7 kJ/mol

Anhydrous salt solution: CuSO 4 (t) à CuSO 4 (p) à CuSO 4 × 5H 2 O (p)

DH solution = DH solv + DH cr = - 78.2 + 11.7 = - 66.5 kJ/mol