Formula for water dissociation. Dissociation of water. Ionic product of water pH and pH of solutions. Hydrogen indicator - pH

Pure water, although poorly (compared to electrolyte solutions), can conduct electric current. This is caused by the ability of a water molecule to disintegrate (dissociate) into two ions, which are conductors of electric current in pure water (below, dissociation means electrolytic dissociation - disintegration into ions):

H 2 O ↔ H + + OH -

For approximately 556,000,000 non-dissociated water molecules, only 1 molecule dissociates, but this is 60,000,000,000 dissociated molecules in 1mm3. Dissociation is reversible, that is, the H + and OH - ions can form a water molecule again. Eventually it comes dynamic equilibrium in which the number of decayed molecules is equal to the number of H + and OH - ions formed. In other words, the speeds of both processes will be equal. For our case, the equation for the rate of a chemical reaction can be written as follows:

υ 1 = κ 1 (for water dissociation)

υ 2 = κ 2 (for the reverse process)

Where υ - speed reaction; κ - reaction rate constant (depending on the nature of the reactants and temperature); , And - concentration (mol/l).

In a state of balance υ 1 = υ 2, hence:

κ 1 = κ 2

Let's do some simple math and get:

κ 1 /κ 2 = /

κ 1 /κ 2 = K

K- equilibrium constant, and in our case, dissociation constant, which depends on the temperature and nature of the substances, and does not depend on concentrations (as well as κ 1 and κ 2). K for water 1.8 10 -16 at 25 °C (reference value).

Due to the very small number of dissociated molecules, the concentration can be taken to be equal to the total concentration of water, and the total concentration of water in dilute solutions as a constant value: =1000(g/l)/18(g/mol)=55.6 mol/l.

Replacing κ 1 / κ 2 on K and using the magnitude , we determine what the product of concentrations is equal to And which is called - ionic product of water:

K = /55.6 mol/l
1.8 10 -16 55.6 mol/l =
10 -14 =

Since, at a certain temperature, the quantities used in calculating the ionic product of water ( K, ) are constant, the value of the ionic product of water just the same all the time. And since the dissociation of a water molecule produces the same number of ions And , it turns out that for pure water the concentration And will be equal 10 -7 mol/l. From the constancy of the ionic product of water, it follows that if the number of H + ions becomes larger, then the number of HO - ions becomes smaller. For example, if a strong acid HCl is added to pure water, it, as a strong electrolyte, will completely dissociate into H + and Cl -, as a result, the concentration of H + ions will increase sharply, and this will lead to an increase in the rate of the process opposite to dissociation, since it depends on the concentration of ions H+ and OH-:

υ 2 = κ 2

During the accelerated process opposite to dissociation, the concentration of HO - ions will decrease to a value corresponding to the new equilibrium, at which there will be so few of them that the rates of dissociation of water and the reverse process will again be equal. If the concentration of the resulting HCl solution is 0.1 mol/l, the equilibrium concentration will be equal to:

= 10 -14 /0.1 = 10 -13 mol/l

When adding the strong base NaOH, the shift will be towards a decrease in the H + concentration.

For approximately 556,000,000 non-dissociated water molecules, only 1 molecule dissociates, but this is 60,000,000,000 dissociated molecules in 1mm3. Dissociation is reversible, that is, the H + and OH - ions can form a water molecule again. As a result, dynamic equilibrium occurs in which the number of decayed molecules is equal to the number of H + and OH - ions formed. In other words, the speeds of both processes will be equal. For our case, the equation for the rate of a chemical reaction can be written as follows:

υ 1 = κ 1 (for water dissociation)

υ 2 = κ 2 (for the reverse process)

where υ is the reaction rate; κ is the reaction rate constant (depending on the nature of the reactants and temperature); , and - concentrations (mol/l).

In a state of equilibrium υ 1 = υ 2, therefore: κ 1 = κ 2

Since, at a certain temperature, the quantities used in calculating the ionic product of water (K, ) are constant, the value of the ionic product of water is also constant. And since the dissociation of a water molecule produces the same number of ions and , it turns out that for pure water the concentrations and will be equal to 10 -7 mol/l. From the constancy of the ionic product of water, it follows that if the number of H + ions becomes larger, then the number of HO - ions becomes smaller. For example, if a strong acid HCl is added to pure water, it, as a strong electrolyte, will completely dissociate into H + and Cl -, as a result, the concentration of H + ions will increase sharply, and this will lead to an increase in the rate of the process opposite to dissociation, since it depends on the concentration of ions H + and OH -: υ 2 = κ 2

Ionic product of wateŕ is the product of the concentrations of hydrogen ions H + and hydroxyl ions OH − in water or in aqueous solutions, the autoprotolysis constant of water.

Water, although a weak electrolyte, dissociates to a small extent:

The equilibrium of this reaction is strongly shifted to the left. The dissociation constant of water can be calculated using the formula:

· - concentration of hydronium ions (protons);

· - concentration of hydroxide ions;

· - concentration of water (in molecular form) in water;

The concentration of water in water, taking into account its low degree of dissociation, is practically constant and amounts to (1000 g/l)/(18 g/mol) = 55.56 mol/l.

At 25 °C, the dissociation constant of water is 1.8 10 −16 mol/l. Equation (1) can be rewritten as:

The constant K in, equal to the product of the concentrations of protons and hydroxide ions, is called the ionic product of water. It is constant not only for pure water, but also for dilute aqueous solutions of substances. With increasing temperature, the dissociation of water increases, therefore, Kv also increases, with decreasing temperature - vice versa. The practical significance of the ionic product of water is great, since it allows, with a known acidity (alkalinity) of any solution (that is, at a known concentration or ), to find the corresponding concentration or . Although in most cases, for convenience of presentation, they do not use absolute values of concentrations, but their decimal logarithms taken with the opposite sign - respectively, the hydrogen index (pH) and the hydroxyl index (pOH).

Since Kb is a constant, when acid (H + ions) is added to the solution, the concentration of hydroxide ions OH − will fall and vice versa. In a neutral environment = = mol/l. At a concentration > 10 −7 mol/l (respectively, the concentration< 10 −7 моль/л) среда будет sour; At a concentration > 10 −7 mol/l (respectively, the concentration< 10 −7 моль/л) - alkaline.

27. Buffer solutions: their composition, properties, mechanism of action. Buffer capacity

Buffer solutions- these are solutions containing buffer systems. Buffer systems are mixtures that contain weak acids and their salts with strong bases or weak bases and their salts with strong acids in a certain quantitative ratio. Such solutions have a stable concentration of H+ ions when diluted with a neutral solvent (water) and a certain amount of strong acids or bases is added to them.

Buffer solutions are found in the waters of the world's oceans, soil solutions and living organisms. These systems perform the functions of regulators that maintain an active reaction of the environment at a certain value necessary for the successful occurrence of metabolic reactions. Buffer solutions are classified into acidic and basic. An example of the former would be an acetate buffer system, and an example of the latter would be an ammonium buffer system. There are natural and artificial buffer solutions. A natural buffer solution is blood, which contains bicarbonate, phosphate, protein, hemoglobin and acid buffer systems. An artificial buffer solution can be an acetate buffer consisting of CH3COOH.

Let us consider the features of the internal composition and mechanism of action of buffer systems using the example of an acetate buffer system: acetate acid/sodium acetate. In an aqueous environment, the components of the buffer system undergo electrolytic dissociation. Sodium acetate, as a salt of a weak acid and a strong base, completely dissociates into ions. The presence of anions in such a buffer mixture depends on the concentration of salt in it and the degree of its dissociation. The concentration of H+ ions in the buffer system is directly proportional to the concentration of the acid in it and inversely proportional to the content of the salt of this acid in it.

Thus, the concentration of H+ ions in the main buffer is directly proportional to the concentration of salt in it and inversely proportional to the concentration of base.

For example, it is necessary to prepare an acetate buffer with several pH values. First, prepare 5M solutions of acetate acid and sodium acetate. To prepare the first solution, take 50 ml of each component. Guided by the formula, determine the concentration of H+ ions in the resulting solution.

For the next buffer solution, take 80 ml of acid solution and 20 ml of salt solution prepared earlier. There are a number of recipes for various buffer solutions used in chemical analysis and laboratory practice.

Buffer solutions are characterized by certain properties. These, first of all, include buffering - the ability to maintain a constant concentration of H+ ions when a certain amount of a strong acid or strong base is added to a buffer solution. For example, if a small amount of chloride acid is added to the acetate buffer, the pH will not shift to the acidic side, since the chloride acid will undergo an exchange decomposition reaction with the salt of the weak acid. As a result of the reaction, a strong acid that can shift the pH to the acidic side is replaced by a weak acid and a neutral salt. The degree of dissociation of a weak electrolyte solution decreases as its concentration increases, tends to zero, and a pH shift does not occur.

Buffer solution capacity(from English buffer- shock absorber, English buff- soften shocks) - the amount of acid or base required to change the pH of the buffer solution by exactly 1.

Buffer mixture, buffer solution, buffer system- a combination of substances, a system that maintains a constant pH.

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

Based on the electrical conductivity of pure water, the concentration of hydrogen and hydroxide ions in water can be calculated. At 25°C it is 10 -7 mol/l.

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

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 almost 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 the concentrate of hydrogen ions and hydroxide ions is a constant value. This constant value is called the ionic product of water. Its numerical value can be easily obtained by substituting the concentrations of hydrogen and hydroxide ions into the last equation. In pure water at 25°C ==1·10 -7 mol/l. Therefore, 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 is higher. 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 increases 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 =10 -11 mol/l

On the contrary, if you add alkali to water and thereby 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 =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, indicate its decimal logarithm, taken with the opposite sign. The last value is called the hydrogen index and is denoted by pH:

For example, if =10 -5 mol/l, then pH=5; if = 10 -9 mol/l, then pH = 9, etc. From here it is clear that in a neutral solution (= 10 -7 mol/l) pH = 7. In acidic solutions pH<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. The approximate 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, and phenolphthalein. In table 17 provides characteristics of some indicators.

For many processes, pH plays an important role. Thus, the pH of human and animal blood has a strictly constant value. Plants can grow normally only at pH values of the soil solution that lie within a certain range characteristic of a given plant type. The properties of natural waters, in particular their corrosiveness, strongly depend on their pH.

Table 17. Key indicators

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Hydrogen index (pH) is a value characterizing the activity or concentration of hydrogen ions in solutions. The hydrogen indicator is designated pH. The hydrogen index is numerically equal to the negative decimal logarithm of the activity or concentration of hydrogen ions, expressed in moles per liter: pH=-log[ H+ ] If [ H+ ]>10-7mol/l, [ OH-]<10-7моль/л -среда кислая; рН<7.Если [ H+ ]<10-7 моль/л, [ OH-]>10-7mol/l - alkaline environment; pH>7. Hydrolysis of salts- this is the chemical interaction of salt ions with water ions, leading to the formation of a weak electrolyte. 1). Hydrolysis is not possibleSalt formed by a strong base and a strong acid ( KBr, NaCl, NaNO3), will not undergo hydrolysis, since in this case a weak electrolyte is not formed. pH of such solutions = 7. The reaction of the medium remains neutral. 2). Hydrolysis by cation (only the cation reacts with water). In a salt formed by a weak base and a strong acid

(FeCl2,NH4Cl, Al2(SO4)3,MgSO4)

The cation undergoes hydrolysis:

FeCl2 + HOH<=>Fe(OH)Cl + HCl Fe2+ + 2Cl- + H+ + OH-<=>FeOH+ + 2Cl- + H+

As a result of hydrolysis, a weak electrolyte, H+ ion and other ions are formed. solution pH< 7 (раствор приобретает кислую реакцию). 3). Гидролиз по аниону (в реакцию с водой вступает только анион). Соль, образованная сильным основанием и слабой кислотой

(KClO, K2SiO3, Na2CO3,CH3COONa)

undergoes hydrolysis at the anion, resulting in the formation of a weak electrolyte, hydroxide ion OH- and other ions.

K2SiO3 + HOH<=>KHSiO3 + KOH 2K+ +SiO32- + H+ + OH-<=>НSiO3- + 2K+ + ОН-

The pH of such solutions is > 7 (the solution becomes alkaline). 4). Joint hydrolysis (both the cation and the anion react with water). Salt formed by a weak base and a weak acid

(CH 3COONH 4, (NН 4)2СО 3, Al2S3),

hydrolyzes both the cation and the anion. As a result, a slightly dissociating base and acid are formed. The pH of solutions of such salts depends on the relative strength of the acid and base. A measure of the strength of an acid and a base is the dissociation constant of the corresponding reagent. The reaction of the medium of these solutions can be neutral, slightly acidic or slightly alkaline:

Al2S3 + 6H2O =>2Al(OH)3v+ 3H2S^

Hydrolysis is a reversible process. Hydrolysis is irreversible if the reaction results in the formation of an insoluble base and (or) a volatile acid

Pure water is a poor conductor of electricity, but still has measurable electrical conductivity, which is explained by the partial dissociation of H2O molecules into hydrogen ions and hydroxide ions:

H 2 O H + + OH –

Based on the electrical conductivity of pure water, one can calculate the concentration of H + and OH – ions in it. At 25 o C it is equal to 10 –7 mol/l.

The H2O dissociation constant 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 of H 2 O or 1000: 18 ≈ 55.56 (mols). 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 constant quantities 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 ionic product of water .

In clean water at 25 o C.
Therefore, for the specified temperature:

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

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

Solutions in which the concentrations of H + and OH – ions are the same are called neutral solutions. IN sour solutions contain more hydrogen ions, and alkaline– hydroxide ions. But whatever the reaction of the medium in the 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 will correspondingly 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 = 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 solution reaction can be quantitatively characterized by the concentration of H + ions:

neutral solution ®

acid 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 quantity is called pH value 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 estimate 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. Characteristics of some of the most common indicators are presented in Table 12.

Table 12. The most important acid-base indicators