Current and voltage dissociation of water. Water dissociation

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

Hydrogen index (pH) is a value that characterizes the activity or concentration of hydrogen ions in solutions. The hydrogen index is denoted by 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=-lg[ H+ ] If [ H+ ]>10-7 mol/l, [ OH-]<10-7моль/л -среда кислая; рН<7.Если [ H+ ]<10-7 моль/л, [ OH-]>10-7mol/l - alkaline environment; pH>7. Salt hydrolysis- this is the chemical interaction of salt ions with water ions, leading to the formation of a weak electrolyte. one). Hydrolysis is not possible A salt 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 at the 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)

cation undergoes hydrolysis:

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

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

K2SiO3 + HOH<=>KHSiO3 + KOH 2K+ +SiO32- + H+ + OH-<=>HSiO3- + 2K+ + OH-

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

(CH 3COONH 4, (NH 4) 2CO 3, Al2S3),

hydrolyzes both cation and anion. As a result, low-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 environment of these solutions can be neutral, slightly acidic or slightly alkaline:

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

Hydrolysis is a reversible process. Hydrolysis proceeds irreversibly if the reaction produces an insoluble base and (or) a volatile acid

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. When it is equal to 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 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. mol). In dilute aqueous solutions, the concentration of zoda can be considered the same. Therefore, replacing the product in the last equation with a new constant, 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. Its numerical value can be easily obtained by substituting the concentrations of hydrogen ions and hydroxide ions into the last equation. In pure water at 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 , as already mentioned, in neutral solutions, the concentration of both hydrogen ions and hydroxide ions is equal to mol / l. In acid 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, so much acid is added to pure water so that the concentration of hydrogen ions rises to mol / l, then the concentration of hydroxide ions will decrease so that the product remains equal. Therefore, in this solution, the concentration of hydroxide ions will be:

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

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 hydrogen index and is denoted by:

For example, if mol/l, then ; if mol / l, then, etc. From this it is clear that in a neutral solution ( mol / l) . In acidic solutions and the less, the more acidic the solution. On the contrary, in alkaline solutions, and the more, the greater the alkalinity of the solution.

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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:

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

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 an 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

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Approximately 556,000,000 non-dissociated water molecules dissociate only 1 molecule, but this is 60,000,000,000 dissociated molecules in 1 mm 3. The dissociation is reversible, that is, the H + and OH - ions can again form a water molecule. As a result, a dynamic equilibrium occurs in which the number of decayed molecules is equal to the number formed from H + and OH - ions. 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 \u003d κ 2 (for the reverse process)

where υ is the reaction rate; κ - 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 the calculation of 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 all 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 of the opposite dissociation, since it depends on the concentrations of ions H + and OH -: υ 2 = κ 2

During the accelerated process of the opposite 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: = 10 -14 / 0.1 = 10 -13 mol/l

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 constant of water autoprotolysis.

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 by the formula:

· - concentration of hydroxonium ions (protons);

- concentration of hydroxide ions;

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

The concentration of water in water, given its low degree of dissociation, is practically constant and is (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 an increase in temperature, the dissociation of water increases, therefore, Kv also increases, with a decrease in temperature, vice versa. The practical significance of the ionic product of water is great, since it allows, at a known acidity (alkalinity) of any solution (that is, at a known concentration or ), to find, respectively, the concentration or . Although in most cases, for convenience of presentation, they use not absolute values of concentrations, but their decimal logarithms taken with the opposite sign - respectively, the hydrogen index (pH) and the hydroxyl index (pOH).

Since K in is a constant, when an acid (H + ions) is added to a solution, the concentration of hydroxide ions OH - will fall and vice versa. In a neutral medium = = 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 are solutions containing buffer systems. Buffer systems are called mixtures, which contain a certain quantitative ratio of weak acids and their salts with strong bases or weak bases and their salts with strong acids. 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 oceans, soil solutions and living organisms. These systems perform the functions of regulators that support the active reaction of the environment at a certain value necessary for the successful flow of metabolic reactions. Buffer solutions are classified into acidic and basic. An example of the first can be an acetate buffer system, the second - ammonium. There are natural and artificial buffer solutions. The natural buffer solution is blood containing bicarbonate, phosphate, protein, hemoglobin and acid buffer systems. The artificial buffer may be an acetate buffer consisting of CH3COOH.

We will 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 the aquatic 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 salt concentration 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 the base.

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

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

Buffer solutions have 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 the buffer solution. For example, if a small amount of perchloric acid is added to an acetate buffer, the pH will not shift to the acid side, since perchloric acid will enter into an exchange decomposition reaction with a salt of a weak acid. As a result of the reaction, a strong acid capable of shifting the pH to the acid side is replaced by a weak acid and a neutral salt. The degree of dissociation of a weak electrolyte solution decreases with an increase in its concentration, tends to zero, and the pH shift does not occur.

Solution buffer tank(from English. buffer- shock absorber buff- soften shocks) - this is the amount of acid or base needed 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.

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 dissociation of H 2 O 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(°С)

Solutions in which the concentrations of H + and OH ions are the same are called neutral solutions. V 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