Water dissociation constant. Electrolytic dissociation of water. Ionic product of water. Hydrogen index of the environment. Concept of indicators Water dissociation reaction
Pure water, although poor (compared to electrolyte solutions), can conduct electricity. 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):
pH value(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
Electrolytic dissociation of water. pH value
Water is a weak electrolyte that dissociates according to the equation: .
This phenomenon is usually called self-ionization or autoprotolysis.
The dissociation constant of water at 25 0 C is:
Since the dissociation constant of water is very small, the concentration of water can be considered constant:
(at 295 K)
The quantity Kw is usually called the ionic product of water.
Ionic product water characterizes the equilibrium between hydrogen ions and hydroxide ions in aqueous solutions and is a constant value at a given temperature.
The acidity or basicity of an aqueous solution must be expressed by the concentration of hydrogen ions or hydroxide ions. Most often, the pH value is used for this purpose, which is related to the concentration of hydrogen ions by the following relationship:
In a neutral environment:
In an acidic environment:
In an alkaline environment:
Calculation of pH and pH of solutions of strong and weak electrolytes.
The concentration of H + ions is determined using the Ostwald equation: = ; similarly for hydroxyl: [ОH – ]= ;
The ability of solutions to maintain a certain pH value is usually called a buffer effect. Solutions with buffering properties are called buffer solutions.
In a broad sense, buffer systems are systems that maintain a certain value of some parameter when the composition changes. Buffer solutions are acidic-basic - they maintain a constant pH when acids or bases are introduced, oxidative-reductive - they keep the potential of systems constant when oxidizing or reducing agents are introduced. The buffer solution is a conjugate pair. Eg:
1. a weak acid and a salt of this acid and a strong base (acetic acid and sodium acetate - acetate buffer)
2. a weak base and a salt of this base and a strong acid (ammonium hydroxide and ammonium chloride - ammonia buffer)
3. solutions containing salts of polybasic acids (sodium hydrogen phosphate and sodium dihydrogen phosphate - phosphate buffer)
Let us consider the mechanism of maintaining pH in acetate buffer. The reactions take place there:
CH 3 COOH ↔ CH 3 COO - + H +
CH 3 COONa ↔ CH 3 COO - + Na +
The first reaction is almost completely suppressed due to the high concentration of acetate ions caused by the dissociation of a strong electrolyte - sodium acetate.
If a strong acid is added to a solution, hydrogen ions will interact with anions to form molecules acetic acid and the reaction of the environment will not change. If a strong base is added to the solution, the hydroxide ions will interact with hydrogen ions (or acetic acid molecules). The formation of water will not affect the pH of the medium. The hydrogen ions that react with OH - ions will be compensated by shifting the equilibrium of the acetic acid dissociation reaction to the right.
Electrolytic dissociation constant of acetic acid:
Hydrogen ion concentration value:
The degree of electrolytic dissociation of acetic acid is insignificant; therefore, its undissociated molecules predominate in the solution. The concentration of undissociated molecules will be almost equal to the concentration of the acid. Then the concentration of undissociated acid can be replaced by the total concentration of acid in solution:
[CH 3 COOH] = [acid],
and the concentration of acetate ions is the concentration of salt in the solution:
[CH 3 COO - ] = [salt].
Substituting these values into expression (2), we obtain the equation for calculating [H + ] for the buffer solution:
Magnitude TO(the electrolytic dissociation constant of the acid) is constant under these conditions.
Taking logarithms of the equations we get:
pK is the negative logarithm of the dissociation constant of acetic acid.
Using the same reasoning, for a mixture of a weak base and a salt of a strong acid, we can derive the equation:
From the equations it follows that the pH of the buffer depends on the value of the constant of a weak acid or weak base, as well as on the ratio of the concentrations of the components of buffer mixtures.
Since the electrolytic dissociation constant under these conditions is constant, the pH of the buffer solution will depend only on the ratio of the concentrations of the acid (or base) and salt taken to prepare the buffer mixture. and does not depend on the absolute value of these concentrations. Experience shows that even with significant dilution of buffer solutions by 10-20 times, the pH changes little.
The ability of buffer solutions to counteract sudden changes in pH is limited. The limit within which the buffering effect manifests itself is usually called the buffer capacity (B). Numerically buffer capacity is determined by the number of mole equivalents of a strong acid or base, which must be added to 1 l buffer mixture to change the pH value by one.
The size of the buffer capacity depends on the concentration of the components of the buffer mixture and their ratio. The higher the concentration of the components of the buffer mixture, the greater its capacity. The maximum buffering effect is observed if the acid and salt are in solution in equivalent quantities.
The presence of buffer mixtures in living organisms determines the constancy of the pH of blood, milk, and plant cell sap. Carbonate and phosphate buffer systems have great importance in the regulation of biochemical processes in the body and soil.
Lecture 5 “Weak and strong electrolytes”
Electrolytes- ϶ᴛᴏ substances whose solutions conduct electric current through ions into which they disintegrate under the influence of polar solvent molecules.
A quantitative characteristic of electrolyte dissociation is the degree of dissociation, which is equal to the ratio of the number of dissociated molecules to total number molecules:
Based on the degree of dissociation, strong electrolytes are distinguished, weak electrolytes and electrolytes of medium strength.
Electrolytic dissociation of water. Hydrogen index - concept and types. Classification and features of the category "Electrolytic dissociation of water. Hydrogen index" 2017, 2018.
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 ions and hydroxide ions in water can be calculated. At it is equal to 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 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. 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 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 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 , as already mentioned, in neutral solutions the concentration of both hydrogen ions and hydroxide ions is equal to mol/l. In sour solutions more concentration hydrogen ions, in alkaline ones - 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 increases 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 you add alkali to water and thereby 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, indicate its decimal logarithm, taken with the opposite sign. The last value is called the hydrogen index and is denoted by:
For example, if mol/l, then ; if mol/l, then etc. From here it is clear that in a neutral solution (mol/l). In acidic solutions, the more acidic the solution, the less. On the contrary, in alkaline solutions the greater the alkalinity of the solution.
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 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(°C)
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. The characteristics of some of the most common indicators are presented in Table 12.
Table 12. The most important acid-base indicators
Conducts electricity very poorly, but still has some measurable electrical conductivity, co which is explained by a slight dissociation of water into hydrogen and hydroxyl ions:
H2O ⇄ H + OH’
Based on the electrical conductivity of pure water, the concentration of hydrogen ions and hydroxyl ions in water can be calculated. It turns out to be equal to 10 -7 G-and he /l.
Applying the law of mass action to the dissociation of water, we can write:
Let's rewrite this equation as follows:
[OH'] = [H 2 O]K
Since there is very little water, the concentration of undissociated H 2 O molecules not only in water, but also in any dilute aqueous solution can be considered a constant value. Therefore, replacing [H 2 O] K with the new constant KH 2 O, we will have:
[H] [OH’] = TO H2O
The resulting equation shows that for water and dilute aqueous solutions at a constant temperature, the product of the concentrations of hydrogen and hydroxyl ions is a constant value. This constant is called the ionic product of water. Its numerical value can be easily obtained by substituting the concentrations of hydrogen and hydroxyl ions into the last equation
TO H2O = 10 -7 10 -7 = 10 -14
Solutions in which the concentration of hydrogen and the concentration of hydroxyl ions are the same and equal to every 10 —7 g-ion/l are called neutral solutions. In acidic solutions the concentration of hydrogen ions is higher, in alkaline solutions the concentration of hydroxyl ions is higher. But whatever the reaction of the solution, the product of the concentrations of H and OH' ions must remain constant.
If, for example, enough acid is added to pure water so that the concentration of hydrogen ions increases to 10 -3, the concentration of hydroxyl ions will have to decrease so that the product [H] [OH'] remains equal to 10 -14. Therefore, in this solution the concentration of hydroxyl ions will be:
10 -14: 10 -3 = 10 -11
On the contrary, if you add alkali to water and thereby increase the concentration of hydroxyl ions, for example, to 10 -5, the concentration of hydrogen ions will become equal to:
10 -14: 10 -5 = 10 -9
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