According to the theory of electrolytic dissociation in an aqueous solution, solute particles interact with water molecules. This interaction can lead to a hydrolysis reaction.
Hydrolysis is a reaction of the exchange decomposition of a substance by water.
Various substances undergo hydrolysis: inorganic - salts, carbides and hydrides of metals, non-metal halides; organic - haloalkanes, esters and fats, carbohydrates, proteins, polynucleotides.
Aqueous solutions of salts have different pH values and different types of media - acidic (pH< 7), щелочную (рН >7), neutral (рН = 7). This is due to the fact that salts in aqueous solutions can undergo hydrolysis.
The essence of hydrolysis is reduced to the exchange chemical interaction of salt cations or anions with water molecules. As a result of this interaction, a low-dissociating compound (weak electrolyte) is formed. And in an aqueous salt solution, an excess of free H + or OH ions appears, and the salt solution becomes acidic or alkaline, respectively.
Salt classification
Any salt can be thought of as the product of the interaction of a base with an acid. For example, the salt KClO is formed by the strong base KOH and the weak acid HClO.
Depending on the strength of the base and acid, one can distinguish four types of salts.
Consider the behavior of salts of various types in solution.
1. Salts formed strong base and weak acid.
For example, the potassium cyanide salt KCN is formed by the strong base KOH and the weak acid HCN:
Two processes take place in an aqueous solution of salt:
2) complete dissociation of salt (strong electrolyte):
The H + and CN ions formed during these processes interact with each other, binding to the molecules of a weak electrolyte - hydrocyanic acid HCN, while the hydroxide - OH ion - remains in solution, thereby causing its alkaline environment. Hydrolysis occurs at the anion CN - .
We write the full ionic equation of the ongoing process (hydrolysis):
This process is reversible, and the chemical equilibrium is shifted to the left (toward the formation of the starting substances), since water is a much weaker electrolyte than hydrocyanic acid HCN:
The equation shows that:
1) there are free hydroxide ions OH - in the solution, and their concentration is greater than in pure water, therefore the KCN salt solution has an alkaline environment (pH> 7);
2) CN ions participate in the reaction with water, in which case they say that anion hydrolysis is taking place. Other examples of weak acid anions that react with water are:
Formic HCOOH - anion HCOO -;
Acetic CH 3 COOH - anion CH 3 COO -;
Nitrogenous HNO 2 - anion NO 2 -;
Hydrogen sulfide H 2 S - anion S 2-;
Coal H 2 CO 3 - CO 3 2- anion;
Sulphurous H 2 SO 3 - SO 3 2- anion.
Consider the hydrolysis of sodium carbonate Na 2 CO 3:
Salt is hydrolyzed by the CO 3 2- anion.
The products of hydrolysis are the acid salt NaHCO 3 and sodium hydroxide NaOH.
The environment of an aqueous solution of sodium carbonate is alkaline (pH> 7), because the concentration of OH - ions increases in the solution. The acid salt NaHCO 3 can also undergo hydrolysis, which proceeds to a very small extent, and it can be neglected.
To summarize what you have learned about anion hydrolysis:
1) according to the anion of the salt, as a rule, they are reversibly hydrolyzed;
2) the chemical equilibrium in such reactions is strongly shifted to the left;
3) the reaction of the medium in solutions of similar salts is alkaline (pH > 7);
4) during the hydrolysis of salts formed by weak polybasic acids, acidic salts are obtained.
2. Salts formed strong acid and weak base.
Consider the hydrolysis of ammonium chloride NH 4 Cl.
Two processes take place in an aqueous solution of salt:
1) a slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be written in a simplified way using the equation:
2) complete dissociation of salt (strong electrolyte):
The resulting OH - and NH 4 ions interact with each other to obtain NH 3 H 2 O (weak electrolyte), while H + ions remain in solution, thereby causing its acidic environment.
Full ionic hydrolysis equation:
The process is reversible, the chemical equilibrium is shifted towards the formation of the initial substances, since water H 2 O is a much weaker electrolyte than ammonia hydrate NH 3 H 2 O.
Abbreviated ionic hydrolysis equation:
The equation shows that:
1) there are free hydrogen ions H + in the solution, and their concentration is greater than in pure water, so the salt solution has an acidic environment (pH< 7);
2) ammonium cations NH + participate in the reaction with water; in this case, we say that there is hydrolysis at the cation.
Multicharged cations can also participate in the reaction with water: doubly charged M 2+ (for example, Ni 2 +, Cu 2 +, Zn 2+ ...), in addition to alkaline earth metal cations, triply charged M 3 + (for example, Fe 3 +, Al 3 + , Cr 3+ …).
Consider the hydrolysis of nickel nitrate Ni(NO 3) 2 , hydrolysis of the salt by the cation:
Salt is hydrolyzed at the Ni 2+ cation.
Full ionic hydrolysis equation:
Abbreviated ionic equation:
The products of hydrolysis are the basic salt NiOHNO 3 and nitric acid HNO 3 .
The medium of an aqueous solution of nickel nitrate is acidic (pH< 7), потому что в растворе увеличивается концентрация ионов Н + .
The hydrolysis of the NiOHNO 3 salt proceeds to a much lesser degree and can be neglected. In this way:
1) according to the cation, the salts, as a rule, are reversibly hydrolyzed;
2) the chemical equilibrium of reactions is strongly shifted to the left;
3) the reaction of the medium in solutions of such salts is acidic (pH< 7);
4) during the hydrolysis of salts formed by weak polyacid bases, basic salts are obtained.
3. Salts formed weak base and weak acid.
Such salts undergo hydrolysis both at the cation and at the anion.
A weak base cation binds OH ions from water molecules, forming a weak base; a weak acid anion binds H+ ions from water molecules, forming a weak acid. The reaction of solutions of these salts can be neutral, slightly acidic or slightly alkaline. It depends on the dissociation constants of two weak electrolytes - acids and bases, which are formed as a result of hydrolysis.
For example, consider the hydrolysis of two salts: ammonium acetate NH 4 CH 3 COO and ammonium formate NH 4 HCCO:
In aqueous solutions of these salts, cations of the weak base NH + interact with hydroxide ions OH - (recall that water dissociates H 2 O \u003d H + + OH -), and anions of weak acids CH 3 COO - and HCOO - interact with cations H + with the formation of molecules of weak acids - acetic CH 3 COOH and formic HCOOH.
Let us write the ionic equations of hydrolysis:
In these cases, hydrolysis is also reversible, but the equilibrium is shifted towards the formation of hydrolysis products - two weak electrolytes.
In the first case, the solution medium is neutral (pH = 7), since K d (CH 3 COOH) = K d (NH 3 H 2 O) = 1.8 10 -5. In the second case, the solution medium will be slightly acidic (pH< 7), т. к. K д (HCOOH) = 2,1 10 -4 и K д (NH 3 H 2 O) < K д HCOOH), где K д - константа диссоциации.
The hydrolysis of most salts is a reversible process. In a state of chemical equilibrium, only part of the salt is hydrolyzed. However, some salts are completely decomposed by water, i.e., their hydrolysis is not reversible process.
Aluminum sulfide Al 2 S 3 in water undergoes irreversible hydrolysis, since the H + ions that appear during hydrolysis by the cation are bound by the OH ions formed during hydrolysis by the anion. This enhances hydrolysis and leads to the formation of insoluble aluminum hydroxide and hydrogen sulfide gas:
Therefore, aluminum sulfide Al 2 S 3 cannot be obtained by an exchange reaction between aqueous solutions of two salts, for example, aluminum chloride AlCl 3 and sodium sulfide Na 2 S.
As a result of hydrolysis both for the cation and for the anion:
1) if salts are hydrolyzed by both the cation and the anion reversibly, then the chemical equilibrium in hydrolysis reactions is shifted to the right; the reaction of the medium in this case is either neutral, or slightly acidic, or slightly alkaline, which depends on the ratio of the dissociation constants of the formed base and acid;
2) salts can be hydrolyzed by both the cation and the anion irreversibly if at least one of the hydrolysis products leaves the reaction sphere.
4. Salts formed strong base and strong acid, do not undergo hydrolysis .
Consider the "behavior" in a solution of potassium chloride KCl.
Salt in an aqueous solution dissociates into ions (KCl \u003d K + + Cl -), but when interacting with water, a weak electrolyte cannot form. The solution medium is neutral (pH = 7), since the concentrations of H + and OH ions in the solution are equal, as in pure water.
Other examples of such salts may be alkali metal halides, nitrates, perchlorates, sulfates, chromates and dichromates, alkaline earth metal halides (other than fluorides), nitrates and perchlorates.
It should also be noted that the reversible hydrolysis reaction is completelyobeys Le Chatelier's principle . Therefore, salt hydrolysis canstrengthen (and even make it irreversible) in the following ways:
1) add water (reduce concentration);
2) heat the solution, thus increasing the endothermic dissociation of water:
This means that the amount of H + and OH - increases, which are necessary for the implementation of salt hydrolysis;
3) bind one of the hydrolysis products into a sparingly soluble compound or remove one of the products into the gas phase; for example, the hydrolysis of ammonium cyanide NH 4 CN will be greatly enhanced by the decomposition of ammonia hydrate to form ammonia NH 3 and water H 2 O:
Hydrolysis cansuppress (significantly reduce the amount of salt undergoing hydrolysis), proceeding as follows:
1) increase the concentration of the solute;
2) cool the solution (to weaken hydrolysis, salt solutions should be stored concentrated and at low temperatures);
3) introduce one of the hydrolysis products into the solution; for example, acidify the solution if its medium is acidic as a result of hydrolysis, or alkalinize if it is alkaline.
Significance of hydrolysis
The hydrolysis of salts has both practical and biological significance.
Since ancient times, ash has been used as a detergent. The ash contains potassium carbonate K 2 CO 3, which hydrolyzes anion in water, the aqueous solution becomes soapy due to the OH - ions formed during hydrolysis.
At present, we use soap, washing powders and other detergents in everyday life. The main component of soap is sodium and potassium salts of higher fatty carboxylic acids: stearates, palmitates, which are hydrolyzed.
The hydrolysis of sodium stearate C 17 H 35 COONa is expressed by the following ionic equation:
i.e., the solution has a slightly alkaline environment.
Salts that create the necessary alkaline environment of the solution are contained in the photographic developer. These are sodium carbonate Na 2 CO 3, potassium carbonate K 2 CO 3, borax Na 2 B 4 O 7 and other salts that hydrolyze along the anion.
If the acidity of the soil is insufficient, the plants develop a disease - chlorosis. Its signs are yellowing or whitening of the leaves, lag in growth and development. If pH> 7.5, then ammonium sulfate (NH 4) 2 SO 4 fertilizer is applied to it, which contributes to an increase in acidity due to hydrolysis by the cation passing in the soil:
The biological role of the hydrolysis of some salts that make up our body is invaluable.
For example, the composition of the blood includes bicarbonate and sodium hydrogen phosphate salts. Their role is to maintain a certain reaction of the environment.
This occurs due to a shift in the equilibrium of hydrolysis processes:
If there is an excess of H + ions in the blood, they bind to OH - hydroxide ions, and the equilibrium shifts to the right. With an excess of OH hydroxide ions, the equilibrium shifts to the left. Due to this, the acidity of the blood of a healthy person fluctuates slightly.
Or for example: human saliva contains HPO 4 - ions. Thanks to them, a certain environment is maintained in the oral cavity (pH = 7-7.5).
Reference material for passing the test:
periodic table
Solubility table
And they show a different reaction of the environment - acidic, alkaline, neutral.
For example, an aqueous solution of aluminum chloride AlCl 3 has an acidic environment (pH< 7), раствор карбоната калия K 2 СО 3 - щелочную среду (pН >7), solutions of sodium chloride NaCl and lead nitrite Pb (NO 2) 2 - a neutral medium (pH = 7). These salts do not contain hydrogen ions H + or hydroxide ions OH - , which determine the medium of the solution. How can one explain the different environments of aqueous solutions of salts? This is due to the fact that in aqueous solutions, salts undergo hydrolysis.
The word "hydrolysis" means decomposition by water ("hydro" - water, "lysis" - decomposition).
Hydrolysis is one of the most important chemical properties.
Salt hydrolysis The interaction of salt ions with water is called, as a result of which weak electrolytes are formed.
The essence of hydrolysis is reduced to the chemical interaction of salt cations or anions with hydroxide ions OH - or hydrogen ions H + from water molecules. As a result of this interaction, a low-dissociating compound (weak electrolyte) is formed. The chemical equilibrium of the water dissociation process shifts to the right.
Therefore, an excess of free H + or OH ions appears in an aqueous salt solution, and the salt solution shows an acidic or alkaline environment.
Hydrolysis is a reversible process for most salts. At equilibrium, only a small fraction of the salt ions are hydrolyzed.
Any salt can be represented as a product of interaction with. For example, the NaClO salt is formed by the weak acid HClO and the strong base NaOH.
Depending on the strength of the original acid and the original base, salts can be divided into 4 types: 
Salts of types I, II, III undergo hydrolysis, salts of type IV do not undergo hydrolysis
Let us consider examples of hydrolysis of various types of salts.
I. Salts formed by a strong base and a weak acid undergo hydrolysis at the anion. These salts are formed by a strong base cation and a weak acid anion, which binds the hydrogen cation H + water molecules, forming a weak electrolyte (acid).
Example: Let us compose the molecular and ionic equations for the hydrolysis of potassium nitrite KNO 2 .
Salt KNO 2 is formed by a weak monobasic acid HNO 2 and a strong base KOH, which can be represented schematically as follows:
Let us write the equation for the hydrolysis of salt KNO 2: 
What is the mechanism of hydrolysis of this salt?
Since H + ions combine into molecules of a weak electrolyte HNO 2, their concentration decreases and the equilibrium of the water dissociation process according to the Le Chatelier principle shifts to the right. The concentration of free hydroxide ions OH - increases in the solution. Therefore, the KNO 2 salt solution has an alkaline reaction (pH > 7).
Conclusion: Salts formed by a strong base and a weak acid, when dissolved in water, show an alkaline reaction of the medium, pH> 7.
II. Salts formed by a weak base and a strong acid hydrolyze at the cation. These salts are formed by a cation of a weak base and an anion of a strong acid. The salt cation binds the hydroxide ion OH - water, forming a weak electrolyte (base).
Example: Let us compose the molecular and ionic equations for the hydrolysis of ammonium iodide NH 4 I.
Salt NH 4 I is formed by a weak monoacid base NH 4 OH and a strong acid HI: 
When the salt NH 4 I is dissolved in water, the ammonium cations NH 4 + bind to the hydroxide ions OH - water, forming a weak electrolyte -. An excess of hydrogen ions H + appears in the solution. Salt solution medium NH 4 I - acidic, pH<7.
Conclusion: Salts formed by a strong acid and a weak base show an acid reaction during hydrolysis, pH< 7.
III. Salts formed by a weak base and a weak acid are hydrolyzed both at the cation and at the anion. These salts are formed by a weak base cation, which binds OH ions from a water molecule and forms a weak base, and a weak acid anion, which binds H + ions from a water molecule and forms a weak acid. The reaction of solutions of these salts can be neutral, slightly acidic or slightly alkaline. It depends on the dissociation constants of the weak acid and the weak base, which are formed as a result of hydrolysis.
Example 1: Let's make the equations of hydrolysis of ammonium acetate CH 3 COONH 4 . This salt is formed by a weak acetic acid CH 3 COOH and a weak base NH 4 OH: 
The reaction of a CH 3 COONH 4 salt solution is neutral (pH = 7), because K d (CH 3 COOH) \u003d K d (NH 4 OH).
Example 2: Let us compose the equations for the hydrolysis of ammonium cyanide NH 4 CN. This salt is formed by the weak acid HCN and the weak base NH 4 OH:
The reaction of the NH 4 CN salt solution is slightly alkaline (pH> 7), because K d (NH 4 OH)> K d (HCN).
As already noted, hydrolysis is a reversible process for most salts. At equilibrium, only a small part of the salt is hydrolyzed. However, some salts are completely decomposed by water, i.e., hydrolysis is irreversible for them.
Irreversible (complete) hydrolysis salts are exposed which are formed by a weak insoluble or volatile base and a weak volatile or insoluble acid. Such salts cannot exist in aqueous solutions. These include, for example: 
Example: Let's make an equation for the hydrolysis of aluminum sulfide Al 2 S 3:
Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S
The hydrolysis of aluminum sulfide proceeds almost completely to the formation of aluminum hydroxide Al (OH) 3 and hydrogen sulfide H 2 S.
Therefore, as a result of exchange reactions between aqueous solutions of some salts, two new salts are not always formed. One of these salts may undergo irreversible hydrolysis to form the corresponding insoluble base and weak volatile (insoluble) acid. For example:
Fe 2 S 3 + 6H 2 O \u003d 2Fe (OH) 3 ↓ + 3H 2 S
Summing these equations, we get:
or in ionic form:
3S 2- + 2Fe 3+ + 6H 2 O \u003d 2Fe (OH) 3 ↓ + 3H 2 S
IV. Salts formed from strong acids and strong bases do not hydrolyze. because the cations and anions of these salts do not bind with H + or OH - water ions, that is, they do not form weak electrolyte molecules with them. The equilibrium of water dissociation does not shift. The environment of solutions of these salts is neutral (pH = 7.0), since the concentrations of H + and OH ions in their solutions are equal, as in pure water.
Conclusion: Salts formed by a strong acid and a strong base do not undergo hydrolysis when dissolved in water and show a neutral reaction of the medium (pH = 7.0).
Step hydrolysis
The hydrolysis of salts can proceed in steps. Let us consider the cases of stepwise hydrolysis.
If the salt is formed by a weak polybasic acid and a strong base, the number of hydrolysis steps depends on the basicity of the weak acid. In an aqueous solution of such salts, an acid salt instead of an acid and a strong base are formed at the first stages of hydrolysis. Salts of Na 2 SO 3, Rb 2 CO 3, K 2 SiO 3, Li 3 PO 4, etc. are stepwise hydrolyzed.
Example: Let us compose the molecular and ionic equations for the hydrolysis of potassium carbonate K 2 CO 3 .
The hydrolysis of the salt K 2 CO 3 proceeds along the anion, because the potassium carbonate salt is formed by a weak acid H 2 CO 3 and a strong base KOH:
Since H 2 CO 3 is a dibasic acid, the hydrolysis of K 2 CO 3 proceeds in two steps.
First stage:
The products of the first stage of hydrolysis of K 2 CO 3 are the acid salt KHCO 3 and potassium hydroxide KOH.
The second stage (hydrolysis of the acid salt, which was formed as a result of the first stage): 
The products of the second stage of hydrolysis of K 2 CO 3 are potassium hydroxide and weak carbonic acid H 2 CO 3. Hydrolysis in the second stage proceeds to a much lesser extent than in the first stage.
The environment of the K 2 CO 3 salt solution is alkaline (pH> 7), because the concentration of OH ions increases in the solution.
If the salt is formed by a weak polyacid base and a strong acid, then the number of hydrolysis steps depends on the acidity of the weak base. In aqueous solutions of such salts, in the first steps, a basic salt is formed instead of a base and a strong acid. Salts of MgSO 4, CoI 2, Al 2 (SO 4) 3, ZnBr 2, etc. are hydrolyzed stepwise.
Example: Let us compose the molecular and ionic equations for the hydrolysis of nickel (II) chloride NiCl2.
The hydrolysis of the NiCl 2 salt proceeds through the cation, since the salt is formed by the weak base Ni(OH) 2 and the strong acid HCl. The Ni 2+ cation binds OH-water hydroxide ions. Ni(OH) 2 is a two-acid base, so the hydrolysis proceeds in two steps.
First stage:
The products of the first stage of hydrolysis of NiCl 2 are the basic salt NiOHCl and the strong acid HCl.
The second stage (hydrolysis of the basic salt, which was formed as a result of the first stage of hydrolysis): 
The products of the second stage of hydrolysis are the weak base nickel(II) hydroxide and the strong hydrochloric acid HCl. However, the degree of hydrolysis in the second stage is much less than in the first stage.
NiCl 2 solution medium - acidic, pH< 7, потому что в растворе увеличивается концентрация ионов Н + .
Not only, but also other inorganic compounds undergo hydrolysis. Carbohydrates, proteins and other substances are also hydrolyzed, the properties of which are studied in the course of organic chemistry. Therefore, a more general definition of the hydrolysis process can be given:
Hydrolysis- This is a reaction of the metabolic decomposition of substances by water.