Potassium nitrate is hydrolyzed by. Hydrolysis of salts
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 metabolic decomposition of a substance with water.
Various substances undergo hydrolysis: inorganic - salts, metal carbides and hydrides, 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 (pH = 7). This is explained by the fact that salts in aqueous solutions can undergo hydrolysis.
The essence of hydrolysis comes down to the exchange chemical interaction of salt cations or anions with water molecules. As a result of this interaction, a slightly 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.
Classification of salts
Any salt can be thought of as the product of the reaction 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 base and acid, we can distinguish four types of salts.
Let's consider the behavior of salts of various types in solution.
1. Salts formed strong foundation And weak acid.
For example, the potassium cyanide salt KCN is formed by the strong base KOH and the weak acid HCN:
In an aqueous salt solution, two processes occur:
2) complete dissociation of salt (strong electrolyte):
The H + and CN - ions formed during these processes interact with each other, binding into molecules of a weak electrolyte - hydrocyanic acid HCN, while the hydroxide - OH ion - remains in the solution, thereby determining its alkaline environment. Hydrolysis occurs at the CN - anion.
Let us write down the complete ionic equation of the ongoing process (hydrolysis):
This process is reversible, and the chemical equilibrium is shifted to the left (towards 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 clean water, therefore, the KCN salt solution has an alkaline environment (pH > 7);
2) CN - ions participate in the reaction with water, in this case they say that hydrolysis occurs at the anion. Other examples of weak acid anions that react with water:
Formic HCOOH - anion HCOO - ;
Acetic CH 3 COOH - anion CH 3 COO - ;
Nitrogen HNO 2 - anion NO 2 -;
Hydrogen sulfide H 2 S - anion S 2-;
Coal H 2 CO 3 - CO 3 2- anion;
Sulfurous H 2 SO 3 is the SO 3 2- anion.
Consider the hydrolysis of sodium carbonate Na 2 CO 3:
Hydrolysis of the salt occurs at the CO 3 2- anion.
The products of hydrolysis are the acid salt NaHCO 3 and sodium hydroxide NaOH.
The medium 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 occurs to a very small extent and can be neglected.
To summarize what you have learned about anion hydrolysis:
1) according to the anion, salts, as a rule, are hydrolyzed reversibly;
2) the chemical equilibrium in such reactions is strongly shifted to the left;
3) the reaction of the medium in solutions of such salts is alkaline (pH > 7);
4) upon hydrolysis of salts formed by weak polybasic acids, acid salts are obtained.
2. Salts formed strong acid And weak foundation.
Let's consider the hydrolysis of ammonium chloride NH 4 Cl.
In an aqueous salt solution, two processes occur:
1) slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be simplified by the equation:
2) complete dissociation of salt (strong electrolyte):
The resulting OH - and NH 4 ions interact with each other to produce NH 3 H 2 O (weak electrolyte), while the H + ions remain in the solution, thereby causing its acidic environment.
The complete ionic equation for hydrolysis is:
The process is reversible, the chemical equilibrium is shifted towards the formation of the starting substances, since water H 2 O is a much weaker electrolyte than ammonia hydrate NH 3 H 2 O.
Abbreviated ionic equation for hydrolysis:
The equation shows that:
1) there are free hydrogen ions H + in the solution, and their concentration is greater than in pure water, therefore the salt solution has an acidic environment (pH< 7);
2) ammonium cations NH + participate in the reaction with water; in this case, they say that hydrolysis occurs at the cation.
Multiply charged 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+ ...).
Let's consider the hydrolysis of nickel nitrate Ni(NO 3) 2, hydrolysis of the salt by cation:
Hydrolysis of the salt occurs at the Ni 2+ cation.
The complete ionic equation for hydrolysis is:
Abbreviated ionic equation:
Hydrolysis products are the main salt NiOHNO 3 and Nitric acid HNO3.
The medium of an aqueous solution of nickel nitrate is acidic (pH< 7), потому что в растворе увеличивается концентрация ионов Н + .
Hydrolysis of the NiOHNO 3 salt occurs to a much lesser extent and can be neglected. Thus:
1) according to the cation, salts, as a rule, are hydrolyzed reversibly;
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) upon hydrolysis of salts formed by weak polyacid bases, basic salts are obtained.
3. Salts formed weak foundation 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; The weak acid anion binds H+ ions from water molecules to form a weak acid. The reaction of solutions of these salts can be neutral, weakly acidic or slightly alkaline. This depends on the dissociation constants of two weak electrolytes - acid and base, 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 = H + + OH -), and the anions of weak acids CH 3 COO - and HCOO - interact with H + cations 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 environment 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 environment 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.
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 an irreversible process.
Aluminum sulfide Al 2 S 3 in water undergoes irreversible hydrolysis, since the H + ions that appear during hydrolysis of the cation are bound by the OH - ions formed during hydrolysis of 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 of both the cation and the anion:
1) if salts are hydrolyzed both at the cation and at the anion reversibly, then the chemical equilibrium in the hydrolysis reactions is shifted to the right; the reaction of the medium is either neutral, or weakly acidic, or weakly alkaline, which depends on the ratio of the dissociation constants of the resulting base and acid;
2) salts can hydrolyze both the cation and the anion irreversibly if at least one of the hydrolysis products leaves the reaction sphere.
4. Salts formed strong foundation And strong acid, do not undergo hydrolysis .
Let's consider the “behavior” of potassium chloride KCl in a solution.
Salt in an aqueous solution dissociates into ions (KCl = K + + Cl -), but when interacting with water, a weak electrolyte cannot be formed. The solution environment 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 include 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 can beenhance (and even make it irreversible) in the following ways:
1) add water (reduce concentration);
2) heat the solution, which increases the endothermic dissociation of water:
This means that the amount of H + and OH - increases, which are necessary for the hydrolysis of the salt;
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 significantly enhanced due to the decomposition of ammonia hydrate to form ammonia NH 3 and water H 2 O:
Hydrolysis is possiblesuppress (significantly reduce the amount of salt undergoing hydrolysis) by proceeding as follows:
1) increase the concentration of the dissolved substance;
2) cool the solution (to reduce 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 environment as a result of hydrolysis is acidic, or alkalize if it is alkaline.
Meaning of hydrolysis
Hydrolysis of salts has both practical and biological significance.
Even in ancient times, ash was used as a detergent. The ash contains potassium carbonate K 2 CO 3, which is hydrolyzed into an anion in water; the aqueous solution becomes soapy due to the OH - ions formed during hydrolysis.
Currently, in everyday life we use soap, washing powders and other detergents. 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 at the anion.
If the acidity of the soil is insufficient, the plants develop a disease - chlorosis. Its signs are yellowing or whitening of leaves, retarded growth and development. If the pH is > 7.5, then ammonium sulfate (NH 4) 2 SO 4 fertilizer is added to the soil, which helps increase acidity due to hydrolysis of the cation occurring in the soil:
The biological role of hydrolysis of certain salts that make up our body is invaluable.
For example, the blood contains sodium 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 hydroxide ions OH -, 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 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 taking the test:
Mendeleev table
Solubility table
And they show different reactions environments - 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 - neutral medium (pH = 7). These salts do not contain hydrogen ions H + or hydroxide ions OH -, which determine the solution environment. How can we explain the different environments of aqueous salt solutions? This is explained by 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 is the interaction of salt ions with water, which results in the formation of weak electrolytes.
The essence of hydrolysis comes down 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 slightly 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 portion of the salt ions are hydrolyzed.
Any salt can be represented as a product of interaction with. For example, the salt NaClO 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's look at examples of hydrolysis various types 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+ of the water molecule, forming a weak electrolyte (acid).
Example: Let's compose the molecular and ionic equations for the hydrolysis of potassium nitrite KNO 2.
The salt KNO 2 is formed by the weak monobasic acid HNO 2 and the strong base KOH, which can be represented schematically as follows:
Let's 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 the weak electrolyte HNO 2, their concentration decreases and the equilibrium of the water dissociation process according to Le Chatelier’s 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, pH > 7.
II. Salts formed by a weak base and a strong acid hydrolyze at the cation. These salts are formed by a weak base cation and a strong acid anion. The salt cation binds the hydroxide ion OH - water, forming a weak electrolyte (base).
Example: Let's create molecular and ionic equations for the hydrolysis of ammonium iodide NH 4 I.
The salt NH 4 I is formed by a weak one-acid base NH 4 OH and a strong acid HI: 
When NH 4 I salt is dissolved in water, ammonium cations NH 4 + bind to the hydroxide ions OH - of water, forming a weak electrolyte -. An excess of hydrogen ions H + appears in the solution. The medium of NH 4 I salt solution is acidic, pH<7.
Conclusion: Salts formed by a strong acid and a weak base show an acidic reaction during hydrolysis, pH< 7.
III. Salts formed by a weak base and a weak acid are hydrolyzed at the same time as both the cation and 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. This depends on the dissociation constants of the weak acid and weak base that are formed as a result of hydrolysis.
Example 1: Let's create the equations for the hydrolysis of ammonium acetate CH 3 COONH 4 . This salt is formed by weak acetic acid CH 3 COOH and weak base NH 4 OH: 
The reaction of the CH 3 COONH 4 salt solution is neutral (pH = 7), because K d (CH 3 COOH) = K d (NH 4 OH).
Example 2: Let's compose the equations for the hydrolysis of ammonium cyanide NH 4 CN. This salt is formed by a weak acid HCN and a 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, for most salts hydrolysis is a reversible process. At equilibrium, only a small portion of the salt is hydrolyzed. However, some salts are completely decomposed by water, i.e. for them hydrolysis is irreversible.
Irreversible (complete) hydrolysis salts that are formed by a weak insoluble or volatile base and a weak volatile or insoluble acid are exposed. Such salts cannot exist in aqueous solutions. For example, they include: 
Example: Let's create an equation for the hydrolysis of aluminum sulfide Al 2 S 3:
Al 2 S 3 + 6H 2 O = 2Al(OH) 3 ↓ + 3H 2 S
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 a weak, volatile (insoluble) acid. For example:
Fe 2 S 3 + 6H 2 O = 2Fe(OH) 3 ↓ + 3H 2 S
Summing these equations, we get:
or in ionic form:
3S 2- + 2Fe 3+ + 6H 2 O = 2Fe(OH) 3 ↓ + 3H 2 S
IV. Salts formed by a strong acid and a strong base do not hydrolyze, because the cations and anions of these salts do not bind to the H + or OH - ions of water, that is, they do not form weak electrolyte molecules with them. The equilibrium of water dissociation does not shift. The solution environment 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 environment (pH = 7.0).
Stepwise hydrolysis
Hydrolysis of salts can occur in stages. Let us consider the cases of stepwise hydrolysis.
If a 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, at the first stages of hydrolysis, an acidic salt is formed instead of an acid and a strong base. Stepwise hydrolyzed salts Na 2 SO 3, Rb 2 CO 3, K 2 SiO 3, Li 3 PO 4, etc.
Example: Let's compose the molecular and ionic equations for the hydrolysis of potassium carbonate K 2 CO 3.
Hydrolysis of the K 2 CO 3 salt proceeds through the anion, because the potassium carbonate salt is formed by the weak acid H 2 CO 3 and the strong base KOH:
Since H 2 CO 3 is a dibasic acid, the hydrolysis of K 2 CO 3 occurs in two stages.
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.
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 occurs to a much lesser extent than in the first stage.
The medium of the K 2 CO 3 salt solution is alkaline (pH > 7), because the concentration of OH - ions in the solution increases.
If a 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 stages a basic salt is formed instead of a base and a strong acid. The salts MgSO 4, CoI 2, Al 2 (SO 4) 3, ZnBr 2, etc. are hydrolyzed stepwise.
Example: Let's create molecular and ionic equations for the hydrolysis of nickel (II) chloride NiCl2.
Hydrolysis of the NiCl 2 salt occurs 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 hydroxide ions OH - water. Ni(OH) 2 is a two-acid base, so hydrolysis occurs in two stages.
First stage:
The products of the first stage of hydrolysis of NiCl 2 are the basic salt NiOHCl and the strong acid HCl.
Second stage (hydrolysis of the main 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, we can give a more general definition of the hydrolysis process:
Hydrolysis- This is a reaction of metabolic decomposition of substances with water.
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