The process of formation of weakly dissociated compounds with a change in the pH of the medium during the interaction of water and salt is called hydrolysis.

The hydrolysis of salts occurs in the case of binding of one water ion with the formation of sparingly soluble or weakly dissociated compounds due to a shift in the equilibrium dissociation. For the most part, this process is reversible and increases with dilution or temperature increase.

Hydrolysis of salts

To find out which salts are undergoing hydrolysis, it is necessary to know which bases and acids were used when it was formed. There are several types of their interactions.

Preparation of salt from base and weak acid

Examples include aluminum sulphide and chromium, as well as accelerated ammonium and ammonium carbonate. These salts, when dissolved in water, form bases and weakly dissociating acids. To trace the reversibility of the process, it is necessary to make up the equation for the salt hydrolysis reaction:

Ammonium acetate + water \u0026 harr; ammonia + acetic acid

In the ionic form, the process looks like:

In the above hydrolysis reaction, ammonia and acetic acid are formed, that is, weakly dissociating substances.

Hydrogen index of aqueous solutions (pH) directly depends on the relative strength, that is, the dissociation constants of the reaction products. The above reaction will be slightly alkaline, since the constant decomposition of acetic acid is less than the ammonium hydroxide constant, that is, 1.75 ∙ 10 - 5 less than 6.3 10 -5. If the bases and acids are removed from the solution, then the process continues to the end.

Consider an example of irreversible hydrolysis:

Aluminum sulfate + water = aluminum hydroxide + hydrogen sulfide

In this case, the process is irreversible, because one of the reaction products is removed, that is, it precipitates.

Hydrolysis of compounds obtained by the interaction of a weak base with a strong acid

This type of hydrolysis describes the decomposition of aluminum sulfate, copper chloride or bromide, as well as ferric chloride or ammonium. Consider the reaction of ferric chloride, which proceeds in two stages:

Ferric chloride + water \u0026 harr; iron hydroxochloride + hydrochloric acid

The ionic equation of hydrolysis of ferric chloride salts takes the form:

Fe 2+ + H2  O + 2Cl - \u0026 harr; Fe (OH) + + H + + 2Cl -

The second stage of hydrolysis:

Due to the deficiency of hydroxo group ions and the accumulation of hydrogen ions, the hydrolysis of FeCl2  proceeds through the first stage. A strong hydrochloric acid and a weak base are formed \u0026 ndash; iron hydroxide. In the case of such reactions, the medium is acidic.

Non-hydrolyzing salts obtained by reacting strong bases and acids

Examples of such salts are calcium or sodium chlorides, potassium sulfate and rubidium bromide. However, the above substances are not hydrolyzed, since they are neutral when dissolved in water. The only low dissociating substance in this case is water. To confirm this statement, you can make the equation of hydrolysis of sodium chloride salts with the formation of hydrochloric acid and sodium hydroxide:

NaCl + H2  O \u0026 harr; NaOH + HCl

Reaction in ionic form:

Na + + Cl - + H2  O \u0026 harr; Na + + HE - + H + + Cl -

Salts as a reaction product of strong alkali and acid of weak strength

In this case, the hydrolysis of salts proceeds along the anion, which corresponds to the alkaline medium of the hydrogen index. Examples include sodium acetate, sulphate and carbonate, potassium silicate and sulphate, as well as sodium hydrocyanic acid. For example, let's make the ion-molecular equations for the hydrolysis of salts of sulfide and sodium acetate:

Dissociation of sodium sulfide:

Na2  S \u0026 harr; 2Na + + S 2-

The first stage of hydrolysis of a polybasic salt, occurs on the cation:

Record in ionic form:

S 2- + H2  O \u0026 harr; HS - + OH -

The second stage is feasible in the case of increasing the reaction temperature:

Consider another hydrolysis reaction using sodium acetate for example:

Sodium acetate + water \u0026 harr; acetic acid + caustic soda.

As a result of the reaction, weak acetic acid is formed. In both cases, the reaction will have an alkaline environment.

Reaction equilibrium according to the Le Chatelier principle

Hydrolysis, like other chemical reactions, is reversible and irreversible. In the case of reversible reactions, one of the reagents is consumed not all, while the irreversible processes proceed with the complete consumption of the substance. This is due to a shift in the equilibrium of the reactions, which is based on changes in physical characteristics such as pressure, temperature, and mass fraction of the reactants.

According to the concept of the Le Chatelier principle, the system will be considered equilibrium until one or several external conditions of the process flow are changed to it. For example, with a decrease in the concentration of one of the substances, the equilibrium of the system will gradually begin to shift towards the formation of the same reagent. Hydrolysis of salts also has the ability to obey the principle of Le Chatelier, with which you can weaken or strengthen the process.

Increased hydrolysis

Hydrolysis can be enhanced to complete irreversibility in several ways:

  • To increase the rate of formation of OH - and H + ions. To do this, the solution is heated, and by increasing the absorption of heat by water, that is, endothermic dissociation, this indicator increases.
  • Add water.
  • Translate one of the products in a gaseous state or bind in a highly soluble substance.

Suppression of hydrolysis

To suppress the process of hydrolysis, as well as to strengthen, in several ways.

Enter into solution one of the substances formed in the process. For example, to alkalize the solution, if it is pH˃7, or vice versa, to acidify, where the reaction medium is less than 7 in terms of pH.

Mutual enhancement of hydrolysis

Mutual enhancement of hydrolization is applied if the system becomes equilibrium. Let us consider a concrete example where systems in different vessels became equilibrium:

Al 3+ + H2  O \u0026 harr; AlOH 2+ + H +

Both systems are little hydrolyzed, so if you mix them with each other, there will be a binding of hydroxins and hydrogen ions. As a result, we obtain the molecular equation of salt hydrolysis:

Aluminum chloride + sodium carbonate + water = sodium chloride + aluminum hydroxide + carbon dioxide.

According to Le Chatelier, the equilibrium of the system will shift to the side of the reaction products, and the hydrolysis will go to the end with the formation of aluminum hydroxide, precipitated. Such an enhancement of the process is possible only if one of the reactions proceeds along the anion, and the other along the cation.

Anion hydrolysis

Hydrolysis of aqueous solutions of salts is carried out by combining their ions with water molecules. One of the methods of hydrolization is performed by anion, that is, the addition of a water ion H +.

Most of this salt is subject to hydrolysis, which is formed through the interaction of a strong hydroxide and a weak acid. An example of salts decomposing in the anion can be sodium sulfate or sodium sulfite, as well as potassium carbonate or phosphate. A hydrogen indicator with more than seven. As an example, consider the dissociation of sodium acetate:

In solution, this compound is divided into a cation \u0026 ndash; Na +. and anion \u0026 ndash; CH3  Soo -.

The cation of dissociated sodium acetate, formed by a strong base, cannot react with water.

At the same time, the anions of the acid easily react with the H molecules.2  ABOUT:

Consequently, the hydrolization is carried out on the anion, and the equation takes the form:


In the event that polybasic acids undergo hydrolysis, the process occurs in several stages. Under normal conditions, these substances hydrolyze in the first stage.

Cation hydrolysis

Salts formed by the interaction of a strong acid and a base of low strength are mainly susceptible to cationic hydrolysis. An example is ammonium bromide, copper nitrate, and zinc chloride. At the same time, the medium in the solution during hydrolysis corresponds to less than seven. Consider the process of hydrolysis by cation on the example of aluminum chloride:

In aqueous solution, it dissociates into an anion \u0026 ndash; 3Cl - and cation \u0026 ndash; Al 3+.

Ions of strong hydrochloric acid do not interact with water.

The ions (cations) of the base, on the contrary, are subject to hydrolysis:

Al 3+ + HON = AlOH 2+ + H +

In the molecular form, the hydrolization of aluminum chloride is as follows:

AlCl3 + H2  O = AlOHCl + HCl

Under normal conditions, it is preferable to neglect the hydrolysis in the second and third stages.

Degree of dissociation

Any reaction of salt hydrolysis is characterized by the degree of dissociation, which shows the relationship between the total number of molecules and molecules capable of passing into the ionic state. The degree of dissociation is characterized by several indicators:

  • The temperature at which the hydrolysis is carried out.
  • The concentration of the dissociable solution.
  • The origin of the dissolved salt.
  • The nature of the solvent itself.

According to the degree of dissociation, all solutions are divided into strong and weak electrolytes, which in turn, when dissolved in different solvents, exhibit different degrees.

  • Substances with a dissociation degree of over 30% are strong electrolytes. For example, sodium hydroxide, potassium hydroxide, hydroxide barium and calcium, as well as sulfuric, hydrochloric and nitric acid.
  • Electrolytes, the degree of which less than 2% are called weak. These include organic acids, ammonium hydroxide, hydrogen sulfide and carbonic acid as well as a number of reasons, R-, d-, f-elements of periodic system.

Dissociation constant

A quantitative indicator of the ability of a substance to decay into ions is the dissociation constant, also called the equilibrium constant. In simple terms, the equilibrium constant is the ratio of electrolytes decomposed into ions to non-dissociated molecules.

Unlike the degree of dissociation, this parameter does not depend on external conditions and the concentration of the salt solution in the process of hydrolysis. During the dissociation of polybasic acids, the degree of dissociation at each step becomes an order of magnitude less.

The indicator of acid-base properties of solutions

PH or pH \u0026 ndash; measure to determine the acid-base properties of the solution. Water in a limited amount dissociates into ions and is a weak electrolyte. When calculating the pH, use the formula, which is the negative decimal logarithm of the accumulation of hydrogen ions in solutions:

  • For alkaline environments, this figure will be more than seven. For example, [H +] = 10 -8 mol / l, then pH = -lg [10 -8] = 8, that is, pH ˃ 7.
  • For acidic conditions, on the contrary, the pH should be less than seven. For example, [H +] = 10 -4 mol / l, then pH = -lg [10 -4] = 4, that is, pH ˂ 7.
  • For a neutral environment, pH = 7.

Very often, to determine the pH-solutions using the rapid method for indicators, which, depending on the pH, change their color. For a more accurate definition, ionomers and pH meters are used.

Quantitative characteristics of hydrolysis

Hydrolysis of salts, like any other chemical process, has a number of characteristics, in accordance with which the course of the process becomes possible. The most significant quantitative characteristics include the constant and degree of hydrolysis. Let us dwell on each of them.

Degree of hydrolysis

To find out which salts are hydrolyzed and in what quantities, use a quantitative indicator \u0026 ndash; the degree of hydrolysis, which characterizes the completeness of the course of hydrolysis. The degree of hydrolysis is called the part of the substance of the total number of molecules capable of hydrolysis, is written in percentage:

where the degree of hydrolysis is \u0026 ndash; h;

the amount of salt particles subjected to hydrolysis \u0026 ndash; n;

the total amount of salt molecules involved in the reaction \u0026 ndash; N.

Factors affecting the degree of hydrolysis include:

  • constant hydrolysis;
  • temperature, with which the degree increases due to the increased interaction of ions;
  • salt concentration in solution.

Hydrolysis constant

It is the second most important quantitative characteristic. In the general form of the equation of salt hydrolysis can be written as:

MA + NON \u0026 harr; MON + ON

From this it follows that the equilibrium constant and the concentration of water in the same solution are constant values. Accordingly, the product of these two indicators will also be a constant value, which means the hydrolysis constant. In general, Kg can be written as:

where is ON \u0026 ndash; acid,

MON \u0026 ndash; base.

In the physical sense, the hydrolysis constant describes the ability of a particular salt to undergo a process of hydrolysis. This parameter depends on the nature of the substance and its concentration.