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

Hydrolysis of salts occurs in the case of binding of one water ion to form hardly soluble or slightly dissociated compounds due to displacement of the dissociation equilibrium. For the most part, this process is reversible and increases with dilution or increase in temperature.

Hydrolysis of salts

To find out which salts are hydrolyzed, it is necessary to know what strengths and acids were used in its formation. There are several types of their interactions.

Preparation of salt from a base and an acid of weak strength

Examples are aluminum and chromium sulphide, as well as ammonium acidate and ammonium carbonate. These salts, when dissolved in water, form bases and poorly dissociating acids. In order to trace the reversibility of the process, it is necessary to formulate the equation for the hydrolysis of salts:

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.

The 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 decay constant of acetic acid is less than the constant of ammonium hydroxide, 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 proceeds to the end.

Consider an example of irreversible hydrolysis:

Aluminum sulphate + water = aluminum hydroxide + hydrogen sulphide

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

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

This type of hydrolysis describes decomposition reactions of aluminum sulfate, copper chloride or bromide, as well as ferric chloride or ammonium chloride. 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 iron chloride salts takes the form:

Fe 2+ + H2  O + 2Cl - \u0026 lt; / RTI \u0026 gt; Fe (OH) + + H + + 2Cl -

The second stage of hydrolysis:

Due to the deficiency of hydroxyl group ions and the accumulation of hydrogen ions, hydrolysis of FeCl2  proceeds through the first stage. A strong hydrochloric acid and a weak base form. \u0026 Ndash; ferric hydroxide. In the case of similar reactions, the medium is acidic.

Non-hydrolyzing salts obtained by the reaction of strong bases and acids

Examples of such salts can be calcium or sodium chloride, potassium sulfate and rubidium bromide. However, these substances do not hydrolyze, since they have a neutral medium when dissolved in water. In this case, the only slightly dissociating substance is water. To confirm this statement, we can formulate the equation for the hydrolysis of sodium chloride salts with the formation of hydrochloric acid and sodium hydroxide:

NaCl + H2  About \u0026 harr; NaOH + HCl

Reaction in ionic form:

Na + + Cl - + H2  About \u0026 harr; Na + + OH - + H + + Cl -

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

In this case, the hydrolysis of the salts proceeds through the anion, which corresponds to the alkaline medium of the hydrogen index. Examples include acetate, sodium sulfate and carbonate, potassium silicate and potassium sulphate, and sodium cyanide. For example, we will compose ion-molecular equations for the hydrolysis of salts of sulfide and sodium acetate:

Dissociation of sodium sulphide:

Na2  S \u0026 harr; 2Na + + S 2-

The first stage of hydrolysis of the polybasic salt takes place on the cation:

Ion recording:

S 2- + H2  About \u0026 harr; HS - + OH -

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

Let's consider one more hydrolysis reaction with the example of sodium acetate:

Sodium Acetic Acid + water \u0026 harr; acetic acid + sodium hydroxide.

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

Equilibrium of the reaction 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 not consumed all, while irreversible processes proceed with the total consumption of the substance. This is due to the shift in the equilibrium of the reactions, which is based on changes in physical characteristics, such as pressure, temperature and mass fraction of the reagents.

According to the concept of the Le Chatelier principle, the system will be considered as an equilibrium system until one or more external conditions for the process are changed. 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 Le Chatelier principle, by means of which it is possible to weaken or intensify the course of the process.

Enhancement of hydrolysis

Hydrolysis can be strengthened to complete irreversibility in several ways:

  • Increase the rate of formation of OH- and H + ions. To do this, heat the solution, and by increasing the absorption of heat by water, i.e., endothermic dissociation, this index rises.
  • Add water.
  • Transfer one of the products into a gaseous state or bind into a heavily soluble substance.

Suppression of hydrolysis

Suppress the process of hydrolysis, as well as strengthen, can be in several ways.

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

Mutual enhancement of hydrolysis

Mutual strengthening of hydrolysis is applied in the event that the system has become equilibrium. Let us analyze a concrete example, where the systems in different vessels become equilibrium:

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

Both systems are poorly hydrolyzed, so if they mix with each other, there will be a binding of hydroxyls and hydrogen ions. As a result, we obtain the molecular equation of hydrolysis of salts:

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

By assuming Le Chatelier, the equilibrium of the system will shift to the side of the reaction products, and 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 through the anion and the other through the cation.

Hydrolysis by anion

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

Mostly, this method of hydrolysis affects salts that are formed by the interaction of a strong hydroxide and a weak acid. An example of salts decomposing over the anion can be sulfate or sodium sulfite, as well as potassium carbonate or phosphate. The hydrogen index is more than seven. As an example, let us analyze the dissociation of sodium acetic acid:

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

The cation of dissociated sodium acetic acid, formed by a strong base, can not react with water.

The acid anions react easily with H molecules2  ABOUT:

Consequently, the hydrolysis is carried out by the anion, and the equation takes the form:


In the case that polybasic acids are hydrolyzed, the process takes place in several stages. Under normal conditions, such substances are hydrolyzed in the first stage.

Hydrolysis by cation

Cationic hydrolysis is mainly affected by salts formed by the interaction of a strong acid and a base of low strength. An example is ammonium bromide, copper nitrate, and also zinc chloride. In this case, the medium in solution under hydrolysis corresponds to less than seven. Let us consider the process of hydrolysis over a cation with the example of aluminum chloride:

In an aqueous solution, it dissociates into an anion \u0026 ndash; 3Cl - and the 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+ + NOH = AlOH 2+ + H +

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

AlCl3 + H2  O = AlOHCl + HCl

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

Degree of dissociation

Any hydrolysis reaction of salts is characterized by the degree of dissociation, which shows the ratio between the total number of molecules and molecules that can transform into the ionic state. The degree of dissociation is characterized by several indicators:

  • The temperature at which hydrolysis is carried out.
  • Concentration of 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 dissolve in different solvents show 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 measure of the possibility 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 saline during hydrolysis. With the dissociation of polybasic acids, the degree of dissociation at each stage becomes an order of magnitude smaller.

The index of acid-base properties of solutions

Hydrogen index or pH \u0026 ndash; A measure for determining the acid-base properties of a solution. Water in a limited amount dissociates into ions and is a weak electrolyte. When calculating the hydrogen index, a formula is used that is a negative decimal logarithm of the accumulation of hydrogen ions in solutions:

  • For an alkaline medium, this figure will be more than seven. For example, [H +] = 10 -8 mole / L, then pH = -lg [10 -8] = 8, that is pH ≤ 7.
  • For an acidic environment, on the contrary, the hydrogen index 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 medium, pH = 7.

Very often an express method for the determination of pH solutions is used for indicators that, depending on the pH, change color. Ionomers and pH meters are used for a more accurate determination.

Quantitative characteristics of hydrolysis

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

Degree of hydrolysis

To find out which salts are hydrolyzed and in what quantity, the quantitative index is used \u0026 ndash; degree of hydrolysis, which characterizes the completeness of the course of hydrolysis. The degree of hydrolysis is called the part of the substance from the total number of molecules capable of hydrolysis, recorded as a percentage:

where the degree of hydrolysis \u0026 ndash; h;

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

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

Factors influencing the degree of hydrolysis include:

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

The hydrolysis constant

It is the second most important quantitative characteristic. In general, the equations of hydrolysis of salts can be written as:

MA + NON \u0026 lt; / RTI \u0026 gt; MON + IN

It follows that the equilibrium constant and the water concentration in the same solution are constant quantities. Accordingly, the product of these two indicators will also be a constant value, which means the hydrolysis constant. In general form, Kr can be written as:

where AT \u0026 ndash; acid,

MON \u0026 ndash; base.

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