2. General notion of Hydrolysis

When various substances are dissolved in water, chemical reaction of an exchange nature often occurs. Such processes are grouped under the heading of hydrolysis. In general, hydrolysis is an exchange reaction of substances with water. The following discussion will be confined to the hydrolysis of salts, which, in practice, is by far the most common case of hydrolysis.

When a salt formed by a strong base and a strong acid, such as NaCl, dissolves, the water dissociation equilibrium does not shift, since the ions of such a salt do not form slightly dissociated products with H⁺ and OH^- ions. Therefore, in the system

NaCl + HOH ↔ NaOH + HCl

the only remaining slightly dissociated compound is water itself. As a result, the equilibrium of the reaction is almost wholly shifted to the left, i.e., practically no hydrolysis of NaCl occurs, and there is no appreciable surplus of either H⁺ or OH^- ions in the solution.

It is different when a salt of a strong base and a weak acid (CH₃COONa), or vice versa, (NH₄Cl) dissolves. In the former case, the H⁺ will be partially bound, while in the latter, the OH^- ion will be partially bound due to the equations:

CH₃COO^- +HOH ↔ CH₃COOH + NaOH;

NH₄⁺ + HOH ↔ NH₄OH + H⁺.

Since both CH₃COOH and NH₄OH are much more dissociated than water, both these equilibria are greatly displaced to the left. Therefore, hydrolysis of the salts in accordance with the equations

CH₃COONa +HOH ↔ CH₃COOH + NaOH;

NH₄Cl + HOH ↔ NH₄OH + HCl

takes place only to an insignificant extent. Nevertheless, the first solution contains a certain surplus of OH^- ions, while the second solution contains a certain surplus of H⁺ ions.

Since both CH₃COOH and NH₄OH are much more dissociated than water, both these equilibria are greatly displaced to the left. Therefore, hydrolysis of the salts in accordance with the equations

CH₃COONH₄ +HOH ↔ CH₃COOH + NH₄OH

should be shifted further to the right. In other words, the hydrolysis of a salt formed by a weak base and a weak acid will be, generally speaking, more complete than if only one of these substances is slightly dissociated.

The acidity or alkalinity of solutions of such salts depends on the relative strength of the acid and the base. If their strengths are equal, the solution may be neutral, which is the case, for example, in the hydrolysis, of CH₃COONH₄. Thus, neutrality of a solution does not, in itself, prove that the salt is not hydrolyzed.

In practice, one mostly has to deal with the hydrolysis of salts with multiply charged ions of the weak constituent (acid or base) and singly charged ions of the strong constituent. Hydrolysis of such compounds, e.g., CuCl₂ and Na₂CO₃ results, as a rule, in basic or acidic salts, respectively:

CuCl₂ + HOH ↔ Cu(OH)Cl + HCl

or Cu²⁺ + HOH ↔ Cu(OH)⁺ + H⁺

Na₂CO₃ + HOH ↔ NaHCO₃ + NaOH

or CO₃^2- + HOH ↔ HCO₃^- + OH^-.

Hydrolysis of such salts does not usually proceed any further, i.e., to formation of free weak base or acid, owing to the accumulation of H⁺ or OH^- ions, respectively, in solution. Exceptions are those cases where the basic or acidic properties of the multivalent constituent are extremely weak. In such case hydrolysis often proceeds practically to completion.