Hydrolysis of Salts

Salt Hydrolysis definition chemistry is Neutralization reaction between an acid and a base results in the formation of salt. Most salts ionize in water to form acids and bases. These salts dissociate into their respective ions, which exist as hydrated ions in aqueous solutions or can interact with the hydrogen or hydroxyl ion of the solvent to form the resulting bases and acids. The mechanism of interaction between the ions of the salts with that of water is called hydrolysis of salts. As per the salt hydrolysis definition and the extent of hydrolysis, salts can be categorized as:

• Basic salt

• Acidic Salt

• Neutral or amphoteric salts.

The formation of these salts depends on the type of salt hydrolysis. They are: 

Salts of a Strong Base and a Strong acid

Salts that are produced by the reaction between a strong base and a strong acid are neutral in nature. The bonds formed between the anion and the cation are strong, and they do not break apart in solution. The electronic distribution of both the cation and the anion after the formation of the bond between them is such that both the ions complete their most stable electronic configuration. These salts are formed by ionic or electrostatic bonds, which cannot be broken in solution. These salts do not get hydrolyzed, but they can get hydrated. Since these salts do not carry any charge, they are called neutral or amphoteric salts. Sodium chloride (NaCl) is a classic example of neutral salt. 

Salts of a Weak Acid and a Strong Base

Salts that are formed by the neutralization of weak acid and strong base have a positive charge. These salts are called a basic salt. These salts hydrolyze easily in water. For example, Sodium acetate (CH3COONa). The salt hydrolysis formula of Sodium acetate is as follows:

CH3COONa (aq) → CH3COO- (aq) + Na+ (aq)

The acetate ion, thus formed, interacts with the hydrogen ion in water to form acetic acid and hydroxyl ion.

CH3COO- (aq) +  HOH → CH3COOH+ OH- (aq)

Since acetic acid is a weak acid, it does not get ionized in water. However, the increase in the OH- ion results in the increased basicity of the solution. Therefore these salts are called basic salts. The pH of the aqueous solution of these salts is always more than 7.

Salts of a Weak Base and a Strong Acid

Salts formed from neutralization reaction between weak bases and strong acids are found to be acidic in nature. Ammonium chloride is an example of acidic salt. 

NH4Cl(aq) → Cl−(aq) + NH4+(aq)

Ammonium ions thus formed then interacts with the hydroxyl ions in the water molecule to form ammonium hydroxide. The hydrogen ions thus formed results in the acidic nature of the solution. Therefore an aqueous solution of these salts is acidic and has a pH of less than 7.

NH+4(aq) + H2O ⇋ NH4OH(aq) + H+(aq)

Salts of Weak Base and Weak Acid

Salts resulting from the neutralization reaction between a weak acid and weak base can be mildly basic, mildly acidic, or neutral depending on the nature of the bases and acids involved. The degree of hydrolysis and ionization of water is independent of the concentration of the solution. An example of such salts is CH3COONH4. The mechanism of formation of such ions are as follows:    

CH3COO− + NH+4 + H2O ⇋ CH3COOH + NH4OH

The pH of the aqueous solution of such salts are denoted as 

pH= 7 + 12 (pKa – pKb)

That is, the pH depends on the pKa and pKb of the acid and the base, respectively.