Hydrolysis of salts

physical chemistry; hydrolysis of salts for A-level students

In chemistry, hydrolysis of salts refers to the reaction of a salt with water, resulting in the formation of an acidic or basic solution. Salts are ionic compounds formed from the neutralization of an acid and a base. When a salt dissolves in water, it dissociates into its component ions, which can either be acidic, basic, or neutral.

The hydrolysis of salts can be classified into two categories: acidic hydrolysis and basic hydrolysis, depending on the nature of the salt and the pH of the resulting solution.

1. Acidic hydrolysis: This type of hydrolysis occurs when the salt formed from a weak acid and a strong base is dissolved in water. The anion of the salt acts as a base and reacts with water to produce hydroxide ions and the corresponding acid. For example, when ammonium chloride, NH4Cl, is dissolved in water, it undergoes hydrolysis according to the equation:

NH4Cl + H2O ⇌ NH3 + HCl

The ammonium ion (NH4+) acts as an acid and donates a proton to the water, forming ammonium hydroxide (NH3) and hydronium ion (H3O+). The chloride ion (Cl-) acts as a base and accepts a proton from water, forming hydrochloric acid (HCl) and hydroxide ion (OH-).

2. Basic hydrolysis: This type of hydrolysis occurs when the salt formed from a strong acid and a weak base is dissolved in water. The cation of the salt acts as an acid and reacts with water to produce hydronium ions and the corresponding base. For example, when sodium acetate, NaC2H3O2, is dissolved in water, it undergoes hydrolysis according to the equation:

NaC2H3O2 + H2O ⇌ Na+ + C2H3O2- + H3O+ + OH-

The sodium ion (Na+) and acetate ion (C2H3O2-) remain unchanged, while water reacts with a small fraction of acetate ions to form acetic acid (HC2H3O2) and hydroxide ion (OH-). At the same time, water reacts with a small fraction of hydronium ions to form hydronium ion (H3O+) and hydroxide ion (OH-).

In summary, hydrolysis of salts can either produce acidic or basic solutions, depending on the nature of the salt and the pH of the resulting solution. Understanding the hydrolysis of salts is important in various chemical and biochemical processes, including the buffering capacity of biological fluids and the maintenance of pH in water treatment processes.