Hydrolysis of Salts and the pH of their Solutions

Hydrolysis of Salts and the pH of their Solutions

When an acid and a base react in definite proportions, they form a salt. In water, these salts undergo ionization, releasing cations and anions. Some of these ions remain simply hydrated — surrounded by water molecules but unchanged. Others, however, interact chemically with water itself, a process called hydrolysis. This interaction reforms the original acid or base from which the salt was derived, and it directly affects the pH of the solution.

The key distinction lies in the strength of the parent acid and base. Ions from strong acids (like $\text{Cl}^-$, $\text{Br}^-$, $\text{NO}_3^-$, $\text{ClO}_4^-$) and strong bases (like $\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{Ba}^{2+}$) do not hydrolyse. They simply get hydrated. Therefore, a salt formed from a strong acid and a strong base — for example, $\text{NaCl}$ — gives a neutral solution with pH = 7.

All other combinations of salt do undergo hydrolysis. The NCERT section examines three specific types, each with a distinct effect on pH.

Type 1: Salt of a Weak Acid and a Strong Base

Consider sodium acetate, $\text{CH}_3\text{COONa}$. This salt comes from acetic acid ($\text{CH}_3\text{COOH}$, a weak acid) and sodium hydroxide ($\text{NaOH}$, a strong base). In water, it dissociates completely:

The sodium ion ($\text{Na}^+$) is from a strong base and does not hydrolyse. The acetate ion ($\text{CH}_3\text{COO}^-$), however, is the conjugate base of a weak acid. It reacts with water:

This is the hydrolysis reaction. Acetic acid is weak ($K_a = 1.8 \times 10^{-5}$), so it remains largely unionised. The production of hydroxide ions ($\text{OH}^-$) increases the solution's basicity. The pH of such a solution is greater than 7.

Type 2: Salt of a Strong Acid and a Weak Base

Now take ammonium chloride, $\text{NH}_4\text{Cl}$. It is formed from hydrochloric acid ($\text{HCl}$, strong) and ammonium hydroxide ($\text{NH}_4\text{OH}$, weak). It dissociates fully:

The chloride ion ($\text{Cl}^-$) is from a strong acid and does not hydrolyse. The ammonium ion ($\text{NH}_4^+$), the conjugate acid of a weak base, undergoes hydrolysis:

Ammonium hydroxide is weak ($K_b = 1.77 \times 10^{-5}$), so it remains largely unionised. The production of hydrogen ions ($\text{H}^+$) makes the solution acidic. The pH of an $\text{NH}_4\text{Cl}$ solution is less than 7.

Type 3: Salt of a Weak Acid and a Weak Base

The most interesting case is a salt like ammonium acetate, $\text{CH}_3\text{COONH}_4$, formed from acetic acid (weak) and ammonium hydroxide (weak). Both ions hydrolyse simultaneously:

Both products — acetic acid and ammonium hydroxide — are weak and remain partially dissociated. The system also involves the dissociation equilibria of water, the weak acid, and the weak base:

The pH of such a solution depends on the relative strengths of the weak acid and weak base. The textbook gives the result without a full derivation: