Question

Explain the common ion effect on dissociation of a weak acid.

Answer 1

(1) Consider the dissociation or ionisation of a weak acid, CH₃COOH in its solution.

CH₃COOH_(aq) ⇌ CH₃COO^-_(aq) + H⁺_(aq)

The dissociation constant Ka for CH₃COOH will be,

K_a = \(\frac{[CH_3COO^-]\times [H^+]}{[CH_3COOH]}\)

K_a is constant for CH₃COOH at constant temperature.

(2) If a strong electrolyte like salt CH₃COONa is added to the solution of CH₃COOH, then on dissociation it gives a common ion CH₃COO^-.

CH₃COONa → CH₃COO^- + Na⁺

(3) Due to common ion CH₃COO^-, overall concentration of CH₃COO^- in the solution is increased, which increases the ratio, \(\frac{[CH_3COO^-]\times [H^+]}{[CH_3COOH]}\).

In order to keep this ratio constant, the concentration of H⁺ is decreased, by shifting the equilibrium to the left hand side according to Le Chatelier’s principle.

(4) Thus the ionisation of a weak acid is suppressed by a common ion.