Question

Define the term common ion effect. Explain with suitable example.

Answer 1

The common ion effect states that the ionisation of a weak electrolyte is suppressed in presence of a strong electrolyte containing an ion common to the weak electrolyte.

1. Consider a solution of weak acid CH₃COOH. It dissociates slightly as CH₃COOH ⇌ CH₃COO⁻ + H⁺.
2. Suppose a strong electrolyte CH₃COONa is added to CH₃COOH. The strong electrolyte dissociates completely CH₃COONa → CH₃COO⁻ + Na⁺ It therefore, provides high concentration of CH3COO⁻ ion which is common to both. CH₃COOH and CH₃COONa.
3. According to Le-Chatelier principle, increase in the concentration of CH₃ COO⁻ due to addition of CH₃COONa, shifts the equilibrium of dissociation of CH₃COOH to the left.
4. Thus reverse reaction is favoured in which CH3COO⁻ ion combines with H⁺ ion to form unionized CH₃COOH Hence, dissociation of CH₃COOH is supressed due to presence of CH₃COONa containing a common ion  CH₃COO⁻