State whether entropy change is positive or negative in the following examples :(i) Melting of ice(ii) Vaporisation of a liquid(b) Explain ‘common ion effect’ with example.

Answer: (a) (i) positive, (ii) positive; (b) explained below.

(a)(i) Melting of ice: ΔS is positive; the ordered solid changes to the more disordered liquid.

(a)(ii) Vaporisation of a liquid: ΔS is positive; the gas has far greater randomness than the liquid.

(b) Common ion effect: The suppression of the degree of dissociation of a weak electrolyte by the addition of a strong electrolyte having an ion common with it.

Example: Acetic acid is a weak acid: \(\mathrm{CH_3COOH \rightleftharpoons CH_3COO^- + H^+}\). Sodium acetate is a strong electrolyte, fully dissociated: \(\mathrm{CH_3COONa \rightarrow CH_3COO^- + Na^+}\). Adding sodium acetate raises [CH₃COO⁻]; by Le Chatelier's principle the equilibrium of acetic acid shifts to the left, so its dissociation and [H⁺] decrease and the pH rises. Similarly NH₄Cl suppresses the dissociation of NH₄OH (used in group III of qualitative analysis).