Explain common ion effect with an example.

Explanation

(a) The dissociation of a weak acid ( ` CH_(3)COOH)` is suppressed in the presence of a salt ` (CH_(3) COONa)` containing an ion common to the weak electrolyte. It is a called the common ion effect.
Eg : Acetic acid is a weak acid. It is not completely dissociated in aqueous solution and hence the following equilibrium exists.
` CH_(3) COOH_(aq) iff H_(aq)^(+) + CH_(3) COO_(aq)^(-)`
However, the added salt, sodium acetate, completely dissociates to produce ` Na^(+) and CH_(3)COO^(-)` ion.
` CH_(3)COONa_(aq) to Na_(aq)^(+) + CH_(3)COO_(aq)^(-)`
Hence, the overall concentration of ` CH_(3)COO_(aq)^(-)` is increased, and the acid dissoication equilbrium is disturted . According to Le chatelier's principle, the excess ` CH_(3)COO^(-)` ions combines with ` H^(+)` ions to produce much more unionized ` CH_(3)COOH` i.e, the eqilibrium will shift towards the left. In other words, the dissociation of ` CH_(3)COOH` is suppressed.