What is common ion effect ? Explain with an example.

Explanation

Common ion effect is the shift in equilibrium on adding a substance. The substance provides an ionic species (common ion) already present in the dissociation equilibrium. It is based on the Le-chatelier.s principle. The common ion suppresses the dissociation of weak electrolyte by the addition of strong electrolyte. Consider an example of acetic acid dissociation equilibrium represented as :
`CH_(3)COOH(aq) hArr H^(+)(aq) + CH_(3)COO^(-)(aq)` or `HAc(aq) hArr H^(+)(aq) + A c^(-)(aq)`
`K_(a) = [H^(+)][A c^(-)]//[HA c]`
Addition of acetate ions to an acetic acid solution results in decreasing the concentration of hydrogen ions, `[H^(+)]`. Similarly if `[H^(+)]` ions are added then equilibrium mixture moves in the direction of undissociated acetic acid.