Common ion effect

The decrease in the ionization of a weak electrolyte by the presence of a common-ion from a strong electrolyte, is called the common ion effect. Ionisation of CH3COOH (weak acid) is decreased by the addition of CH3COONa (CH3COO- being the common ion)
CH3COOH <======> CH3COO- + H+ ………………………..(A)
CH3COONa -----------> CH3COO- + Na+
In the presence of CH3COO- equilibrium (A) shifts in backward direction.
Ionisation of H2S (weak acid) is decreased by the addition of HCl (H+ being the common ion)
H2S <=======> 2H+ + s2-
HCl <=======> H+ + Cl-
Ionisation of NH4OH (weak base) is decreased by the addition of NH4Cl (NH4 + being the common ion)
NH4OH <=======> NH4+ + OH-
NH4Cl --------------> NH4+ + Cl-
Solubility of a sparingly soluble salt is decreased by the addition of common ion. Presence of AgNO3 or KCl decreases the solubility of AgCl.
AgCl <=======> Ag+ + Cl-
AgNO3 <========> Ag+ (common ion) + NO3-
KCl <=======> K+ + Cl-(common ion)
The common ion effect is thus based on Le-Chatelier’s principle in which the stress on the equilibrium that results from raising one of the product concentrations is relieved by shifting the equilibrium to left.

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