Common Ion Effect

common ion effect

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Common Ion Effect
” The decrease in the  degree of dissociation of a weak electrolyte  on addition of a strong electrolyte containing a common ion . This is known as common ion effect”
Dissociation of  acetic acid [CH₃COOH ]decreases in presence of sodium acetate
Ex.1–

CH₃COONa—–> CH₃COO^– + Na⁺
CH₃COOH—–> CH₃COO^– + H⁺
Weak Electrolyte      Common ion
A/c to law of mass action,

CH₃COONa is a strong electrolyte, it is almost completely ionised into   acetate ion [CH₃COO^– ]&  sodium ion [Na⁺ ].When CH₃COONa is added to the solution of acetic Acid the concentration of CH₃COO^– increases.

Concentration of (H⁺) ions depend upon the concentration of (CH₃COO^–)
when Concentration  of (CH₃COO^–) increases then concentration of H⁺ decreases. It means dissociation of CH₃COOH is decreased.
Ex.2- Dissociation of NH₄OH decreases in presence of NH₄Cl. Explain

NH₄Cl               —–>  NH₄⁺          +  Cl^–

NH₄OH               —–>   NH₄⁺       +   OH^–

weak electrolyte            Common ion
A/c to Law of mass action-

NH₄Cl is a strong electrolyte & ionises to give   NH₄⁺ &  Cl^–. After the addition of NH₄OH, the Concentration of  NH₄⁺ increases .
Concentration of (OH^–) depends upon  the concentration of NH₄⁺. So concentration of [OH^–] decreases. Hence dissociation of NH₄OH decreases.

Hence in presence of common ion, dissociation of weak electrolyte decreases.