What do you mean by common ion effect?

We know that weak acids and weak bases slightly ionize in water and equilibrium is established in their solutions. The phenomenon of suppression of the degree of dissociation of a weak acid or a weak base by the addition of a strong electrolyte containing a common ion is known as common ion effect.

For example: let us consider the ionization of a weak base ammonium hydroxide i.e. NH4OH as:
NH4OH (aq) ⇌ NH4+ (aq) + OH (aq)

The ionization constant for the base will be:
Kb = [NH4+] [OH] / [NH4OH]
If solid NH4Cl is added to the solution, the concentration of NH4+ ions increases. According to Le-Chateliers’s principle, the equilibrium shifts to the left. As a result, the concentration of OH is considerably decreased and the weak base NH4OH becomes even weaker in the presence of its salt.

NH4OH ⇌ NH4+ + OH
(Equilibrium shifts to left side)
NH4Cl ⇌ NH4+ + Cl
This phenomenon is known as common ion effect.

Category: Ionic Equilibria

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