Common Ion Effect

The phenomenon in which the degree of dissociation of a weak electrolyte is suppressed by the addition of a strong electrolyte having an ion common to weak electrolyte is known as common ion effect. This phenomenon is a direct application of Le Chatelier's Principle.

Let us consider dissociation of a weak electrolyte (acetic acid)-

CH₃COOH ⇌ CH₃COO⁻ + H⁺

The equilibrium constant-

K_a = [CH₃COO⁻] [H⁺]/[CH₃COOH]

Now, if sodium acetate is added to this solution-

The concentration of CH₃COO⁻ ion in the solution increases and thus, in order to have K_a constant [H⁺] must decrease or the concentration of undissociated acetic acid must increase. In other words, the dissociation of acetic acid is suppressed on addition of CH₃COONa to its solution.

Summary: The presence of the "common ion" effectively "pushes" the reaction backward, making the weak acid even less dissociated than it would be on its own.

Test Your Knowledge

Which of the following pairs would demonstrate the Common Ion Effect?

A. HCl and NaCl
B. NaOH and KCl
C. CH₃COOH and HCl
D. NH₄OH and NH₄Cl

View Answer

D. NH₄OH and NH₄Cl
Ammonium hydroxide is a weak base and ammonium chloride is a strong electrolyte providing the common NH₄⁺ ion.

What happens to the dissociation of acetic acid (CH₃COOH) when sodium acetate (CH₃COONa) is added to the solution?

A. Sodium acetate reacts with water to form more acid
B. The dissociation is suppressed
C. The pH of the solution decreases significantly
D. The dissociation increases

View Answer

B. The dissociation is suppressed
The addition of a common ion (acetate) shifts the equilibrium toward the undissociated acid side, reducing dissociation.