Common Ion Effect

When a strong electrolyte is a added to the solution of a weak electrolyte having common ion, the ionization of weak electrolyte is highly suppressed. This phenomenon is known as common ion effect. Let us consider the ionization of a weak electrolyte NH4OH.

$$\ce{NH4OH<=>NH4^+ + OH^-}$$

At equilibrium, we can calculate the value of ionization constant (K) of this electrolyte by using law of mass action.

$$\text{Ionization Constant(K)} = \frac{[NH_4^+] [OH^-]}{[NH_4OH]}$$

Now, if a strong electrolyte with a common ion (say NH4OH) is added to the above equilibrium, then the concentration of NH4+ ions in the solution is increased due to the dissociation of NH4Cl as given below:

$$\ce{NH4Cl->NH4^+ + Cl^-}$$

Due to increase in the concentration of NH4+ ions in the solution, the equilibrium gets disturbed. The disturbed equilibrium tries to regain equilibrium according to Le-Chatelier’s principle. According to Le-Chatelier, the system can regain its equilibrium when the concentration of NH4+ ions is decreased. For this, OH ion in the solution combines with NH4+ ions to form ammonium hydroxide (NH4OH) as follows:

$$\ce{OH- + NH4OH+ <=> NH4OH}$$

Then the degree of ionization of NH4OH decreases and the dissociated NH4OH in the solution regenerates due to the addition of NH4Cl that has common ion NH4+. So, this is a good example of common ion effect.

Common ion effect is also observed when CH3COONa is added to CH3COOH solution.  Similarly when KCl is added to the NaCl solution this phenomenon is observed.

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