Q19.3P - Chemistry: Molecular Nature Of Matter And Change

Question

What is the common-ion effect? How is it related to Le Châtelier’s principle? Explain with equations that include HF and NaF

Step-by-Step Solution

Answer

The solubility of a sparingly soluble salt can be decreased further by adding a common ion to it. 

Step 1: The given buffer mixture

The given buffer solution is HF and NaF. HF is the weak acid and the following equilibrium reaction occurs:

${\text{HF(aq) + H}}_2\text{O(l)}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{ + }}{\text{(aq) + F}}^{\text{ - }}\text{(aq) }$

When NaF is added, to this solution, then the concentration of ${\mathrm{F}}^{-}\left(\mathrm{aq}\right)$ increases.

$\text{NaF(aq)}\to {\text{Na}}^{\text{ + }}{\text{(aq) + F}}^{\text{ - }}\text{(aq)}$

Step 2: LeChatlier’s principle

When NaF is added, to this solution, then the concentration of ${\mathrm{F}}^{-}\left(\mathrm{aq}\right)$ increases.

So, the right side of the equilibrium, the concentration is more. According to LeChatleir’s principle, to nullify this change the equilibrium will move towards left side. Hence, the dissociation of HF decreases further.

This is called common ion effect. The solubility of a sparingly soluble salt can be decreased further by adding a common ion to it.