Question

What is common ion effect? Give an example.

Answer 1

Common ion effect

• The term "common-ion effect" describes the decrease in solubility of an ionic precipitate caused by the addition of a soluble compound that has an ion with the precipitate to the solution.
• The common ion effect is seen when weak and strong electrolytes are mixed.
• Since ${\mathrm{NH}}_4\mathrm{OH}$ is a weak electrolyte and ${\mathrm{NH}}_4\mathrm{Cl}$ is a strong electrolyte, the common ion effect is seen.

Example of common ion effect:

• Solutions containing both Sodium chloride $\left(\mathrm{NaCl}\right)$ and Silver chloride $\left(\mathrm{AgCl}\right)$ also contain a common ion, ${\mathrm{Cl}}^{-}$ ion.
• The equilibrium is as follows:

$\mathrm{AgCl}\rightleftharpoons {\mathrm{Ag}}^{+}+{\mathrm{Cl}}^{-}$

• The equilibrium will be moved to the left to generate more solid $\mathrm{AgCl}$.
• As a result, the solubility of $\mathrm{AgCl}$ will decrease.