Question

What is the common ion effect? Give an example.

Answer 1

Hint: The phenomenon in which ionization of one electrolyte gets suppressed by the presence of another electrolyte is called the common ion effect.

Complete step-by-step answer:
In technical terms we define common ion effect as-
If we have a solution containing several types of ions and equilibrium is achieved, when we add another species containing the same ion, to the existing solution, reduction in the degree of dissociation of the first species is observed. This effect is called the common ion effect.

Let us take an example of a Silver chloride solution in which equilibrium is achieved.
\[\text{AgC}{{\text{l}}_{(aq)}}\leftrightharpoons\text{ }\text{ A}{{\text{g}}^{+}}\text{ + C}{{\text{l}}^{-}}\]
If we add sodium chloride to this, naturally we will get more concentration of Chlorine. Due to this excess chlorine, the equilibrium will get shifted to the left side and hence more AgCl will be formed. This will also decrease the solubility of silver chloride.

Additional information:
Common ion effect is a consequence of Le Chatelier’s principle for equilibrium reaction of ionic association or dissociation reaction.
Common ion effect also influences the solubility of a compound. However, the advantage of this phenomenon can also be taken. Aquifers which contain chalk and limestone, a common ion effect is used in these to obtain drinking water.
Calcium carbonate is sparingly soluble in water, and it can be precipitated out by adding sodium chloride in the solution. In this way, the common ion effect is used in treatment of water.

Note: Compounds of transition metal ions do not show common ion effects as they have a tendency to form complex ions.