Ncert Exercises & Solutions

What do you understand by the term 'auto-protolysis' of water? What is its significance?

Auto-protolysis means self ionisation of water.

\underset{{Acid}_{1}}{H_{2} O (l)} + \underset{{Base}_{2}}{H_{2} O (l)} ⇌ \underset{{Acid}_{2}}{H_{3} O^{+}} + \underset{{Base}_{1}}{{OH}^{-} (aq)}

Due to auto-protolysis, water is amphoteric in nature. It reacts with both acids and bases. e.g.,

\underset{{Acid}_{1}}{H_{2} O (l)} + \underset{{Base}_{2}}{{NH}_{3} (aq)} \to \underset{{Acid}_{2}}{{NH}_{4}^{+} (aq)} + \underset{{Base}_{1}}{{OH}^{-} (aq)}

\underset{{Base}_{1}}{H_{2} O (l)} + \underset{{Acid}_{2}}{H_{2} S (aq)} \to \underset{{Acid}_{1}}{H_{3} O^{+} (aq)} + \underset{{Base}_{2}}{{HS}^{-} (aq)}