Question

Define or explain the meaning of the term 'acid salt'.

Answer 1

An acid salt is formed by the partial neutralisation of a dibasic or tribasic acid with replaceable hydrogen atoms present in it. For example, sodium bicarbonate or ${\mathrm{NaHCO}}_3$ is formed by the reaction between dibasic carbonic acid and sodium hydroxide. Let us now understand how an acid salt is formed in a dibasic acid and not in a monobasic acid.

Carbonic acid is a dibasic acid. One molecule of the acid generates two hydronium ions when ionised in water. A dibasic acid has two replaceable hydrogen atoms that form two types of salts when reacted with base: an acid salt and a normal salt.

Ionisation of carbonic acid:

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{HCO}}_3^{-} \\ \mathrm{Carbonic}\mathrm{acid}\mathrm{Bicarbonate}\mathrm{ion} \\ {\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CO}}_3^{2-} \\ \mathrm{Bicarbonate}\mathrm{ion}\mathrm{Carbonate}\mathrm{ion} \end{gathered}$$

Reaction with base:

NaOH + H₂CO₃ → NaHCO₃ + H₂O
Acid salt

2NaOH + H₂CO₃ → Na₂CO₃ + H₂O
Normal salt