Question

Carbonic acid gives an acid salt but hydrochloric does not. Explain.

Answer 1

Carbonic acid is a dibasic acid. One molecule of the acid forms two hydronium ions when ionised in water. As dibasic acids have two replaceable hydrogen atoms, they form two types of salts when reacted with a base. They are acid and normal salts.

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:

$$\begin{gathered} \mathrm{NaOH}+{\mathrm{H}}_2{\mathrm{CO}}_3\stackrel{}{\to }{\mathrm{NaHCO}}_3+{\mathrm{H}}_2\mathrm{O} \\ (\mathrm{Acid}\mathrm{salt}) \\ 2\mathrm{NaOH}+{\mathrm{H}}_2{\mathrm{CO}}_3\stackrel{}{\to }{\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O} \\ (\mathrm{Normal}\mathrm{salt}) \end{gathered}$$

Hydrochloric acid is a monobasic acid. ​One molecule of the acid forms one hydronium ion when ionised in water. As monobasic acid has only one replaceable hydrogen atom, it forms only normal salt when reacted with a base.

Ionisation of hydrochloric acid:

$$\begin{gathered} \mathrm{HCl}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{Cl}}^{-} \\ \mathrm{Hydrochloric}\mathrm{Water}\mathrm{Hydronium}\mathrm{ion}\mathrm{Chloride}\mathrm{ion} \\ \mathrm{acid} \end{gathered}$$

Reaction with base:

$$\begin{gathered} \mathrm{NaOH}+\mathrm{HCl}\stackrel{}{\to }\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O} \\ (\mathrm{Normal}\mathrm{salt}) \end{gathered}$$

Thus, carbonic acid gives an acid salt but hydrochloric acid does not.