Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:

$$HCl(aq)+H_2O(l)⟶H_3O^+(aq)+Cl^-(aq)$$ $$HNO_3(aq)+H_2O(l)⟶H_3O^+(aq)+NO_3^-(aq)$$ $$HCN(aq)+H_2O(l)⇌H_3O^+(aq)+CN^-(aq)$$

Even though it contains four hydrogen atoms, acetic acid, CH3CO2H, is also monoprotic because only the hydrogen atom from the carboxyl group (COOH) reacts with bases:

Similarly, monoprotic bases are bases that will accept a single proton.

Diprotic acids contain two ionizable hydrogen atoms per molecule; ionization of such acids occurs in two steps. The first ionization always takes place to a greater extent than the second ionization. For example, sulfuric acid, a strong acid, ionizes as follows:

$$\text{First ionization: }H_2SO_4(aq)+H_2O(l)⇌H_3O^+(aq)+HSO_4^-(aq)\qquad K_{a1}=\text{more than }10^2\text{; complete dissociation}$$ $$\text{Second ionization: }HSO_4^-(aq)+H_2O(l)⇌H_3O^+(aq)+SO_4^{2-}(aq)\qquad K_{a2}=1.2×10^{-2}$$

This stepwise ionization process occurs for all polyprotic acids. Carbonic acid, H2CO3, is an example of a weak diprotic acid. The first ionization of carbonic acid yields hydronium ions and bicarbonate ions in small amounts.

First Ionization: $$H_2CO_3(aq)+H_2O(l)⇌H_3O^+(aq)+HCO_3^-(aq)\qquad K_{H_2CO_3}=\frac{[H_3O^+][HCO_3^-]}{[H_2CO_3]}=4.3×10^{-7}$$

The bicarbonate ion can also act as an acid. It ionizes and forms hydronium ions and carbonate ions in even smaller quantities.

Second Ionization: $$HCO_3^-(aq)+H_2O(l)⇌H_3O^+(aq)+CO_3^{2-}(aq)\qquad K_{HCO_3^-}=\frac{[H_3O^+][CO_3^{2-}]}{[HCO_3^-]}=4.7×10^{-11}$$

$K_{H_2CO_3}$ is larger than $K_{HCO_3^-}$ by a factor of 104, so H2CO3 is the dominant producer of hydronium ion in the solution. This means that little of the $HCO_3^-$ formed by the ionization of H2CO3 ionizes to give hydronium ions (and carbonate ions), and the concentrations of H3O+ and $HCO_3^-$ are practically equal in a pure aqueous solution of H2CO3.