85 Polyprotic Acids
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Learning Objectives
By the end of this section, you will be able to:
- Extend previously introduced equilibrium concepts to acids and bases that may donate or accept more than one proton
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:
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:
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.
The bicarbonate ion can also act as an acid. It ionizes and forms hydronium ions and carbonate ions in even smaller quantities.
\({K}_{{\text{H}}_{2}{\text{CO}}_{3}}\) is larger than \({K}_{{\text{HCO}}_{3}{}^{\text{−}}}\) by a factor of 104, so H2CO3 is the dominant producer of hydronium ion in the solution. This means that little of the \({\text{HCO}}_{3}{}^{\text{−}}\) formed by the ionization of H2CO3 ionizes to give hydronium ions (and carbonate ions), and the concentrations of H3O+ and \({\text{HCO}}_{3}{}^{\text{−}}\) are practically equal in a pure aqueous solution of H2CO3.
If the first ionization constant of a weak diprotic acid is larger than the second by a factor of at least 20, it is appropriate to treat the first ionization separately and calculate concentrations resulting from it before calculating concentrations of species resulting from subsequent ionization. This approach is demonstrated in the following example exercise.
Ionization of a Diprotic Acid “Carbonated water” contains a palatable amount of dissolved carbon dioxide. The solution is acidic because CO2 reacts with water to form carbonic acid, H2CO3. What are \(\left[{\text{H}}_{3}{\text{O}}^{\text{+}}\right],\) \(\left[{\text{HCO}}_{3}{}^{\text{−}}\right],\) and \(\left[{\text{CO}}_{3}{}^{2-}\right]\) in a saturated solution of CO2 with an initial [H2CO3] = 0.033 M?
Solution As indicated by the ionization constants, H2CO3 is a much stronger acid than \({\text{HCO}}_{3}{}^{\text{−}},\) so the stepwise ionization reactions may be treated separately.
The first ionization reaction is
Using provided information, an ICE table for this first step is prepared:
Substituting the equilibrium concentrations into the equilibrium equation gives
Assuming x << 0.033 and solving the simplified equation yields
The I