7.7 Oxyacids
Oxyacids (or oxoacids) are a class of inorganic acids containing oxygen, hydrogen, and another atom. There are many types of oxyacids.
7.7.1 Oxidation State
The strength of an oxyacid decreases with the oxidation state of the element. Below we see a series of oxyacids with the element chlorine. The acidic proton is given in red.
| Name | Formula | Structure | Oxidation State | Ka | pKa |
|---|---|---|---|---|---|
| Perchloric acid | HClO4 |  |
7 | 1 × 108 | –8 |
| Chloric acid | HClO3 |  |
5 | 1 × 101 | –1 |
| Chlorous acid | HClO2 |  |
3 | 1 × 10-2 | 2.0 |
| Hypochlorous acid | HClO |  |
1 | 2.95 × 10-8 | 7.53 |
Notice that the oxidation state decreases for every oxygen that is removed. As the oxidation state of chlorine decreases, the pKa increases (i.e. the strength of the acid decreases). To put another way, as each oxygen is removed, the ability of the acidic hydrogen to dissociate from the complex decreases.
7.7.2 Electronegativity of Central Atom
The electronegativity of the central atom also plays a role in the strength of the oxyacid. As the electronegativity increases, the acid strength increases. This is due to the ability for the central atom to pull surrounding electron density toward itself, thereby increasing the polarity of the O–H bond and making the acidic hydrogen more easily ionizable.
Below is a series of oxyacids with a central halogen atom. The oxidation state for the central atom is 1 for all of these. The electronegativity of the central atom is given.
| Name | Formula | Structure | Electronegativity | Ka | pKa |
|---|---|---|---|---|---|
| Hypochlorous acid | HClO | |
3.16 | 4.0 × 10-8 | 7.40 |
| Hypobromous acid | HBrO |  |
2.96 | 2.8 × 10-9 | 8.55 |
| Hypoiodous acid | HIO |  |
2.66 | 3.2 × 10-11 | 10.5 |
7.7.3 Polyprotic oxyacids
Polyprotic oxyacids contain more than one acidic proton. The strength of the acid decreases with the removal of each successive proton. Consider sulfuric acid, H2SO4. There are two acidic protons. Removal of these protons, in sequence, gives
\[\begin{align*} \mathrm{H_2SO_4}(aq) + \mathrm{H_2O}(l) &\rightleftharpoons \mathrm{H_3O^+}(aq) + \mathrm{HSO_4^-} &&\quad K_{\mathrm{a}} = 1.0\times 10^{3}\\ \mathrm{HSO_4^-}(aq) + \mathrm{H_2O}(l) &\rightleftharpoons \mathrm{H_3O^+}(aq) + \mathrm{SO_4^{2-}} &&\quad K_{\mathrm{a}} = 1.2\times 10^{-2}\\ \end{align*}\]
Notice the dramatic decrease in the equilibrium constant as each proton is removed. H2SO4 is a much stronger acid than HSO4–. The data is tabulated below.
| Name | Formula | Structure | Ka | pKa |
|---|---|---|---|---|
| Sulfuric acid | H2SO4 |  |
1.0 × 103 | –3.00 |
| Hydrogen sulfate | HSO4– |  |
1.2 × 10-2 | 1.92 |
The table below outlines the acid strength of phosphoric acid, H3PO4, and its deprotonated analogues.
| Name | Formula | Structure | Ka | pKa |
|---|---|---|---|---|
| Phosphoric acid | H3PO4 |  |
7.1 × 10-3 | 2.15 |
| Dihydrogen phosphate | H2PO4– |  |
6.3 × 10-8 | 7.20 |
| Hydrogen phosphate | HPO42– |  |
4.2 × 10-13 | 12.38 |