The basicity or alkalinity of an acid is a measure of its ability to donate protons (H+ ions). Although strong acids produce high concentrations of H+, this alone is not a measure of their basicity.
The basicity of an acid is determined by the:
- Stability of its conjugate base - acids that yield stable conjugate bases are weak acids.
- Charge on the conjugate base - conjugate bases with higher charge are generally weaker bases than neutral conjugate bases.
- Number of acidic protons - acids that can donate more than one proton per molecule, termed polyprotic acids, are generally more acidic than monoprotic acids.
For example, HCl has a very strong conjugate base, Cl-, which is very stable. In contrast, HF has a weak conjugate base, F-, which is relatively unstable because it is a small, highly electronegative ion with the high hydration energy. HCl is therefore a stronger acid than HF.
Similarly, the polyprotic acid H2SO4 is a stronger acid than the monoprotic acid HCl because it can donate more than one proton. The first proton of H2SO4 dissociates to form H+ and HSO4-, with the HSO4- ion being a relatively weak base. The second proton of H2SO4 is more acidic because the conjugate base SO42- is a weaker base than HSO4-.
The basicity of acids can be measured by their pKa values, which represent the negative log of their acid dissociation constant (Ka). The lower the pKa, the stronger the acid.
For example, HCl has a very low pKa (-7), meaning that it is completely dissociated in water and has a strong conjugate base.
In contrast, HF has a much higher pKa (3.17), indicating that it only dissociates to a limited extent and has a weak conjugate base.
A higher basicity means that an acid can donate a proton (H+) to a stronger base. Acids that can donate multiple protons are called polyprotic acids. The basicity of an acid depends on the strength of the conjugate base.
- Strong acids have weak conjugate bases.
- Weak acids have strong conjugate bases.
The following are some examples of strong and weak acids:
| Strong Acids | Weak Acids |
|---|---|
| HCl | HCN |
| H2SO4 | HF |
| HNO3 | H2CO3 |
| HClO4 | CH3COOH |
The strength or weakness of an acid is also affected by the solvent. For example, HCl is a strong acid in water but is a weak acid in ethanol.
