4.2.6 Strong and weak acids

Strong acids fully ionise; weak acids only partly ionise

1. A strong acid is completely ionised in aqueous solution, so nearly all its molecules split into ions.
2. Hydrochloric acid ($HCl$), sulfuric acid ($H_2{SO}_4$) and nitric acid (${HNO}_3$) are strong acids.
3. A weak acid is only partly ionised, so only a small fraction of its molecules form ions at any moment.
4. Ethanoic acid (${CH}_{3}COOH$), citric acid and carbonic acid ($H_2{CO}_3$) are weak acids.
5. The ionisation of a weak acid is reversible, and the equilibrium lies well to the left, so few ions form at any moment.
6. $${CH}_{3}COOH(aq)\rightleftharpoons {CH}_3{COO}^{-}(aq)+H^{+}(aq)$$

pH depends on the hydrogen ion concentration

1. A lower pH means a higher hydrogen ion concentration.
2. As the pH falls by 1 unit, the hydrogen ion concentration increases by a factor of 10.
3. At the same concentration, a strong acid has a lower pH than a weak acid because it releases more hydrogen ions.

Concentration is not the same as strength

1. Strength is the degree of ionisation: how completely the acid splits into ions.
2. Concentration is how much acid is dissolved in a given volume of solution.
3. A weak acid can be concentrated, and a strong acid can be dilute; the two ideas are independent.
4. So a dilute strong acid may have a lower pH than a concentrated weak acid.