Acid strength

The strength of an acid in Arrhenius definition, the Brønsted-Lowry definition refers to its ability or tendency to donate hydrogen cation. A strong acid is one that completely ionizes (dissociates) in a solution (provided there is sufficient solvent). In water, one mole of a strong acid HA dissolves yielding one mole of H⁺ (as hydronium ion H₃O⁺) and one mole of the conjugate base, A⁻. Essentially, none of the non-ionized acid HA remains. Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO₄), nitric acid (HNO₃) and sulfuric acid (H₂SO₄). In aqueous solution, each of these essentially ionizes 100%.

In contrast, a weak acid only partially dissociates. Examples in water include carbonic acid (H₂CO₃) and acetic acid (CH₃COOH). At equilibrium, both the acid and the conjugate base are present in solution.

Stronger acids have a larger acid dissociation constant (K_a) and a smaller logarithmic constant (pK_a = - log K_a) than weaker acids. The stronger an acid is, the more easily it loses a proton, H⁺. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base.