Acid strength

Acids and bases
Acceptor number Acid Acid–base reaction Acid–base homeostasis Acid strength Acidity function Amphoterism Base Buffer solutions Dissociation constant Donor number Equilibrium chemistry Extraction Hammett acidity function pH Proton affinity Self-ionization of water Titration Lewis acid catalysis Frustrated Lewis pair Chiral Lewis acid ECW model
Acid types
Brønsted–Lowry Lewis Mineral Organic Oxide Strong Superacids Weak Solid
Base types
Brønsted–Lowry Lewis Organic Oxide Strong Superbases Non-nucleophilic Weak
v t e

Acid strength is the tendency of an acid, symbolised by the chemical formula ${\displaystyle {\ce {HA}}}$, to dissociate into a proton, ${\displaystyle {\ce {H+}}}$, and an anion, ${\displaystyle {\ce {A-}}}$. The dissociation or ionization of a strong acid in solution is effectively complete, except in its most concentrated solutions.

${\displaystyle {\ce {HA -> H+ + A-}}}$

Examples of strong acids are hydrochloric acid ${\displaystyle {\ce {(HCl)}}}$, perchloric acid ${\displaystyle {\ce {(HClO4)}}}$, nitric acid ${\displaystyle {\ce {(HNO3)}}}$ and sulfuric acid ${\displaystyle {\ce {(H2SO4)}}}$.

A weak acid is only partially dissociated, or is partly ionized in water with both the undissociated acid and its dissociation products being present, in solution, in equilibrium with each other.

${\displaystyle {\ce {HA <=> H+ + A-}}}$

Acetic acid (${\displaystyle {\ce {CH3COOH}}}$) is an example of a weak acid. The strength of a weak acid is quantified by its acid dissociation constant, ${\displaystyle K_{{\ce {a}}}}$ value.

The strength of a weak organic acid may depend on substituent effects. The strength of an inorganic acid is dependent on the oxidation state for the atom to which the proton may be attached. Acid strength is solvent-dependent. For example, hydrogen chloride is a strong acid in aqueous solution, but is a weak acid when dissolved in glacial acetic acid.