Strong and Weak Acids and Bases

A strong acid  or a strong base completely ionizes (dissociates) in a solution. 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.

Strong acid:                   HA + H₂O  →  A^-(aq)  +  H₃O⁺(aq)

Strong base:                 BOH + H₂O  →  B⁺(aq)  +  OH^-(aq)

Examples of strong acids and bases are given in the table below.  In aqueous solution, each of these essentially ionizes 100%.

A weak acid or a weak base only partially dissociates. At equilibrium, both the acid and the conjugate base are present in solution

Weak acid:                 AH  + H₂O  ↔   A^-(aq)  +  H₃O⁺(aq)

   Weak base:              BOH  +   H₂O   ↔    B⁺(aq)   +   OH^-(aq)   or

                                    B  +  H₂O   ↔    BH⁺(aq)   +  OH^-(aq)

Examples of weak acids and bases are given in the table below.

Stronger acids have a larger acid dissociation constant (Ka) and a smaller logarithmic constant (pKa = −log Ka) 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.

See also Acid-base properties of aqueous solutions of salts with ions from both acids and bases, Buffer solutions,  pKa of inorganic acids and bases, pKa of phenols, alcohols and carboxylic acids and pKa of amines, diamines and cyclic organic nitrogen compounds, as well as Acid and base pH indicators or pH values of some solutions of acids and bases.