Brønsted–Lowry acid–base theory

The Brønsted–Lowry acid–base theory (also called proton theory of acids and bases)^[1] is one of the two main definitions of acids and bases in modern chemistry, alongside the Lewis acid-base theory.^[2] Johannes Nicolaus Brønsted and Thomas Martin Lowry developed the theory in 1923.^[3][4]

A chemical is a Brønsted–Lowry acid if it can give a hydrogen ion to another chemical,^[5] and a chemical is a Brønsted–Lowry base^[6] if it can take a hydrogen ion from another chemical. When an acid loses a hydrogen, it becomes its conjugate base, and when a base gains a hydrogen ion, it becomes its conjugate acid.^[7]

According to these definitions, an acid-base reaction always takes the form

AH + B → A⁻ + BH⁺

where AH is a Brønsted-Lowry acid, B is a Brønsted-Lowry base, A⁻ is the acid's conjugate base, and BH⁺ is the base's conjugate acid.

Comparison to Arrhenius theory

The Brønsted-Lowry theory (adds to or) expands the Arrhenius theory to understand amphoteric compounds, compounds like ammonia that act like a base without containing hydroxide, and solvents other than water.

In the Arrhenius theory, sodium hydroxide is a base, and hydrochloric acid is an acid, which has the Arrhenius neutralization reaction

NaOH + HCl → NaCl + H₂O

that produces a salt (sodium chloride) and water.

In the Brønsted-Lowry theory, sodium hydroxide is no longer considered a base, but rather an equivalent of the actual base, the hydroxide ion OH⁻. When sodium hydroxide dissolves, it splits into OH⁻ and Na⁺. The sodium is considered a spectator ion that does not participate in the actual reaction. The acid-base reaction is

HCl + OH⁻ → Cl⁻ + H₂O

Here, the chloride ion is the conjugate base of HCl, while water is the conjugate acid of OH⁻. The dissolved ions making sodium chloride is not part of the acid-base reaction.