Brønsted–Lowry acid–base theory

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

A chemical is a Brønsted–Lowry acid if it can give a hydrogen ion to another chemical, and a chemical is a Brønsted–Lowry base 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.

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

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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.

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