Theories of Acids and Bases

An overview of three theories used to describe acids and bases.

Arrhenius Theory

Arrhenius Acids - produce H+ ions in water H+(aq) Arrhenius Bases - produce OH– ions in water OH-(aq)

The theory explains the reaction between H+ and OH- to give water (neutralisation)

Limitation

The definition is not applicable to:

• a base that doesn’t contain OH– for example ammonia

• species that are part of gaseous systems, such as hydrogen chloride

• species that are insoluble.

Brønsted–Lowry Theory

THE definition for A Level Chemistry:

Brønsted acid - H+/proton donor Brønsted base - H+/proton acceptor

Much more general than the Arrhenius Theory

Species need not be soluble in water.

A Brønsted base doesn’t need to form OH-.

Lewis Theory

Lewis acids are electron pair acceptors Lewis bases are electron pair donors

There’s no need for the Lewis acid to form H+.

In the graphic above, NH3 is a Lewis base - the nitrogen in NH3 donates a lone pair of electrons.