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.
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