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Concept
Brønsted–Lowry acid–base theory
Summary
The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was first developed by Johannes Nicolaus Brønsted and Thomas Martin Lowry independently in 1923. The basic concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory generalises the Arrhenius theory.
In the Arrhenius theory, acids are defined as substances that split up in aqueous solutions to give H+ (hydrogen ions or protons), while bases are defined as substances that split up in aqueous solutions to give OH− (hydroxide ions).
In 1923 physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England both independently proposed the theory named after them. In the Brønsted–Lowry theory acids and bases are defined by the way they react with each other, generalising them. This is best illustrated by an equilibrium equation.
acid + base ⇌ conjugate base + conjugate acid.
With an acid, HA, the equation can be written symbolically as:
HA + B A- + HB+
The equilibrium sign, ⇌, is used because the reaction can occur in both forward and backward directions (is reversible). The acid, HA, is a proton donor which can lose a proton to become its conjugate base, A−. The base, B, is a proton recipient which can become its conjugate acid, HB+. Most acid–base reactions are fast, so the substances in the reaction are usually in dynamic equilibrium with each other.
Consider the following acid–base reaction:
CH3 COOH + H2O CH3 COO- + H3O+
Ethanoic acid, , is an acid because it donates a proton to water () and becomes its conjugate base, the ethanoate ion (). is a base because it accepts a proton from and becomes its conjugate acid, the hydronium ion, ().
The reverse of an acid–base reaction is also an acid–base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid.
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