Acid and Base Theories

By Namkha Sherpa

The Arrhenius theory

Acids are substances which produce hydrogen ions in solution.

Bases are substances which produce hydroxide ions in solution.

The Bronsted-Lowry Theory

An acid is a proton (hydrogen ion) donor.

A base is a proton (hydrogen ion) acceptor.

The Lewis Theory

An acid is an electron pair acceptor.

A base is an electron pair donor.

The relationship between the three theories

The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it. Hydroxide ions are still bases because they accept hydrogen ions from acids and form water. An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them.

The Bronsted-Lowry theory says that they are acting as bases because they are combining with hydrogen ions. The reason they are combining with hydrogen ions is that they have lone pairs of electrons - which is what the Lewis theory says. The two are entirely consistent.

Lewis' theory extends that of Bronsted-Lowry by considering not the hydrogen ions, but rather the electrons pairs involved. In Bronsted-Lowry and Arrhenius the hydrogen ion is attracted to a pair of electrons in the neutralization process. Lewis rationalized that the electron pair itself was the essential characteristic of a base.