10.1 Concepts of Acids and Bases (Bronsted-Lowry Concept)

10.1.2 Bronsted-Lowry Concept

In 1923, the Danish chemist Bronsted and the English chemist Lowry independently presented their theories of acids and bases on the basis of proton- transfer. According to this concept:

An acid is a substance (molecule or ion) that can donate a proton $({\mathrm{H}}^{+})$$(\mathrm{H}^{+})$ to another substance. A base is a substance that can accept a proton $({\mathrm{H}}^{+})$$(\mathrm{H}^{+})$ from another substance. For example, HCl acts as an acid while ${\mathrm{N}\mathrm{H}}_3$$\mathrm{NH}_{3}$ acts as a base:

It is a reversible reaction. In the forward reaction, HCl is an acid as it donates a proton, whereas $\mathsf{H}_{2}\mathsf{O}$ is a base as it accepts a proton. In the reverse reaction, $\mathsf{Cl}^{- }$ ion is a base as it accepts a proton from acid $\mathsf{H}_{2}\mathsf{O}^{+}$ ion. $\mathsf{Cl}^{- }$ ion is called a conjugate base of acid HCl and $\mathsf{H}_{2}\mathsf{O}^{+}$ ion is called a conjugate acid of base $\mathsf{H}_{2}\mathsf{O}$ . It means every acid produces a conjugate base and every base produces a conjugate acid such that there is conjugate acid- base pair. Conjugate means joined together as a pair.

A conjugate acid is a specie formed by accepting a proton by a base. A conjugate base is a specie formed by donating a proton by an acid.