ABSTRACT
In Brønsted-Lowry theory, an acid is a proton (H+) donor and a base is a proton (H+) acceptor. Examples of acids are HCl, CH3COOH, H3O+ and H2O. Examples of bases are NH3, CH3COO−, H2O and OH−. H2O can therefore act as either an acid or a base. A
reaction between an acid and a base (an acid-base reaction) involves the exchange of a proton, for example:
HCl+NH3→Cl−+NH4+ acid base
These definitions apply under all conditions, but the most important acid-base systems use water as a solvent. In this case, equilibria are set up in water, which for HA (a general acid) is:
and for B (a general base) is:
In the first equilibrium, the H2O molecule acts as a base, accepts a proton and forms the hydronium ion, H3O+, which is the hydrated form of the proton in solution. In the second, the H2O molecule acts as an acid and forms the hydroxide ion, OH−. The base that results from the transfer of a proton from the acid is called the conjugate base of the acid. Therefore, A− is the conjugate base of HA and OH− is the conjugate base of H2O. Similarly, the acid that results from the acceptance of a proton by the base is called the conjugate acid of the base. This means that BH+ is the conjugate acid of B and H3O+is the conjugate acid of H2O.
