Brønsted and Lowry Concept of Acids and Bases
In 1923, Brønsted and Lowry pointed out independently that acid-base reactions can be interpreted as proton-transfer reactions. An acid is defined as a proton (H+) donor, and a base is defined as a proton acceptor.
Any hydrogen-containing molecule or ion capable of donating or transferring a proton is an acid, while any molecule or ion that can accept a proton is a base.
For example, in a reaction between ammonia (NH3) and hydrogen fluoride (HF), ammonia acts as a base (accepts a proton) while HF behaves as an acid (donates a proton).
NH3 + HF → NH4+ + F-
In the Brønsted-Lowry Concept, acids and bases can be either ions or molecular substances.
Acid Base Equilibrium
In any acid-base equilibrium both forward and reverse reactions involve proton transfers. For example,
H2O (l) + NH3 (aq) ⇌ NH4+ (aq) + OH- (aq)
In the forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid. In the reverse reaction, NH4+ donates a proton to –OH. Thus NH4+ ion acts as an acid and –OH as a base.
The species NH4+ and NH3 are called conjugate acid-base pair. A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the gain or loss of a proton. In a conjugate acid-base pair the weaker an acid is, stronger is its conjugate base. Likewise, the weaker a base is, the stronger is its conjugate acid.
Amphiprotic Species
Some species can act both as an acid and as a base. For example, H2O.