In 1923, a Danish Chemist J.H. Bronsted and an English Chemist T.M. Lowry independently proposed new definitions for acids and bases. They proposed that:
An acid is a substance that can donate a proton.
A base is a substance that can accept a proton.
These definitions are more general than Arrhenius definitions because according to these definitions even ions can behave as acids or bases. Moreover, these definitions are not restricted to reactions taking place in aqueous solution only. In order to understand this concept of acids and bases, let take some specific examples.
From the above equations, it is obvious that acid base reactions according to Bronsted-Lowry concept involve transfer of proton from the acid to a base. A substance can act as an acid only if another substance capable of accepting a proton, is present.
CONJUGATE ACID-BASE PAIRS
An acid after losing a proton becomes a base whereas a base after accepting the proton becomes an acid. For example, let us consider the reaction between water and ammonia as represented by the following equilibrium
In the forward reaction, water donates a proton to ammonia (base) and acts as acid. In the reverse reaction, NH4 + ions donate a proton to the OH- ions (base) and act as acid. A base formed by the loss of proton by an acid is called conjugate base of the acid whereas an acid formed by gain. of a proton by the base is called conjugate acid of the base. In the above example, OH-is the conjugate base of H2O and NH4 + is conjugate acid of NH3. Acid-base pairs such as H2O/OH – and NH4 + INH3 which are formed by loss or gain of a proton are called conjugate acid-base pairs.
A strong acid would have large tendency to donate a proton. Thus, conjugate base of a strong acid would be a weak base. Similarly, conjugate base of a weak acid would be a strong base.
Some more conjugated acid-base pairs have been given in the following equations:
It may be noticed that in equation ( 1) H2O is behaving as a base whereas in equation (2) it is behaving as an acid. Similarly, HCO3- ion in equation (3) acts as an acid and in equation (4) it acts as a base. Such substances which can act as acids as well as bases are called amphoteric substances.