What do you understand by conjugate acid base pairs? Give examples.

Solution

A species formed by adding a proton to Bronsted base is called conjugate acid and a species formed by losing a proton from Bronsted acid is called conjugate base.
$Brosted space plus space straight H to the power of plus space space space rightwards arrow space space space space Conjugate space acid space space space space space Bronsted space acid space space space rightwards arrow space space space space space straight H to the power of plus space plus space Conjugate space base$
Thus, every acid has its conjugate base and every base has its conjugate acid. Consider a reaction between HCl and H₂O.
$HCl with acid below space plus space stack straight H subscript 2 straight O with base below space space space space leftwards harpoon over rightwards harpoon space space stack straight H subscript 3 straight O to the power of plus with acid below space plus space stack Cl to the power of minus with base below$
In the forward reaction, HCl donates a proton to water.
∴ HCl is an acid and H₂O is a base.
In the backward reaction, H₃O⁺ ion donates a proton to Cl^– ion.
∴ H₃O⁺ is an acid and Cl^– is a base.
Thus Cl^– ion is a conjugate base of HCl and H₃O⁺is the conjugate acid of base H₂O. Such pairs of substances which differ from one another by a proton are known as conjugate acid-base pairs. Thus any acid-base reaction really involves two acids and two bases, forming conjugate pairs.