Consider a reaction of an acid and a base.
HCl (aq) + NH3 (aq) ⇌ NH4+ (aq) + Cl– (aq)
It is clear from the reaction that HCl donates a proton which act as an acid and forms Cl– ion which has tendency to accept a proton which act as a base. Similarly, in reverse direction, NH4+ ion accepts a proton and acts as a base. Hence, an acid donates a proton and becomes a base while base accepts a proton and becomes an acid.
Such pairs of substances which can be formed from one another by the gain or loss of a proton are known as conjugate acid and base pairs. Hence, in the equation 9, Cl– is the conjugate base of acid HCl and NH4+ is the conjugate acid of the base NH3.
Hence, Cl–, HCl, NH4+ and NH3 are conjugate acid-base pairs.
From above discussion we can conclude that:
Acid = conjugate base + H+
Base + H+ = conjugate acid
I.e. each conjugate acid has one extra proton and each conjugate base has one proton less.
