Question

Carbonic acid is a weak acid. The pK_​a for the dissociation of a first proton is 3.6, while the pK_^​​a for the dissociation of the second proton, represented by this chemical equation, is 10.3.

reaction: HCO_​3^​- (aq) + H_​2O (l) <-----------> CO^​​₃^​2- ​(aq) + H​​_^​3O⁺​ (aq)

Question: Provide a reasonable explanation for the difference in the pK_a​ values for the first and second dissociations.

Answer 1

H₂CO_​3^​ (aq) + H_​2O (l) <-----------> HCO^​​₃^- ​(aq) + H​​_^​3O⁺​ (aq)

here dissociation of H₂CO_​3 reaches to equilibrium.so that if 1 M H2CO3 used after reaching equilibrium only some amount of HCO^​​₃^- ​(aq) will produce, which is less than 1 M.

Ka1 = [HCO3^-][H3O+]/[H2CO3]

HCO_​3^​- (aq) + H_​2O (l) <-----------> CO^​​₃^​2- ​(aq) + H​​_^​3O⁺​ (aq)

Now what ever the HCO^​​₃^- ​(aq) in step-1.which involves in dissociation in aqueous medium and again it reaches to equilibrium.so that the concentration of dissociation is very less compared to step-1.so that Ka2 value is very less,pka2 value is very high.

Ka2 = [CO3^2-][H3O+]/[HCO3^-]