Question

For the Brønsted-Lowry acid base reactions shown below, list the stronger acid, stronger base, weaker acid, and weaker base

H2CO3 + CN- (Arrows) HCO3- + HCN

Answer 1

H₂CO₃ (aq) + CN^- (aq)   HCO₃^- (aq) + HCN (aq)

pKa values are H₂CO₃ (aq) = 6.35

pKa HCN = 9.31

Lower the pKa value stronger is the acid and vice-versa.

pKa of H₂CO₃ (aq) is smaller than pKa of HCN. Hence H₂CO₃ (aq) is stronger acid than HCN.

Now, stroger acid on ionization gives weaker base than the conjugate base formed by relatively weaker acid.

NC^- is conjugate base of relatively weak acid HCN and hence CN^- (aq) is stronger base. Whereas, HCO₃^- (aq) is a conjugate base formed from relatively stronger acid H₂CO₃ (aq).

Hence we have,

Strong Acid : H₂CO₃ (aq)

Strong base: CN^- (aq)

Weak Base: HCO₃^- (aq)

Weak Acid: HCN (aq)

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