Question

State reasons for the following :

(i) Monochloroethanoic acid has a higher pK_a value than dichloroethanoic acid.

(ii) Ethanoic acid is a weaker acid than benzoic acid.

Answer 1

(i) The strength of an acid is indicated by pK_a value, where, pK_a = – log K_a

Since monochloroethanoic acid is weaker than dichloroethanoic acid so it has lower value of dissociation constant K_a.

Therefore, it has higher value of pK_a.

(ii) The —COOH group in benzoic acid is attached to sp² - carbon of the phenyl ring and is more acidic than acetic acid in which —COOH group is attached to sp³ – carbon atom of CH₃ group. So, benzoic acid is stronger than acetic or acetic acid is weaker acid than benzoic acid.