Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect on covalent bonds. The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid.
The inductive effect is cumulative: trichloroacetic acid is 1000x stronger than chloroacetic acid.
The impact of the EWG group on pKa decreases with distances from the carboxylic group.
For benzoic acids, the effect is quantified by the Hammett equation:
where
: = Reference constant
: = Substituent constant
: = Reaction rate constant