Both fluorine and chlorine are involved in -I effect as wll as as +R effect with the benzene ring. Whereas -I effect tends to increase the acidic strengh, +R effect decreases the same. Fluroine has greater -I effect than chlorine since it is more electronegative in nature. But it has also greater magnitude of +R effect (opposing factor) compared to chlorine. This may be attributed to the reason that 2p orbitals of carbon and fluorine have comparable sizes while this is not so in case of carbon and chlorine atoms. In the light of this, p-fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid.
