F is more electronegative than Cl which means that it holds the electrons of a bond with C closer to itself than does Cl. This is in part a product of the small size of the F atom. The bonded electrons feel a stronger force from the positively charged F nucleus than do Cl’s bonded electrons because F’s are closer and have fewer electrons in lower energy levels shielding the positive nuclear charge from these bonded electrons. Therefore, F exerts a stronger force on these electrons in a bond with C than does Cl. The more intense hogging of the bonded electrons acts like an electron sucking mechanism on the whole molecule, withdrawing electron density away from the carboxyl end more effectively than Cl would. This in turn makes the negatively charged carboxylate more stable with the F group than with the Cl group.
Just because F has a stronger pull on bonded electrons than Cl does not make it more stable with a full negative charge in the case of a conjugate base. If we refer back to the relative sizes of the anions, we know that F- is smaller than Cl-. This means we have F- and Cl- with equal charges, only F- has its charge more densely concentrated. Therefore, there is a stronger electrostatic force between H+ and F- than there is between a H+ and Cl-. This makes F- more reactive than Cl- and therefore a stronger base. HF is therefore a weaker acid than HCl.
