# Is HCl a strong acid than HF?

The fluorine atom is smaller than the chlorine atom, and the resultant $H - X$ bond is weaker for $X = C l$, because there is less effective overlap. Upon protonolysis with water, the fluoride ion is much more polarizing than chloride, and induces much more solvent order. Both entropy and enthalpy thus conspire to reduce the acidity of hydrogen fluoride.