Question

Why is HCl a strong acid but not HF? Is it because of the electronegativity of Flourine?

Answer 1

The reduced acidity of `HF` with respect to the lower hydrogen halides is largely an entropy effect. Why and how?

Explanation:

The acid base reaction of the hydrogen halides in water is so represented:

`HX(g) + H_2O(l) rightleftharpoons H_3O^+(aq) + X^(-)(aq)`

The `H-F` bond is undoubtedly stronger than that of the lower hydrogen halides, so this marginally favours the reverse reaction. The `F^-` ion is SMALLER and more POLARIZING than the lower halides, and causes much more solvent order upon hydration. Because of this entropy diminution, the free energy of this reaction as written is disfavoured. The lower halides, `Cl^-`, `Br^-`, are less efficiently solvated so entropy does not differentiate.

What studies there are of gas phase acidities (which is a different concept to these aqueous acidities and which removes solvation) suggest that the acidities of the hydrogen halides are roughly comparable. Thus solvation seems to operate in the case of hydrogen fluoride acidities.