Question

The hydrogen halides of HCl, HBr and HI are all considered strong acids. However, HF is a weak acid. What factor is most responsible for this difference?

Answer 1

`H-X(aq) rightleftharpoons H^+(aq) + X^(-)(aq)`

Explanation:

Good question, and you're not likely to get a wholly satisfactory answer. Two factors are important here: (i) the strength of the `H-X` bond; and (ii), the charge density of the anion.

Now, it is well known that the `H-F` bond is stronger than the `H-Cl`, and `H-Br` bonds. The forward direction of the equilibrium is thus disfavoured for `HF`.

The charge density of the fluoride anion is also differentiating, and this is probably the decisive effect. The `F^-` is very strongly polarizing on the basis of its size. Thus the fluoride anion causes more solvent order, and is thus disfavoured by entropy with respect to the lower halides.

This is a very old inorganic chesnut. The consensus of opinion is that the entropy effect dominates, and that the lower acidity of `HF` is an entropy effect.

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