Question

Why is `HF` a strong acid?

Answer 1

In aqueous solution, hydrogen fluoride is in fact a weak acid.

Explanation:

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

For hydrogen chloride, hydrogen bromide, hydrogen iodide, the given equilibrium lies strongly, almost quantitatively to the right.

I do not have the `K_a` values to hand, but `H-F` is a much weaker acid than its Group 17 homologues. Why? (i) the `H-F` bond is stronger; and (ii), the fluoride ion is small and strongly polarizing, and thus dissociation is disfavoured on the basis of entropy. Both entropy and enthalpy thus conspire to reduce the acidity of `HF` versus `HX`. The entropy effect, i.e. the large charge density of the anion, is probably most significant.

Now in all of this, I have assumed an aqueous solution. There are other solvent systems, for instance liquid ammonia, or even liquid hydrogen fluoride. This, however, is the preserve of the specialist. I am perfectly justified in confining discussion to aqueous media.