Why is HF a weak acid, and not a strong acid?

1 Answer
Aug 8, 2016

Answer:

Both entropy and enthalpy reduce the acidity of #HF# with respect to the lower hydrogen halides.

Explanation:

We assess the extent of the following rxn:

#H-X(aq) + H_2O rightleftharpoons H_3O^+ + X^-#

For the lower hydrogen halides, #X=Cl, Br, I#, the equilibrium lies strongly to the right. For #X=F#, the equilibrium lies to the left (you will have to get your own quantitative data). So why?

i. It is a fact that the #H-F# bond is stronger than #H-Cl#, and #H-Br#. Enthalpy favours the reverse reaction for #X=F#.

ii. It is also a fact that the #F^-# is smaller and more polarizing, and thus more likely to induce solvent order. Entropy favours the reverse reaction for #F^-#.

And thus both enthalpy and entropy conspire to REDUCE the acidity of #HF# relative to #HCl# and #HBr# and #HI#, which three are all strong Bronsted acids. The entropy effect is probably the most significant. I am happy to entertain further questions if you have doubts.