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QUESTION

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

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

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 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.