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?

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?

Sarah Angotti · Accepted Answer

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

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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Emilia Alvarez · Answer

undefined The difference in acidity between hydrogen halides is primarily attributed to the strength of the bond between hydrogen and the halogen atom. In the case of HF, the hydrogen-fluorine bond is relatively strong due to the high electronegativity of fluorine, making it more difficult for the hydrogen ion to dissociate in water compared to the other hydrogen halides. This results in HF being a weaker acid compared to HCl, HBr, and HI.