Question

Does acid-strength correlate to the electronegativity of the counter ion for the hydrogen halides?

Answer 1

Acidity correlates INVERSELY to electronegativity of the hydrogen halides.

Explanation:

Consider the hydrogen halides, `HF`, `HCl`, `HBr`, `HI`. Fluorine is the MOST electronegative atom; yet `HF` is also the weakest acid, i.e. the following equilibrium `(X=F)` lies to left:

`HX + H_2O rightleftharpoons H_3O^+ + X^-`

For `X=Cl, Br, I`, there is poor overlap in the `H-X` bond, and for these lower halides the equilibrium lies strongly to the right. Acid strength increases down the Group due to poorer overlap, even though the electronegativity of the heteroatom is reduced down the group. The charge density of the anion, an ENTROPY effect, is probably the defining factor in the spontaneity of the reaction.