Question

Why does the acidity of `HX` `"(X=halogen)"` INCREASE down the Group?

Answer 1

This is conceived to be an entropy effect..........

Explanation:

We interrogate the equilibrium:

`HX(aq) + H_2O(l) rightleftharpoonsH_3O^(+) + X^-`

Both the hydronium ion, and the halide anion are solvated species. That `"iodide"` anion is less charge dense, and less effectively solvated as a consequence, means that entropy favours dissociation. This entropy effect, this solvation effect is probably more significant than the enthalpy effect, which in any case also favours cleavage of the WEAKER `H-I` bond.

On the other hand, hydrogen fluoride is the weakest acid of the hydrogen halides. Here, the `H-F` bond is stronger than that of `H-Cl`, and `H-Br`, because the fluorine atom is smaller, and more effectively overlaps the hydrogen atom.

Certainly fluoride salts give rise to basic solutions:

`F^(-) + H_2O(l) rightleftharpoons HF(aq)+HO^(-)`

How is this consistent with the prior argument?