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

1 Answer
Feb 23, 2017

Answer:

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?