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

Feb 23, 2017

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

#### Explanation:

We interrogate the equilibrium:

$H X \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

Both the hydronium ion, and the halide anion are solvated species. That $\text{iodide}$ 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 $H - C l$, and $H - B r$, because the fluorine atom is smaller, and more effectively overlaps the hydrogen atom.

Certainly fluoride salts give rise to basic solutions:

${F}^{-} + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s H F \left(a q\right) + H {O}^{-}$

How is this consistent with the prior argument?