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

Jun 3, 2016

Consider the hydrogen halides, $H F$, $H C l$, $H B r$, $H I$. Fluorine is the MOST electronegative atom; yet $H F$ is also the weakest acid, i.e. the following equilibrium $\left(X = F\right)$ lies to left:
$H X + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$
For $X = C l , B r , I$, 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.