Which is the strongest acid among these: HBr, HF, HI, and HCl?

Jun 29, 2016

Acidity increases in the order: $H F , H C l , H B r , H I$. Note that in water dissociation is fairly complete.

Explanation:

Acidity may be related to the reaction:

$H A r i g h t \le f t h a r p \infty n s {H}^{+} + {A}^{-}$

The stronger the acid; the further to the right lies this equilibrium. Of course, the acidity is modified by the identity of the solvent. In water, the acid base reaction is often represented as:

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

In water, for all the hydrogen halides, EXCEPT for $H F$, the equilibrium would lie almost quantitatively to the right. We could measure $\left[{H}^{+}\right]$ or its equivalent in another solvent, say acetic acid, and we would find the order of acidity as $H I > H B r > H C l > H F$.

This clearly relates to the enthalpy of the $H - X$ bond, and also to an entropy effect. The larger halides, ${X}^{-}$, are entropically favoured in that they are less charge dense, and cause less solvent order upon solvolysis.