Question

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

Answer 1

Acidity increases in the order: `HF, HCl, HBr, HI`. Note that in water dissociation is fairly complete.

Explanation:

Acidity may be related to the reaction:

`HA rightleftharpoonsH^+ + 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:

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

In water, for all the hydrogen halides, EXCEPT for `HF`, the equilibrium would lie almost quantitatively to the right. We could measure `[H^+]` or its equivalent in another solvent, say acetic acid, and we would find the order of acidity as `HI>HBr>HCl>HF`.

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.