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

1 Answer
Jun 29, 2016

Answer:

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.