As early as 1938, G. N. Lewis pointed out that the relative strength of an acid or base depended upon the base or acid against which it was measured. No single rank order of acid or base strength can predict the energetics of the cross reaction. Consider the following pair of acid–base reactions:.
: 4F-CHOH + OEt   −ΔH = 5.94 kcal/mole
: 4F-CHOH + SMe   −ΔH = 4.73 kcal/mole
These data suggest that OEt is a stronger base than SMe.  The opposite is found, however, when I is the acid:
: I + OEt   −ΔH = 4.16 kcal/mole
: I + SMe   −ΔH = 7.63 kcal/mole