The heat of vaporization at the normal boiling point is an approximate measure of the strength of intermolecular forces in the liquid phase. Why? The compounds methane and water have similar molar masses and similar values of Trn. Yet for methane ∆Hlnv = 8.17 kJ mol-1, and for water ∆Hlnv = 40.66 kJ mol-1. Explain the large difference.