Standard enthalpy of vapourisation `Delta_("vap")H^( )` for water at `100^@C` is `40.66 kJ mol^( 1)`. The internal energy of vapourisation of water at `100^@C` (in kJ `mol^( 1)`) is (Assume water vapour to behave like The difference between `Delta`H and `Delta`U at constant volume is equal to Solved Heat of vaporization of water at 100 degree C and Chegg.com Use Hess's law to prove that Delta subH = Delta vapH + Delta fusH. Homework.Study.com