The difference between `Delta`H and `Delta`U at constant volume is equal to 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 Calculated and Experimental Enthalpies vap H and vap H exp of Download Table Use Hess's law to prove that Delta subH = Delta vapH + Delta fusH. Homework.Study.com