Standard enthalpy of vaporisation \Delta _ { v a p } H ^ { \circ } for a .. The difference between `Delta`H and `Delta`U at constant volume is equal to For the vaporisation of water : `H_(2)O (I) hArr H_(2)O(g)` [1 atm. Pressure] Given : `Delta S = 120 JK^( 1)` and `Delta H =+45.0 kJ`. Evaluate the temperature at which liquid water Heat of Fusion Explained ChemTalk Heat of Vaporization from Vapor Pressure (Example)