The difference between `Delta`H and `Delta`U at constant volume is equal to Delta Vap H Latent Heat of Vaporization Delta Hvap of Water calculated by corresponding states correlation Vapor Pressure Standard enthalpy of vaporization (vapH) for water at 100C is 40.66 kJ.. 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