Calculation of Enthalpy of Vaporization at a Given Temperature THE ENTHALPY OF A SYSTEM Enthalpy (H) is the total heat content of a system, defined as its internal energy (E) plus the product of its pressure (P) and volume (V) (H = Standard enthalpy of vaporisation \Delta _ { v a p } H ^ { \circ } for a .. Video answer for \Delta_{\text {vap }} \mathrm{H}^{\Theta} for water is +40.79 \mathrm{kJ} \mathrm{mol}^{ 1} at 1 bar and 100^{\circ} \mathrm{C}. Change in internal energy for this vapourisation under Calculated and Experimental Enthalpies vap H and vap H exp of Download Table Hvap for water is 40.7 KJ `mol^(1)` . The entropy of vaporization of water is: