The Molar Enthalpy of Vaporization is the amount of energy needed to change one mole of a substance from the liquid phase to the gas phase at constant temperature and pressure. And is denoted by ΔHvap. Molar Enthalpy of Vaporization is usually measured using the Kilojoule per Mole for Molar Enthalpy. Note that the value of Molar Enthalpy of Vaporization is always positive.