Enthalpy Change of Vapourisation

The standard enthalpy change of vapourisation is a physical property of substances. It is defined as the amount of heat (or energy) required per unit mass of a substance to completely vapourize the substance at its boiling point. The heat of vapourization is expressed in kJ/mol (kJ mol-1), or calories/gram. The use of kJ/kg is also possible, but less customary. Other units still in use in certain places include Btu/lb (British Thermal Units per pound).

Because vapourisation is the opposite process of condensation, the term heat of condensation is also used. The latter is defined as the heat released when a unit mass of a substance is completely condensed at its boiling point.

The standard enthalpy change of vapourization of water is about 2260 kJ/kg which is equal to 40.8 kJ/mol. This is quite a lot: it is five times the energy needed for heating the water from 0°C to 100°C.