# Molar Enthalpy of Vaporization Definition

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. The usual unit is kilojoules per mole (kJ/mol).

Because energy is required to vaporize a liquid, molar enthalpy of vaporization has a positive sign. This indicates energy is absorbed by the system to get the molecules into the gas state.

## Molar Enthalpy of Vaporization Formula

The mathematical formula used to calculate molar enthalpy of vaporization is:

q = n⋅ΔHvap

• q is the amount of absorbed heat
• n is the number of moles
• ΔHvap is the molar enthalpy change of vaporization

This equation is rearranged to give:

ΔHvap = q/n

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Helmenstine, Anne Marie, Ph.D. "Molar Enthalpy of Vaporization Definition." ThoughtCo, Aug. 25, 2020, thoughtco.com/definition-of-molar-enthalpy-of-vaporization-605361. Helmenstine, Anne Marie, Ph.D. (2020, August 25). Molar Enthalpy of Vaporization Definition. Retrieved from https://www.thoughtco.com/definition-of-molar-enthalpy-of-vaporization-605361 Helmenstine, Anne Marie, Ph.D. "Molar Enthalpy of Vaporization Definition." ThoughtCo. https://www.thoughtco.com/definition-of-molar-enthalpy-of-vaporization-605361 (accessed July 6, 2022).