Molar Enthalpy of Vaporization Definition

Vapor on a black background
Molar enthalpy of vaporization is the enthalpy needed to change one mole of liquid into a vapor.

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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