Molar Enthalpy of Vaporization Definition

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

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By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on July 11, 2019

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 April 23, 2024).