Hess's Law Definition

Hess's law states the enthalpy of a reaction is independent of the path between the initial and final states.
Hess's law states the enthalpy of a reaction is independent of the path between the initial and final states. John M Lund Photography Inc / Getty Images
By
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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Anne Marie Helmenstine, Ph.D.
Updated March 13, 2019

Hess's law states that the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it. In other words, the enthalpy change of a chemical reaction (the heat of reaction at constant pressure) does not depend on the pathway between the initial and final states. The law is a variation of the first law of thermodynamics and conservation of energy.

Importance of Hess's Law

Because Hess's law holds true, it's possible to break a chemical reaction into multiple steps and use the standard enthalpies of formation to find the overall energy of a chemical reaction. Standard enthalpy tables are compiled from empirical data, usually acquired using calorimetry. Using these tables, it's possible to calculate whether or not a more complex reaction is thermodynamically favorable or not.

Applications of Hess's Law

In addition to calculating the enthalpy of a reaction rather than directly measuring it, Hess's law is used to:

  • Find electron affinities based on theoretical lattice energy.
  • Calculate heat change of phase transitions.
  • Calculate heat change when a substance changes allotropes.
  • Find the heat of formation of an unstable intermediate in a reaction.
  • Find the lattice energy of ionic compounds.

Sources

  • Chakrabarty, D.K. (2001). An Introduction to Physical Chemistry. Mumbai: Alpha Science. pp. 34–37. ISBN 1-84265-059-9.
  • Leicester, Henry M. (1951). "Germain Henri Hess and the Foundations of Thermochemistry". The Journal of Chemical Education. 28 (11): 581–583. doi:10.1021/ed028p581