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What is Hess's Law?

Can you explain Hess's Law of Summation?
Terry W. Holbrook

Hess's law says that reaction enthalpy doesn't depend on the route you take from the reactants to the products. (A more compact way to state Hess's law is "enthalpy is a state function".) The law is a direct consequence of the fact that energy is conserved. If a reaction could proceed by two different routes that gave two different reaction enthalpies, reversing the reaction by one route and going forward by the other would give you a way to create energy out of nothing- and that's not possible.

Hess's law is important because it provides a practical way to combine thermochemical equations for known 'step' reactions to get a thermochemical equation for some 'target' reaction. The basic procedure is this:

  1. Write out the thermochemical equations for the step reactions.
  2. Write the balanced chemical equation for the target reaction.
  3. Reverse step reactions so products/reactants match the target reaction.
  4. Scale step reactions so products/reactants that don't appear in the target reaction will cancel out.
  5. Add the step reactions.
  6. Scale the resulting reaction so it matches the target reaction.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! What is Hess's Law?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/hesss-law.shtml