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Summary Hess's Law
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  • Chemistry
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  • Junior / 11th Grade

The PDF explains Hess's Law, which states that the total enthalpy change of a chemical reaction is independent of the reaction pathway as long as the initial and final conditions are the same. It introduces Hess cycles, illustrating how to use them to calculate enthalpy changes that cannot be deter...

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  • October 3, 2024
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  • 2024/2025
  • Summary

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  • Junior / 11th grade
  • Chemistry
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CIE AS Chemistry Your notes

5.2 Hess’s Law
Contents
Hess’s Law




Page 1 of 9

© 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers

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Hess’s Law
Your notes
Hess Cycles
Hess’s Law states that:
"The total enthalpy change in a chemical reaction is independent of the route by which the chemical
reaction takes place as long as the initial and final conditions are the same."
This means that whether the reaction takes place in one or two steps, the total enthalpy change of the
reaction will still be the same
Illustration of Hess's Law




According to Hess’ Law, the enthalpy change of the direct route, going from reactants (A+B) to
product (C), is equal to the enthalpy change of the indirect routes
Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using
calorimetry, e.g.:
3C (s) + 4H2 (g) → C3H8(g)
ΔHf (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard
conditions

Calculating ΔHr from ΔHf using Hess’s Law energy cycles
The products can be directly formed from the elements = ΔH2
OR
Page 2 of 9

© 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers

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