Hess law: the statement and the two forms
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
Hess law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
Words that must appear
- Independent of the route
- The same initial and final conditions
The sign
Using formation data: delta H = sum of the formation enthalpies of the products minus the sum for the reactants. Using combustion data: delta H = sum for the reactants minus the sum for the products.
Do not confuse with
A single reaction step. Hess law relates the total of several routes.
Common questions
What is Hess law?
The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
What is the Hess law expression using formation data?
Delta H equals the sum of the formation enthalpies of the products minus the sum for the reactants.
What is the Hess law expression using combustion data?
Delta H equals the sum of the combustion enthalpies of the reactants minus the sum for the products.
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