Using the model
Symbols, assumptions and limitations
Enthalpy is a state function, so Hess steps may be reversed and multiplied before addition. Reversing a reaction changes the sign of ΔH; multiplying the entire equation multiplies ΔH by the same factor. Formation data must refer to the stated phases and a common reference temperature, normally 298.15 K.
Worked examples
See the method in practice
Methane combustion
Using standard formation enthalpies, products minus reactants gives a negative ΔH for CH₄ combustion, identifying an exothermic reaction.
Reversing a Hess step
If a required intermediate reaction is written backward, its enthalpy sign must also be reversed before the steps are added.
Questions
Frequently asked
Why is elemental standard-state formation enthalpy zero?
The standard enthalpy of formation of an element in its reference form is defined as zero, not measured as an absence of energy.
Do phases matter?
Yes. H₂O(l) and H₂O(g) have different formation enthalpies, so phase labels must match the intended reaction.
Does Hess’s law require the same reaction pathway?
No. Because enthalpy is a state function, only the initial and final states determine the total enthalpy change.
Sources & review
Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.
OpenStax Chemistry — Enthalpy OpenStax Chemistry — Hess’s LawWritten by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology