Enthalpy Change Calculator (Hess's Law)
Calculate reaction enthalpy from formation enthalpies with Hess's law and correct ΔH for temperature with Kirchhoff's law. Shows if it is exothermic.
What this calculator does
Add the formation enthalpies of products and reactants (with coefficients) to get ΔH of the reaction, see whether it is exothermic or endothermic, and adjust it to another temperature.
When you need it
Reaction enthalpies tell you how much heat a process releases or absorbs. They are used for fuel comparisons, calorimetry problems and for deciding how much cooling a reactor needs.
Formulas
- ΔH°rxn = Σ n ΔH°f(products) − Σ n ΔH°f(reactants)
- Kirchhoff: ΔH(T₂) = ΔH(T₁) + ΔCp (T₂ − T₁)
How to use it
- Enter each reactant and product with its coefficient and ΔH°f.
- Read the sums for products and reactants and the reaction enthalpy.
- Optionally correct ΔH to another temperature with ΔCp.
Worked example
Find ΔH° for CH4 + 2 O2 → CO2 + 2 H2O(l), with ΔH°f in kJ/mol: CH4 −74.8, CO2 −393.5, H2O(l) −285.8, O2 0.
- Products: −393.5 + 2(−285.8) = −965.1 kJ
- Reactants: −74.8 + 2(0) = −74.8 kJ
- ΔH° = −965.1 − (−74.8) = −890.3 kJ
Answer
ΔH° = −890.3 kJ per mole of methane, so the reaction is exothermic.
Common mistakes
- Forgetting to multiply ΔH°f by the coefficient.
- Subtracting in the wrong order (products minus reactants).
- Using H2O(g) instead of H2O(l) when the problem gives liquid water.
Related tools
Frequently asked questions
What is Hess's law?
The enthalpy change of a reaction is the same whether it happens in one step or several, so ΔH can be built from other reactions.
What is the enthalpy of formation of an element?
Zero, for the element in its standard state, for example O2(g) or C(graphite).
How do I know if a reaction is exothermic?
A negative ΔH means heat is released (exothermic); a positive ΔH means heat is absorbed (endothermic).