Hess's Law Calculator
Reaction enthalpy from standard enthalpies of formation — enter ΔHf° and coefficients for reactants and products, get ΔH°rxn, per-species contributions and whether the reaction is exo- or endothermic.
How it works
ΔH°rxn = Σ nₚ·ΔHf°(products) − Σ nᵣ·ΔHf°(reactants); elements in their standard state have ΔHf° = 0
Enthalpy is a state function, so the heat of a reaction does not depend on the route — Hess’s law. The most useful form combines tabulated standard enthalpies of formation: imagine breaking every reactant into elements and reassembling the products, and the bookkeeping reduces to products minus reactants, each weighted by its coefficient. The same principle lets you add and subtract measured reactions to obtain one that cannot be run directly, such as the formation of carbon monoxide.
Worked example
Combustion of methane, CH₄ + 2O₂ → CO₂ + 2H₂O(l): [−393.5 + 2(−285.8)] − [−74.8 + 0] = −890.3 kJ/mol — the standard heat of combustion.
Frequently asked questions
Liquid or gaseous water?
It matters by 44 kJ per mole of water (the heat of vaporisation): −285.8 for liquid, −241.8 for vapour. Use the state your reaction produces.
What if I have reaction enthalpies instead of ΔHf?
Reverse, scale and add the equations so they sum to the target; the ΔH values combine the same way.
Related calculators
For study and planning — check units and conditions against your data source before using a result in the lab.