Enthalpy of Reaction Calculator
The enthalpy change of a reaction from the standard formation enthalpies of its products and reactants.
Hess's law: ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants), each weighted by its coefficient.
How the enthalpy of reaction calculator works
Hess's law: ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants), each weighted by its coefficient. Because enthalpy is a state function, the path does not matter — only the endpoints.
Elements in their standard states have ΔH°f = 0 by definition, which is why they drop out of the sum. Negative means exothermic; positive means the reaction absorbs heat.
Formula: ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants)
Worked examples
| Inputs | ΔH of reaction | Note |
|---|---|---|
| Methane combustion | -890.5 kJ/mol | −890.5 kJ/mol — strongly exothermic |
| A thermoneutral reaction | 0 kJ/mol | ΔH = 0 |
| An endothermic one | 100 kJ/mol | +100 kJ/mol |
FAQFrequently asked questions
What is Hess's law?
That enthalpy change depends only on the initial and final states, not the route. It lets you compute a reaction enthalpy from tabulated formation values.
Why are elements zero?
By definition. The standard formation enthalpy of an element in its standard state is defined as zero, which fixes the reference point.
What does a negative ΔH mean?
Exothermic — the reaction releases heat. Methane combustion releases 890 kJ per mole.
Is ΔH the same as heat released?
At constant pressure, yes. At constant volume the internal energy change is the relevant quantity, and it differs by the work done against the atmosphere.
Does ΔH tell me if the reaction happens?
No — that is ΔG. Most exothermic reactions are spontaneous, but not all, and enthalpy alone cannot decide it.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.