Hess's Law Enthalpy
Enter a balanced equation and get the standard enthalpy of reaction ΔH°rxn from tabulated standard enthalpies of formation (298 K). All species must be in the built-in table.
How to use this tool
Find out how much heat a reaction releases or absorbs. Enter a balanced equation and the tool applies Hess's law to the tabulated formation enthalpies and returns ΔH° for the reaction.
What to enter
- Balanced equation: formulas with coefficients, using -> or = between sides, e.g. 2 H2 + O2 -> 2 H2O. It must already be balanced.
- Examples: pick a ready-made reaction from the list. Every species must appear in the built-in table shown below the input.
Reading the result
The headline is ΔH°rxn in kJ. A negative value means the reaction gives off heat (exothermic); a positive value means it absorbs heat (endothermic). The bigger the magnitude, the more energetic the reaction.
Worked example
Methane combustion CH4 + 2 O2 -> CO2 + 2 H2O returns ΔH°rxn ≈ −890 kJ, strongly exothermic.
Reaction
Result
A negative ΔH° means the reaction releases heat (exothermic); a positive value means it absorbs heat (endothermic). The figure is per mole of reaction exactly as written, double every coefficient and you double ΔH°. Elements in their standard state contribute zero.
Methodology
By Hess's law, ΔH°rxn = Σ(coefficient × ΔH°f)products − Σ(coefficient × ΔH°f)reactants. Elements in their standard state have ΔH°f = 0. A negative result is exothermic; positive is endothermic.
Sources
- Standard enthalpies of formation (298.15 K) from NIST and the CRC Handbook. Water is taken as liquid.
Known limits
- The equation must be balanced and every species present in the table; physical states are fixed to the tabulated standard states (no gas/liquid choice).