How this calculator works
Enthalpy is U + pV. At constant pressure its change is deltaU + p deltaV; if pressure changes, use the full difference in pV instead. A reaction calculation applies Hess law: sum signed stoichiometric coefficients times standard formation enthalpies. Included tokens use OpenStax Appendix G reference values at 298.15 K, with water phase specified. Enter a balanced reaction and consistent standard states; this tool does not check atom or charge balance.
Formula
H = U + pV; at constant p, deltaH = deltaU + p deltaV; deltaHreaction = sum(products nu Hf) - sum(reactants nu Hf).
Worked example
At 100000 Pa, a 200 J internal-energy increase and 0.002 m³ expansion give deltaH = 400 J. Using the listed reference data, CH4 + 2 O2 -> CO2 + 2 H2O(liquid) gives -890570 J/mol reaction.
Advertisement
Frequently asked questions
When is enthalpy change equal to heat?
For a constant-pressure process with only pressure-volume work and negligible kinetic and potential energy changes.
Can an enthalpy value be negative?
Yes. Absolute reference levels are conventional; changes and consistent reaction sums carry physical meaning.
Does liquid versus gas water matter?
Yes. Their formation enthalpies differ because vaporization requires energy. Choose the correct phase.