Reaction Enthalpy Calculator — ΔH° from formation enthalpies

Hess's law in one line: add up the formation enthalpies of everything produced, subtract the sum for everything consumed. Enter the two sums (in kJ) and get the standard reaction enthalpy.

Reaction Enthalpy Calculator — ΔH° from formation enthalpies

$\Delta H^\circ = \Sigma\Delta H_f(products) - \Sigma\Delta H_f(reactants)$

Solve for

Sum of ΔH_f (products) (ΣP)

Unit

Sum of ΔH_f (reactants) (ΣR)

Unit

Result unit

How to interpret the result

Worked check: methane combustion CH₄ + 2O₂ → CO₂ + 2H₂O(l). Products: −393.5 + 2(−285.8) = −965.1 kJ; reactants: −74.8 kJ. ΔH° = −965.1 − (−74.8) = −890.3 kJ per mole of CH₄ — strongly exothermic.

Assumptions

Standard states (1 bar, 298 K) and tabulated ΔH_f values
The equation is balanced before summing