Work out heat of formation instantly with clear inputs, formula shown and shareable results.
An unknown formation enthalpy is recovered by rearranging the Hess summation: if the measured reaction enthalpy and every other species' formation enthalpy are known, the missing one is forced. The back-calculated reaction enthalpy is shown so you can confirm the answer reproduces the measurement you started from.
Rearranged Hess summation
dHf(unknown) = (dH(rxn) + sum dHf(reactants) - sum known dHf(products)) / coefficient
Verification
dH(rxn) = sum dHf(products) - sum dHf(reactants)
Formation enthalpies are per mole of substance. If the species appears twice in the balanced equation, its contribution to the sum is doubled.
Enter its coefficient and swap the two sums; the sign convention then places the unknown on the reactant side.