Heat Of Formation Calculator

Work out heat of formation instantly with clear inputs, formula shown and shareable results.

About Heat Of Formation Calculator

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.

Formulas

Rearranged Hess summation

dHf(unknown) = (dH(rxn) + sum dHf(reactants) - sum known dHf(products)) / coefficient

Verification

dH(rxn) = sum dHf(products) - sum dHf(reactants)

Worked example

With the default values below, Heat Of Formation Calculator returns Formation Enthalpy Of The Unknown of -285.55. Change any input above and every figure here is recalculated in your browser.

Inputs used

Measured reaction enthalpy: -890 kJ/mol
Sum of the known product formation enthalpies: -393.5 kJ/mol
Sum of the reactant formation enthalpies: -74.6 kJ/mol
Coefficient of the unknown speciesminimum 0: 2

Results

Formation Enthalpy Of The Unknown: -285.55 kJ/mol
Total Product Formation Enthalpy: -964.60 kJ/mol
Back-Calculated Reaction Enthalpy: -890.00 kJ/mol
Same Value In kcal/mol: -68.248

Step by step

1Values used
2Rearranged Hess summation
3Verification
4Formation Enthalpy Of The Unknown= -285.55 kJ/mol
5Total Product Formation Enthalpy= -964.60 kJ/mol
6Back-Calculated Reaction Enthalpy= -890.00 kJ/mol
7Same Value In kcal/mol= -68.248

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why divide by the coefficient?

Formation enthalpies are per mole of substance. If the species appears twice in the balanced equation, its contribution to the sum is doubled.

What if the unknown is a reactant instead?

Enter its coefficient and swap the two sums; the sign convention then places the unknown on the reactant side.