In order to calculate a reaction enthalpy from enthalpis of formation, one simply adds up the enthalpies of the products time their stoichiometric coefficients, and substracts a similar sum for the reactants:
| Enthalpies of Formation (kJ/mol) | |
|---|---|
| A | -241 |
| B | 290 |
| C | 19 |
| D | -323 |