Heat of Combustion

In a general combustion reaction between initial reactants the process may be written as:
fuel + oxidizer Þ products + energy release

If the reaction takes place at constant volume, no work is done and application of the first law indicates that the energy release, Q_v, is the change in the internal energy of the system: Q_v = (DU)_v = (U_products - U_reactants).

If the reaction takes place at constant pressure, both work and heat interactions can occur and the energy release, Q_p , is the change in the enthaply of the system so that:

Q_p = (DH)_p = (H_products - H_reactants) .

The quantities Q_i are the heats of combustion under the stated condition, i .