The enthalpy of combustion is the energy released by a combustion reaction between hydrocarbons, oxygen and a heat source. The method for calculating the enthalpy of combustion is to take the enthalpies of formation of the products and subtract the enthalpies of formation of the reactants. This energy is usually in the form of heat, expressed as kilojoules.
For instance, the enthalpy of combustion for ethene is minus 900 kilojoules per mole. The equation for this reaction is C2H4 + 3O2 --> 2CO2 + 2H2O, along with minus 900 kilojoules per mole of heat given off. Glucose, a much heavier hydrocarbon molecule, gives off minus 2,801 kilojoules per mole.
Enthalpies expressed in negative numbers are exothermic, meaning the reaction gives off heat. All enthalpies of formation of the products in combustion are the same basic numbers because carbon dioxide and water are always the products of combustible hydrocarbons. The standard enthalpy of formation for pure oxygen is zero because the substance is in its most elemental state. Charts that show standard enthalpies of formation can be used to determine how much energy is produced during combustion reactions.
The enthalpy of formation is how much energy, or heat, is needed to produce a molecule of a substance. A mole is the ratio of molecules or atoms per a standard weight of a substance. A kilojoule is a unit of energy.