Heat of combustion
The heat of combustion is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities:
energy/mole of fuel (kJ/mol)
energy/mass of fuel
energy/volume of the fuel
The heat of combustion is conventionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation 
of the products and reactants (though this approach is purely empirical since most heats of formation are calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the magnitude of the heat of combustion is 418 kJ/mol (c + 0.3 h – 0.5 o) to a good approximation (±3%). The heat of combustion of all organic compounds has the sign corresponding to an exothermic reaction (negative in the standard chemical convention) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in O2 to the stronger bonds in CO2 and H2O releases energy as heat.
