The heat of reaction for the complete combustion of a mole or unit mass of a fuel in oxygen at the standard state of 25ºC and 101.3 kPa. For the combustion of (p. 179) a hydrocarbon, the carbon is converted to carbon dioxide and the hydrogen is converted to water vapour. For fuels that contain sulphur and nitrogen, the combustion products are sulphur dioxide and nitrogen. It is also known as the standard enthalpy of combustion. See calorific value.