The standard enthalpy of formation, denoted as ΔfH⦵, is the change in enthalpy when one mole of a substance in its standard state is formed from its constituent elements in their reference state, with all substances in their standard states. It is measured in units of energy per amount of substance, usually stated in kilojoules per mole (kJ mol−1), and is used to predict heats of formation for simple unstrained organic compounds with the heat of formation group additivity method
. The standard state refers to atmospheric pressure of one bar and a specified temperature, which is usually 25 degrees Celsius
