<P> The standard enthalpy of formation is measured in units of energy per amount of substance, usually stated in kilojoule per mole (kJ mol), but also in kilocalorie per mole, joule per mole or kilocalorie per gram (any combination of these units conforming to the energy per mass or amount guideline). </P> <P> In physics the energy per particle is often expressed in electronvolts (eV), where 1 eV corresponds to 96.485 kJ mol . </P> <P> All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc .) have a standard enthalpy of formation of zero, as there is no change involved in their formation . </P> <P> The formation reaction is a constant pressure and constant temperature process . Since the pressure of the standard formation reaction is fixed at 1 atm, the standard formation enthalpy or reaction heat is a function of temperature . For tabulation purposes, standard formation enthalpies are all given at a single temperature: 298 K, represented by the symbol Δ H ⊖ 298 K . </P>

Why is the enthalpy of formation of an element zero