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The standard enthalpy of formation is measured in units of energy per amount of substance, usually stated in kilojoule per mole (kJ mol −1), 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).Īll 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. This is true for all enthalpies of formation. įor example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the following reaction under the above conditions:Īll elements are written in their standard states, and one mole of product is formed. One exception is phosphorus, for which the most stable form at 1 bar is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation. For an element: the form in which the element is most stable under 1 bar of pressure.

For a pure substance or a solvent in a condensed state (a liquid or a solid): the standard state is the pure liquid or solid under a pressure of 1 bar.

For a gaseous or solid solute present in a diluted ideal solution: the hypothetical state of concentration of the solute of exactly one mole per liter (1 M) at a pressure of 1 bar extrapolated from infinite dilution.

For a gas: the hypothetical state it would have assuming it obeyed the ideal gas equation at a pressure of 1 bar.

The superscript Plimsoll on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 ☌ or 298.15 K). The standard pressure value p ⦵ = 10 5 Pa (= 100 kPa = 1 bar) is recommended by IUPAC, although prior to 1982 the value 1.00 atm (101.325 kPa) was used. The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.