Standard enthalpy change of formation

In chemistry, the standard enthalpy change of formation (denoted ΔH_f⁰ or Δ_fH⁰) is the enthalpy change that occurs when 1 mol of a compound is formed from its elements in their standard states under standard conditions. If heat is released in the process of formation (for example burning carbon in oxygen to form carbon dioxide), then the sign will be negative. The SI unit for any enthalpy change is kJ/mol (kilojoules per mole).

Due to conservation of energy and Hess's Law, enthalpies of formation can be used to calculate the heat absorbed or released in any chemical reaction. Thus, enthalpies of formation have been determined and tabulated for a wide variety of compounds.

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Notational note: The plimsol sign 'O' was first used in the 19th century. Recently, textbooks have begun to replace the plimsol superscript with a zero superscript (pronounced nought). This may cause some confusion. The standard state does not refer to zero temperatures, pressures and concentrations but to an arbitrarily chosen set of finite ones.