Standard enthalpy change of formation

In chemistry, the standard enthalpy change of formation is the enthalpy change (i.e. heat absorbed) when some amount 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. It is typically given the symbol ΔH_f⁰ or Δ_fH⁰, and measured in kJ/mol (kilojoules per mole).

Due to conservation of energy, enthalpies of formation can be used to calculate the heat absorbed or released in any chemical reaction.

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Notational note: In the above definition the plimsol sign O is used. This is the original notation of the thermodynamicists of the 19th century. In more recent textbooks there has been a tendency to replace the plimsol by a superscript 0 (zero). This replacement is unfortunate, since the thermodynamicists had good reasons to avoid a zero in the notation: the standard state does not refer to zero temperatures, pressures and concentrations but to an arbitrarily chosen set of finite ones. It is an arbitrary reference state, just like the plimsol line on ships, where the symbol was originally used.