, the standard enthalpy change of formation
) 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
to form carbon dioxide
), then the sign will be negative. The SI
unit for any enthalpy change is kJ/mol (kilojoules
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.
The plimsol sign '
' 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.