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
fH
0, 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.
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.