Standard enthalpy change of formation
In
chemistry, the
standard enthalpy change of formation (denoted ΔH
f0 or Δ
fH
0) 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 (kilo
joules 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.
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.
