For example, at 298 K the reference state of nitrogen is a gas of N2 molecules, that of
mercury is liquid mercury, that of carbon is graphite, and that of tin is the white
(metallic) form. There is one exception to this general prescription of reference states:
the reference state of phosphorus is taken to be white phosphorus despite this
allotrope not being the most stable form but simply the more reproducible form of
the element. Standard enthalpies of formation are expressed as enthalpies per mole
of molecules or (for ionic substances) formula units of the compound. The standard
enthalpy of formation of liquid benzene at 298 K, for example, refers to the reaction