It is actually allowed everywhere in a stoichiometric equation. Whether or not you'll apply it is a matter of taste. read more
There is no such thing as 3/8 atom of sulfur, but there is 3/8 moles of sulfur. In a stoichiometric equation, thinking in terms of atoms is helpful but only such limited. If you think of them as the latter (as it is supposed to be), it makes absolute sense that fractional moles are allowed in enthalpy. read more
As a brief reminder, here is the chemical reaction for the standard enthalpy of glucose: 6C(s, graphite) + 6H 2 (g) + 3O 2 (g) ---> C 6 H 12 O 6 (s) Each standard enthalpy value is associated with a chemical reaction. The reaction will always form one mole of the target substance (glucose in the example) in its standard state. read more