I frequently encounter two ways of writing enthalpies of some process, e.g. for the enthalpy of formation:

  1. $\Delta_{r} G$ (or $\Delta_{R} G$)
  2. $\Delta G_{r}$ (or $\Delta G_{R}$)

On the IUPAC Gold Book webpage they used the first notation (although the use of upper- and lower-case descriptors varies, see here and here). Is this the accepted way of doing it (and the other one is simply a relic) or might this be a matter of preference. And how about the upper- or lower-case descriptor: Is there any convention for that?


I think you have already answered your question… IUPAC's usage should be a pretty good indicator of the proper notation.

There is, however, an even better reference than the Gold Book: IUPAC's Green Book, or Quantities, Units and Symbols in Physical Chemistry (full text here), on section 2.11.1 Other symbols and conventions in chemical thermodynamics (page 59 and following):

(iii) Examples of the use of the symbol ∆
The symbol ∆ denotes a change in an extensive thermodynamic quantity for a process. The addition of a subscript to the ∆ denotes a change in the property.

It says later:

[∆vapH] can also be written ∆Hvap, but this usage is not recommended

The reason is that ∆Hvap could be understand as a change in Hvap, which it is not. It a change of H during a process, so the subscript should be on the ∆.

| improve this answer | |
  • 2
    $\begingroup$ Thanks for making me aware of the Green Book. I shouldn't have stopped looking after finding the Gold Book. $\endgroup$ – Philipp Nov 12 '13 at 13:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.