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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?

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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 ∆.

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    $\begingroup$ Thanks for making me aware of the Green Book. I shouldn't have stopped looking after finding the Gold Book. $\endgroup$
    – Philipp
    Nov 12, 2013 at 13:21

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