No, since this was chemistry, I was talking about enthalpy of an exothermic reaction (negative enthalpy), and interstellar space is pretty close to constant pressure, but not constant entropy for Gibbs free energy.
In any case:
dH=TdS + Vdp note dp is small in space but V can be large and dS is the change in entropy.
...and enthalpy of ideal (interstellar H & O) gases does not depend on pressure, unlike entropy and Gibbs energy. If you really just mean free energy U, then they are basically the same thing in open space (but not in a plasma), a distinction without difference.
For further pedantics I recommend Wikipedia, since I doubt we are helping anyone else.
In any case: dH=TdS + Vdp note dp is small in space but V can be large and dS is the change in entropy.
...and enthalpy of ideal (interstellar H & O) gases does not depend on pressure, unlike entropy and Gibbs energy. If you really just mean free energy U, then they are basically the same thing in open space (but not in a plasma), a distinction without difference.
For further pedantics I recommend Wikipedia, since I doubt we are helping anyone else.