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  • $\begingroup$ Yes, if you accept the idea that, even if the system is not quite at equilibrium, you can still calculate its Gibbs free energy. Some people would balk at this, but, personally, not me. We do this all the time when we use the open system, time dependent version of the first law of thermodynamics, with respect to U. $\endgroup$
    – Chet Miller
    Commented Oct 15, 2020 at 14:09
  • $\begingroup$ @ChetMiller Can we say therefore that the thermodynamic potentials such as $G$, $U$, $H$ etc. can be well defined (we can measure them) even if the system is not in equilibrium? Because for example we can measure $U$ in principle (the sum of kinetic and potential energy) even if the system hasn't fixed temperature pressure etc . $\endgroup$
    – Antonios Sarikas
    Commented Oct 15, 2020 at 16:20
  • $\begingroup$ @Chet Miller so you are talking about the realm of non equilibrium thermodynamics? $\endgroup$
    – Bob D
    Commented Oct 15, 2020 at 19:46
  • $\begingroup$ I suppose that this approximation would be considered that. $\endgroup$
    – Chet Miller
    Commented Oct 15, 2020 at 20:08
  • $\begingroup$ @BobD Of course, as the minimum is approached, the rate of entropy generation decreases (I'm guessing, in proportion to the square of the overall reaction rate), so that, in the vicinity of the minimum in G, entropy generation due to deviation from equilibrium strongly approaches zero. $\endgroup$
    – Chet Miller
    Commented Oct 15, 2020 at 22:27