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The second law of thermodynamics states that $dS_{universe}>0$ for all processes.

In addition to the second law, gibbs free energy is used to consider the feasibility of a reaction. $dG=dH_{syst}-T*dS_{syst}$. If $dG_{syst}<0$, this means that the reaction is spontaneous, but a reaction with $dG_{syst}$ can also occur if energy is input into the system (for example electrolysis).

Therefore, $dS_{univ}>0$ does not imply $dG_{syst}<0$.

However, is there such a quantity as $dG_{univ}$, and if so is the relationship $dG_{univ}<=>dS_{univ}$ true?

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  • $\begingroup$ There are a lot of different notation you use here and also "rxn" if you can clarify this I would be glad. I'm not sure what you mean in the first sentence, what I'm sure yet is that $\mathrm{d}S>0$ always for isolated system. $\endgroup$ – ParaH2 Dec 9 '16 at 20:35
  • $\begingroup$ You mix up things. Entropy extreme and similar principles are applied for certain systems, eg entropy max is only true for isolated systems! $\endgroup$ – Greg Dec 10 '16 at 7:24
  • $\begingroup$ @Greg Is the whole univserse not be defined as an isolated system? $\endgroup$ – Adroit Dec 10 '16 at 8:54
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Didn't you ask the last question about this, too?

The issue here is that when $\Delta G > 0$, the reaction is indeed non-spontaneous as written. If you wish to add energy to the system, for example, via electrolysis, you should include the additional changes in the computation of the free energy change. You will find that after you sum up everything $\Delta G < 0$.

Also, note that $\Delta S_{\mathrm{universe}} < 0$ does not mean that the process will not happen; it only means that it's (fairly) unlikely to happen.

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  • $\begingroup$ It is praiseworthy that you have included the last line. Unlike thermodynamics, Statistical Mechanics has no inherent time asymmetric ingredient. So, indeed the reversal of the Second Law is , though highly unlikely, possible. $\endgroup$ – user5764 Dec 10 '16 at 3:41
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The relationship is: $\Delta G = - T \Delta S_{univ} $

Also, if you want to consider electrolysis in this context, your system needs to include the power input in some form. Your entropy increase from obtaining that power is large enough to offset the entropy penalty of an unfavorable process making it spontaneous.

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  • $\begingroup$ It seems to me now, that dG is always defined to deal with only the system. Is this the case? $\endgroup$ – Adroit Dec 10 '16 at 15:17
  • $\begingroup$ That is just the equation, G is defined based on entropy of the universe. Which you use just depends on how you are thinking about the problem. You can substitute in $-T/Delta S_{univ} $ for $/Delta G$ if you prefer to think about the problem that way. $\endgroup$ – brose Dec 10 '16 at 16:42
  • $\begingroup$ $\Delta G$ is a more convenient way for chemists to think about is since typically we only care about a system and whether the change in that system is favorable or not. In a crude practical sense who cares about the entropy of the universe, I just want to know if a process in a system is spontaneous and if not how to make it spontaneous. $\endgroup$ – brose Dec 10 '16 at 16:48

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