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How is Gibbs free energy for a reaction different from that in a process? Is it because a reaction generally occurs at a constant temperature and pressure, and so dG = dH - TdS, while for a process temperature may or may not be fixed for a process, say expansion of a gas, and so dG = dH - TdS - SdT? Also, how do we define H at non-constant pressure? Is it dH = dU + d(PV)?

Also, why do we say that The G at any point is the max non-mechanical work that can be drawn out? Why does this not include mechanical work too?

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  • $\begingroup$ The second part has been addressed multiple times already - just a search for "gibbs work" will bring up several questions asking the same thing. chemistry.stackexchange.com/questions/32873/… $\endgroup$ – orthocresol♦ Apr 20 '16 at 10:46
  • $\begingroup$ We look at dG at constant temperature and pressure only as a vehicle for determining the relationship we need to use to calculate the equilibrium constant for the reaction. That relationship can be used for all temperatures and pressures. $\endgroup$ – Chet Miller Apr 20 '16 at 11:25

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