Could you explain, on a conceptual level, why we include ΔG° In the equation ΔG = ΔG° + RTln(Q)? Why does the change in Gibbs energy at standard conditions have to be included whenever we want to determine ΔG for a system?

You may check out this thread:

http://www.chemicalforums.com/index.php?topic=69815.0

Your response in the other forum was outstanding! Really helped me understand the difference between ΔG° and ΔG! I am still a little unclear on one point, and I'm really hoping you can help clarify it for me.

Given that ΔG = ΔG° + RTln(Q), and ΔG° = -RTln(K), ΔG = -RTln(K) + RTln(Q)

This makes sense, and if we were in standard conditions (everything held at 1M), then ΔG = -RTln(K) + RTln(1/1), which would mean that ΔG = -RTln(K) (in this instance, ΔG is essentially ΔG° (i.e. ΔG° = ΔG° + 0)

I also understand that when different concentrations are used the ΔG will deviate from ΔG° (because the RTln(Q) value will change) , and we can use this different number for ΔG to predict what will happen in the reaction. (All of this follows from your post in the other forum, Corribus).

However, the other forum also asks how the equation ΔG = ΔG° + RTln(Q) is derived/related to ΔG = ΔH - TΔS. I have read a lot about ΔG = ΔH - TΔS, and I understand how it measures thermal energy transfer between the system and the environment; however, I still can't understand how ΔG = ΔG° + RTln(Q) was derived, particularly why we are relying on ΔG°. I completely agree that the equation works (as just shown), but I still don't understand why we would include ΔG°. Why does the free energy at standard conditions matter/impact the results for other free energy calculations? If I want to calculate the free energy for a reaction at a certain time with a certain concentration/temperature, why should I care about/include the free energy of the equation at standard conditions? It seems somewhat random to throw that into the equation.

Edit: Does it somehow fit into this derived form of the Gibbs Equation? ΔS(universe) = - ΔH/T(system) + ΔS(system) ; Is ΔG° perhaps a representation of the change in entropy of the system (i.e. the reversible process)?