Standard corrections to GFE for uni and bi molecular rxns

[Einstein Mcfly]

Hello everyone. I have a weird question regarding the calculation of free energy changes in a multi-step mechanism. Now, if you have a bunch of calculated free energies in gas phase at 1 atm (say, from an electronic structure program) and you want to model in solvent, you need to get them in a 1M standard state and this requires adding an additional term RT*ln(RT). Now, if every step in your mechanism is either unimolecular or bimolecular, there will be no change in the concentration of anything as the total number of moles of things will all stay constant and these corrections will all cancel when you calculate the delta Gs. However, if you have some mixing of them, such as
1) AB--> A +B
2) B+C->D
etc.
then in the first case you've added a mole and in the second case you've removed a mole. Therefore, when I calculate the delta G for these steps, the sign of RT*ln(RT) should be positive in the first case and negative in the second case, correct? You won't get cancellation when you calculate the delta Gs and these corrections will tweek things by three or so kcal/mol. Am I on the right track here or am I missing something?

Thanks for any help you can give.

 

[Dr Uma Sharma]

How can you predict the sign of delta G only by counting moles on the reactant side and product side?

 

[Einstein Mcfly]

Sorry, I'm not talking about getting the sign of delta G, I'm talking only about the correction in going from 1 atm to 1M. In that case delta G is the difference between the Gs at STP plus the difference between the correction terms for each G. Clearly the effect of the correction term will be zero if one mole goes to one mole or two moles go to two moles, but what about one mole going to two (as in A-->B+C) or two going to one (C+D-->F)?

I just want to make sure that I'm correct about when the correction term will figure in (that is, not cancel out).