Gibbs Free Energy Change/Entropy

[sidnake]

Hi, seen as it's Physical Chemistry I am asking about and Physical Chemistry is essentially applied Physics I figured that it would be okay to ask here. I have an exam this tuesday, and my lecturer went over some seminar questions. He provided answers to the calculations but did not provide answers to the worded questions, and I'm getting a bit confused on this one.

As I understand so far, as a reaction proceeds if the Gibbs free Energy change decreases then the reaction is spontaenous.
Δ_rG^θ=Ʃ_prodvΔ_fG^θ-Ʃ_reactvΔ_fG^θ
So if the products had a Gibbs Free Energy that was lower than the reactants Gibbs Free Energy then the Standard Gibbs energy of the reaction Δ_rG^θ would be a negative number. Indicating the reaction proceeded in the forward direction.

In my seminar however we were asked the following question,
"What is the second law of thermodynamics? By considering changes in both the system of
interest and the surroundings, explain how this law leads to the fact that the Gibbs free energy change for a reaction is negative when the reaction is likely to proceed. "
I've been looking in my textbook, but to be honest I'm a bit confused.

Could someone help me in understanding this question, and maybe providing a model answer so I can understand this concept for my exam on tuesday. Even if this is unlikely to come up because it is on the seminar, I feel it would be unwise to proceed to the next year not understanding this. Especially as physical chemistry is my weak spot.

Thanks Alex

 

[Ygggdrasil]

The fact that ΔG < 0 for any spontaneous process is derived from the second law of thermodynamics. See the discussion here:

https://www.physicsforums.com/showthread.php?t=585631

Let me know if you have any more questions.

 

[sidnake]

Thankyou very much for replying, I'm new to this forum and totally overlooked searching for previously asked questions. Thankyou for responding in a kind manner.

I think I understand the concept now, am I right in thinking that you could have a reaction that proceeded spontaenously even with a positive Gibbs Free energy, if Delta H was very large, and T and Delta S very small?

 

[Jorriss]

Thankyou very much for replying, I'm new to this forum and totally overlooked searching for previously asked questions. Thankyou for responding in a kind manner.

I think I understand the concept now, am I right in thinking that you could have a reaction that proceeded spontaenously even with a positive Gibbs Free energy, if Delta H was very large, and T and Delta S very small?

If the reaction is at constant T and P, no, a reaction can not proceed if deltaG is positive. Entropy is effectively the definition of a process being spontaneous or not and thermodynamic free energies are different ways to dress up the second law in terms of system variables. If deltaG is positive, at constant T and P, then the reaction will not proceed spontaneously.

If deltaH is very large, TdeltaS must be even larger to compensate and vice versa.