Note: the following question is from a recent biochemistry exam, for which the grades have been posted and the exam questions and answers have been made available to students. I asked my professor about the answer to this question, and when I explained the issue, we resolved that we would investigate the question further to determine the correct answer.
I have a question regarding spontaneity:
1. Please select the TRUE statement regarding thermodynamics in reversible reactions:
a. When ∆G for a reaction is zero, the reverse and forward reactions will proceed spontaneously equally.
b. When ∆G for a reaction is more than zero, the reaction will proceed spontaneously in the reverse direction.
c. Both a and b are true.
d. Neither a nor b is true.
I think that the answer should be "b," but the key says that the answer is "c."
Answer "b" is clearly correct, but what about choice "a"? The question I am exploring is whether a reaction at equilibrium is (or can be) spontaneous. I thought that spontaneity is a process by which a system moves to a lower, more thermodynamically favorable energy state, and as such is controlled by the sign of ∆G.
Part of the issue is the clarification as to whether spontaneity describes the micro- or macro- state of the system. It is clear that chemical equilibrium is a dynamic process, and that at equilibrium the forward and reverse reaction rates are equal. (But "rates" takes us away from thermodynamics.)
So, when we say that ∆G<0 and a reaction is spontaneous in the forward direction, it is not spontaneous in the reverse direction, even though some molecules on a micro-state level may be converting in the reverse direction, the overall macro-state trend is that the system moves to a lower, more thermodynamically favorable energy state.
I am having a hard time finding links that address this question directly. Most say that ∆G determines spontaneity, and give the usual "if ∆G<0, spontaneous...∆G>0, non-spontaneous... ∆G=0, equilibrium." I want to know when ∆G=0, can we even say that such a reaction is spontaneous?
Please post what you think is the answer to the above multiple choice question is and any relevant support (such as textbook pages/reputable web pages) would be greatly appreciated.
The Attempt at a Solution
Please see above