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Name: Stefanie
Status: Student
Age: 16
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Question:
How does Gibb's Free energy tie in with reduction reactions and glycolysis and equilibrium???



Replies:
Hidden in your question is a far deeper one, "What is thermodynamics all about?" The Gibbs Free Energy is a measure of the amount of work that can be extracted from some process operating at constant pressure. In the literature it is given the symbol "G" or "F". You will see both. The term "Free Energy" should be understood in the context of "available" not in the context of "getting something without any cost".

The Gibbs Free Energy is comprised of two terms: G = H - T*S, where 'H' is the "enthalpy", "T" is the absolute temperature, and "S" is the "entropy". The change in Free Energy call it dG = dH - TdS at a constant temperature. The dH term is the energy change (at constant pressure), and the dS term is the entropy change (a measure of the change in the amount of disorder produced by the process.

If dG < 0, (which means dH < 0 and/or dS > 0) that process, say a chemical reaction will occur spontaneously. Here again, you need to be careful about the definition of the term "spontaneously". In the context here it means that the process, given a pathway, occurs spontaneously; it says nothing about the rate at which the process might occur. So for example, you could mix H2 and O2 gas in a balloon, and if you are very careful, the two gases will mix without any appreciable formation of H2O. None the less, that reaction is still considered "spontaneous" in the context of thermodynamics, because if you provide a pathway -- a spark, a match, or a wandering cosmic ray -- the reaction to form water occurs explosively.

A negative value of dG occurs if dH < 0, that means the process releases energy in some form to the surroundings. The H2 + O2 reaction above is an example. If the process tends to lead to a final state (or products) that is more disorganized than the starting state (or reactants), the process (reaction) will also tend to occur spontaneously. A simple example of this is the mixing of two gases (A and B) initially in different containers separated by a barrier. When the barrier (valve or whatever) is removed or opened the two gases will mix to form a uniform mixture of A+B, even if the pressure and temperature of the gasses are the same and the gases do not undergo a chemical reaction.

If dG should happen to be zero, dG = 0, the process (or reaction) is at equilibrium and no change in the system will/can occur. So the criterion for determine if a system (process, chemical reaction) will occur, is at equilibrium, or will not occur is a matter of determining whether dG < 0, dG = 0, dG > 0, respectively. There are ways of doing this, but that is to much detail for a forum such as this.

Returning now to your question about the reduction reactions and glycolysis. Those are just two chemical reactions to which the same general principles of Gibbs Free Energy apply. If dG < 0, the reaction occurs (assuming some pathway or catalyst is present); if dG = 0 the reaction is at equilibrium, and if dG > 0, the reaction, as written will not occur.

I hope you followed this through. Like so many questions we receive, the question is easy to ask, but whose explanation is much more complicated. We always run the risk of "over explaining" a question. I hope I didn't do that here.

Vince Calder



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