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Gibb's Free Energy
Name: Stefanie
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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