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Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please?

From Borrell and Dixon, 1984 "Electrode potential diagrams and their use in the Hill-Bendall or Z-scheme for photosynthesis":

enter image description here

And their statement in the conclusions section:

This article has outlined the use of electrode potential diagrams in a simple system, and in the Hill-Bendall scheme for photosynthesis, for which the diagram provides a basis for understanding the mechanism. Emphasis has been placed (1) on the use of arrows to depict electron transfer, with a rule to give the correct direction of the coupled reactions; (2) on the fact that the diagram is a free-energy diagram and not an energy diagram, and (3) on the fact that the energy quantities corresponding to heights represent the maximum work available or the minimum work required for particular processes.

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    $\begingroup$ What do you mean with diagram? Diagram of energy vs what, diagram of free energy vs what? $\endgroup$
    – Lagerbaer
    Commented Mar 4, 2012 at 3:10
  • $\begingroup$ It came from the conclusions section of this paper by Borrell and Dixon, 1984 "Electrode potential diagrams and their use in the Hill-Bendall or Z-scheme for photosynthesis" where they say: "Emphasis has been placed (1) on the use of arrows to depict electron transfer, with a rule to give the correct direction of the coupled reactions; (2) on the fact that the diagram is a free-energy diagram and not an energy diagram." - see question for the "free-energy diagram" figure they use in the paper. $\endgroup$
    – AJP
    Commented Mar 5, 2012 at 8:36
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    $\begingroup$ One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $T\cdot S$ where $T$ is temperature and $S$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) $\endgroup$
    – Lagerbaer
    Commented Mar 5, 2012 at 23:43
  • $\begingroup$ That's actually really (!) useful as well. Thanks Lagerbaer. I've already marked John's as correct but this is the perfect complement to it so I've added it to his answer. $\endgroup$
    – AJP
    Commented Mar 6, 2012 at 9:55

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"Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help.

From Lagerbaer in a comment:

One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $T\cdot S$ where $T$ is temperature and $S$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat)

I did a quick search for "what is energy" and What Is Energy? Where did it come from? looks an informative thread. have a look at this discussion and see if it casts any light on your question.

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