SECOND LAW & ENTROPY 2 - G and S, the Hidden Variables...

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G and S, the Hidden Variables Controlling Chemistry and Cell Voltage If the sign of Δ H does not necessarily predict the direction of “downhill” for a chemical process, what quantity does? Let us suppose that there is some quantity that does tell us the direction of any chemical process, i.e. when it is negative the process goes forward, and when it is positive, the process goes backwards. Let us call this quantity Δ G, and name it the “free energy” or the “driving force”. In thermodynamic jargon, when Δ G is negative, the process is also said to be “spontaneous” but a better term might be “eager”. “Spontaneous” sounds like nothing can stop it, but “eager” is more like a ball on top of hill with a wedge in front of it. “Eventually”, the ball will jiggle around and find a path to roll to the bottom. When it finally reaches the bottom it is at equilibrium. It turns out that, from the study of heat engines, gas expansions, etc, the free energy can be written as a combination of two other functions, one of which is our familiar heat change, Δ H, and the other is a new function, Δ S, the entropy change. The functional form is: Δ G = Δ H - T Δ S (constant T and P) where T is the absolute, or Kelvin, temperature. The entropy is sometimes considered a bit mysterious, but it can be simply described. It proportional to the log of the number of
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This note was uploaded on 01/16/2012 for the course ENVIRON 404 taught by Professor Rasmussen during the Fall '11 term at University of Michigan.

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SECOND LAW & ENTROPY 2 - G and S, the Hidden Variables...

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