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Novice question about Gibbs energy
ASmith:
Yes, ΔG doesn't change and neither does ΔH, the heat lost from the system, so TΔS must be the same in both situations. But ΔS does change. Moreover this change can't be compensated by a change in T. So the traditional interpretation doesn't make sense.
Orcio_87:
--- Quote ---ΔG doesn't change and neither does ΔH, the heat lost from the system, so TΔS must be the same in both situations.
--- End quote ---
Are you 100 % sure ? I think that for 100 % conversion of heat (fuel) into energy of battery overall ΔH = 0 and TΔS = 0, while for 0 % conversion - ΔH < 0 and TΔS > 0. Conversion of heat into energy of a battery reduces growth of entropy.
If you 100 % sure I can't help you any further.
ASmith:
"Conversion of heat into energy of a battery reduces growth of entropy."
Exactly. The entropy varies, it depends on how ΔH is used, but ΔH is constant. Therefore ΔH can't be equated with TΔS. ΔH = TΔS is false.
The answer seems fairly simple. The chemical bonds in the fuel neither know nor care how the energy released may be used. It's energy that matters (as well as temperature) not entropy.
Does anyone else wish to defend the use of entropy instead of energy?
Borek:
--- Quote from: ASmith on June 09, 2021, 05:46:01 PM ---Why is the heat flow from a system divided by T equated to the entropy change in the surroundings?
--- End quote ---
Only for reversible processes. General form is [itex]dS \geq \frac{dQ}T[/itex].
ASmith:
I thought all reactions were reversible - depending on concentrations etc. - but I don't see what this has to do with the point I'm making. How can the physics at the instant of a reaction depend on the fate of the energy that's released?
To try to make this clearer suppose there is negative Gibbs energy for a mole of products from reactions in the Sun. The energy could all be radiated into space and hence be said to contribute to the entropy of the surroundings. The energy though could fall on, for example, a solar panel on a spacecraft and some of it stored in a battery. So not all of this energy would increase the entropy of the surroundings. Hence ΔS is variable but ΔH and ΔG aren't, so the equality is false. Or do you think the Gibbs energy varies to take account of what will happen to the radiated energy at some time in the future?
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