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Old Apr 28th 2019, 08:14 AM   #1
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Origin of Gibb's free energy

Gibb's free energy gives the energy which is easily available in the system, it can be used to do work. But I wonder what is the origin of the Gibb's free energy? Can anyone explain at the molecular point of view? As in, say the electrons are in a random state of motion, so the collective energy they have at that point of time, can be used for work -- Or, it is that we cannot actually explain it like this, cuz afterall Gibb's free energy is explained from a a macroscopic point of view.
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Old Apr 29th 2019, 10:35 AM   #2
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I don't know much about the Gibbs free energy
(which is why I didn't respond sooner)
However I have had a little poke about on the web <Wikipedia : Gibbs Free Energy >

One thing I notice is that in your post you indicate:
Gibb's free energy gives the energy which is easily available in the system
However, Wikipedia seems to indicate that it is the maximum energy theoretically available from the system.
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Old Apr 29th 2019, 12:08 PM   #3
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Enthalpy is the maximum energy in a system. Gibbs free energy G = H - TS, where H is the enthalpy of the system. Due to an inherent disorder in the system (entropy, S) which cannot be converted to useful work, we have to subtract it from H
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Old Apr 30th 2019, 06:45 AM   #4
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Originally Posted by TheClimber View Post
Enthalpy is the maximum energy in a system. Gibbs free energy G = H - TS, where H is the enthalpy of the system. Due to an inherent disorder in the system (entropy, S) which cannot be converted to useful work, we have to subtract it from H
It was good you made this reply since it brings out your misunderstanding of the subject.

Woody told you true and was only trying to help.
I think he deserves thanks for that, don't you?

If heat content (enthalpy) is the maximum energy of a system I suggest you stand in front of a cannonball travelling at 200 Kilometers per hour and try to convince me that it has the same energy as a similar cannonball standing quietly by your left foot, but is at the same temperature as the travelling one.


A bit of mathematics for you.

yes, dG = dH - TdS with appropriate sign conventions.

BUT

T is always positive, and if dS is positive the process can be spontaneous.
If dH is negative (ie the process evolves heat) then dH and -Tds add and this makes dG more negative but greater in absolute value than dH.

This is the condition for a spontaneous process - that dG is negative (evolves energy)

As regards to your question, Gibbs free energy is no different than any other sort of energy.
It is a property of a body or system, a form of accounting, not a real entity like a cannonball. The caloric theory was discredited more than 250 years ago.

As an aside note that eneergy is not a conserved property under Einstinian relativity, the first law holds within any given frame but the energy sum will be different from frame to frame.
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