Physics Help Forum Origin of Gibb's free energy

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Apr 28th 2019, 08:14 AM #1 Junior Member   Join Date: Apr 2019 Posts: 2 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.
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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 Apr 29th 2019, 12:08 PM #3 Junior Member   Join Date: Apr 2019 Posts: 2 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
Apr 30th 2019, 06:45 AM   #4
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 Originally Posted by TheClimber 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

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.

 Tags energy, free, gibb, gibbs free energy, origin

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