# Gibbs Free Energy and maximum work

1. Sep 30, 2015

### tonyjk

Hello,
The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system; this maximum can be attained only in a completely reversible process. This maximum work is equal to H-TS. My question is this TS energy is what kind of energy? and from where it will come if it is not from the internal energy of the system?

Thank you

2. Sep 30, 2015

### DrDu

The change of G is only the maximum work if the process is isothermal. Then $\Delta TS=T\Delta S$ and, as the process is reversible, the change of entropy of the system has to equal the change of entropy of the environment, which is $\Delta S = Q/T$. So basically you are substracting the heat released by the system from the change in energy to get the work, only.

3. Sep 30, 2015

### tonyjk

But the change of internal energy is it also the heat generated by the system?

4. Sep 30, 2015

### tonyjk

Ah you mean $T\Delta S$ is part of change of internal energy of the system thus subtracting to $\Delta H$ you will get the useful work from internal energy right?

5. Sep 30, 2015

### DrDu

Yes, exactly.

6. Sep 30, 2015

### tonyjk

Just a last question,

Gibbs free energy used in chemical reactions; excluding oxydo-reduction reaction, is only used to know if the reaction is spontaneous or not and does not have any physical meaning? If It has, what is work done by a chemical reaction? Is it related to potential energy due to chemical bonding?

Thank you

Last edited: Sep 30, 2015