4
$\begingroup$

I'm wondering if there is any difference between Gibbs Free Energy and Exergy because their definition seems to me the same $\Delta H - T_\text{env}\cdot\Delta S$.

Exergy represents the maximum amount of work that can be extracted from a system going in equilibrium with its environment, while the Gibbs Free Energy is evaluated at $T,P$ of the system in equilibrium with the environment.

Practically what is the difference, if any?

|improve this question
$\endgroup$
2
$\begingroup$

Exergy is equal to the Gibbs free energy if you happen to be in the isothermal--isobaric ensemble $(N, P, T)$, i.e. in a closed system if the pressure $P$ and temperature $T$ are constant. This is indicated in Eq. 7 of the Wikipedia page for exergy.

|improve this answer
$\endgroup$
  • 1
    $\begingroup$ I would add a personal note: I've done thermodynamics (and taught it) for many years without ever using the concept of exergy… Although it has some historical relevance, I am not sure how useful it is in a modern presentation of statistical thermodynamics. $\endgroup$ – F'x Aug 11 '16 at 11:35
  • $\begingroup$ Thanks for your precise and clear answer. So Exergy is a way to "call" the free energies depending on the process considered (constant volume-->Helmholtz free energy, constant pressure-->Gibbs free energy)? For example if I considered a stream of hot gas, the maximum work that can be extracted, i.e. its exergy, is the Gibbs free energy if I assume to be at constant pressure? $\endgroup$ – horowitz Aug 11 '16 at 12:46
0
$\begingroup$

Gibbs free energy is calculated without possible PV-work, ie. without volume changes.

|improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.