3 replies to this topic

[Karen123]

I've started an statistical thermodynamic course and although my knowledge is premature, there is something I don't quite understand.

It is stated that for a system to be in equilibrium ds = 0. Is this true for all systems, including phase changes? Because when deriving the Claudius clapeyron equation, it is stated that for a single component in phase equilibrium the following must be satisfied,
The chemical potentials must be equal between the liquid and gas phase and dG = 0. This I understand because both component must have the same Gibbs free energy during a phase change, but what about ds=0 for phase changes? Is a phase change not an equilibrium state?

[Francisco Angel]

Dear Karen123

Check this link for example:

http://web.gps.calte.../tutorial1.html

I'm not an expert, but I remember from my thermodynamic class back at university, that you can derive different equilibrium criteria, depending of which variables are kept constant in the system: P, T, E,V etc, which translate to different quantities being minimized or maximized: S, H, G, A (Helmholtz free energy). In particular G is minimized in a spontaneous process at constant pressure and temperature, a restriction that we tend to forgot fast xd.

 

Best regards.

[Ganesh Buddha]

two phases in equilibrium does not entail that there is no phase change, but phase change always does not mean that the phases are at equilibrium... please think over it...

[MrShorty]

I feel like there must be some context missing from this. It is pretty clear that, in a two phase system, the entropy of the vapor and the entropy of the liquid will be different. This is easy to see using a calculator like NIST's (http://webbook.nist....hemistry/fluid/ pick any fluid and compute equilibrium state). You should see that entropy of the vapor phase and entropy in the liquid phase are different. Either the statement is specifically talking about equilibrium in a single phase system, or there is something else being discussed that gives that condition.