Maxwell's relations (thermodynamics): validity

[plmokn2]

Homework Statement

I came across a slightly unusual question today. It started out fine, just asking me to derive a maxwell relation but then asked under what conditions is this relationship valid.

Homework Equations

The Attempt at a Solution

In deriving the relation I need to assume U, V and p are functions of state, but I can't think of any system where this wouldn't be the case. Also the first law must take the form TdS-pdV which only applies for simple gas systems without any extra freedom. But is that is?

If someone could let me know if I'm missing something obvious here (or just confirm I'm right) I'd appreciate it.
Thanks

 

[Shooting Star]

Probably, it's the mathematical fact that the order of partial differentiation of an analytic function of two variables does not matter.

del/del x(del f/del y) = del/del y(del f/del x).

 

[plmokn2]

Thanks