# Van der Waals Equation and specific heat

1. Nov 14, 2013

Consider a gas with constant specific heat cv and the van der waals equation of state
(P+ a/v2)(v-b)=RT , where v=V/n

A) Find du and the specific internal energy u=U/n
B) Find ds and the specifi entropy s=S/n

Here's what I've tried so far:

A) I took the initial equation and subbed in the v=V/n :
(P+ a/(V/n)2)(V/n-b)=RT

I know that du= cvdT which can be rearranged cv = du/dT so that is where I can find the du is from the specific heat. However, I don't understand how the van der waals equation can be related mathematically to specific heat?
I know that a=a measure of attraction between particles and b=the volume excluded by a mol of particles. Do I need to solve for these first?

Any help is appreciated!

2. Nov 14, 2013

### Staff: Mentor

dU = cv only for an ideal gas. For a non-ideal gas, u also depends on pressure P. Have you learned the general equation for du in terms of dT and dP?

3. Nov 18, 2013

I found this equation:
dU=CvdT+ [T(∂P/∂T)v - P]dV
but it doesn't mention anything about being related to van der waals' equation so I'm not sure if this is relevant?? Honestly, our text doesn't go much into detail on van der waals so I'm pretty lost..

4. Nov 18, 2013

### Staff: Mentor

Your equation is supposed to apply to any gas (or liquid), so it should be applicable to a vdW gas. Yes, the vdW equation is relevant. So you substitute the vdW equation into the term in parenthesis. Incidentally, the dV should be dv.