# Homework Help: Relation between heat capacities and van der Waals equation

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1. Oct 26, 2017

### patrickmoloney

1. The problem statement, all variables and given/known data
Find the expression for $c_p - c_v$ for a van-der-waals gas, with the equation of state

$$\Bigg{(}p+\dfrac{a}{V^2}\Bigg{)}(V-b)=RT$$

2. Relevant equations

3. The attempt at a solution
Basically I've proved

$$c_p - c_v = \Bigg{[} p + \Bigg{(}\dfrac{\partial E}{\partial V}\Bigg{)}_T \Bigg{]}\Bigg{(}\dfrac{\partial V}{\partial T}\Bigg{)}_p$$

($E$ and $V$ are the energy and volume of one mole).

The question states that

$$p + \Bigg{(}\dfrac{\partial E}{\partial V}\Bigg{)}_T = T \Bigg{(}\dfrac{\partial p}{\partial T}\Bigg{)}$$

I don't need to prove this I just need to use it.

So far the only thing that I can come up with is rearranging the van der waals equation in terms of $p$ which gives

$$p = \dfrac{RT}{(V-b)}-\dfrac{a}{V^2}$$

Then

$$\Bigg{(}\dfrac{\partial p}{\partial T}\Bigg{)}_V = \dfrac{R}{(V-b)}$$

Which can be substituted into the equation $c_p - c_v$ but rearranging the van der waals formula for $V$ is very tedious. Which leads to believe there is a an easier step. It's an exam question so there much be a neat way to solve this. I want to ask here rather than look up someone else's long winded solution. I'm not looking for the solution I genuinely want to be able to tackle this problem or another problem like it should it come up in the exam

2. Oct 26, 2017

### Staff: Mentor

Why can't you just differentiate the left side of the equation by the product rule?

3. Oct 26, 2017

### patrickmoloney

The van der waals equation?

4. Oct 26, 2017

### Staff: Mentor

Sure.

5. Oct 26, 2017

### patrickmoloney

Differentiating the LHS with respect to $T$ and keeping the volume constant I get

$$\Bigg{(}\dfrac{\partial p}{\partial T}\Bigg{)}_V= \dfrac{R}{V-b}$$

Substituting that into my desired equation I get

$$c_p - c_v = \Bigg{[}T\Bigg{(}\dfrac{R}{V-b}\Bigg{)}\Bigg{]}\Bigg{(}\dfrac{\partial V}{\partial T}\Bigg{)}_p$$

6. Oct 26, 2017

### TSny

You might be able to avoid solving for V by using the "reciprocal rule" for partial derivatives $$\left( \frac{\partial x}{\partial y} \right)_z = \frac{1}{\left( \frac{\partial y}{\partial x} \right)_z}$$

7. Oct 26, 2017

### patrickmoloney

Would that mean I could do $$\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p$$ to solve the VDW equation?

8. Oct 26, 2017

### TSny

Yes, but I'm not sure what you mean when you say "solve the VDW equation".

9. Oct 26, 2017

### patrickmoloney

Van der waals equation of state. Like I did with $$\Bigg{(}\dfrac{\partial p}{\partial T}\Bigg{)}_V$$

10. Oct 26, 2017

### TSny

Yes, you use the VdW equation of state to find an expression for the partial derivative.

11. Oct 26, 2017

### patrickmoloney

I got $$\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p = -\Bigg{(}\dfrac{2ab}{V^3} +\dfrac{a}{V^2}- P\Bigg{)} \cdot \dfrac{1}{R}$$

Then inverting this equation yields

$$\Bigg{(}\dfrac{\partial V}{\partial T}\Bigg{)}_p = -\Bigg{(}\dfrac{V^3}{2ab} +\dfrac{V^2}{a}-\dfrac{1}{P}\Bigg{)} \cdot R$$

12. Oct 26, 2017

### TSny

The first equation looks correct. It involves the three variables P, V, and T. You could use the VdW equation of state to express one of these variables in terms of the other two so that you reduce the number of variables in the expression. For example, you can express P in terms of V and T if you want the final answer to be in terms of V and T.

[EDIT: Sorry. Your first equation does not involve T. But if you use the first equation as you wrote it, your final answer for the difference in specific heats will involve the three variables P, V, and T. You can use the VdW equation to eliminate one of these variables.]

You did not invert the equation correctly. The inverse of the expression $\frac{2}{3} + \frac{4}{5}$ is not $\frac{3}{2} + \frac{5}{4}$. Rather, it would be $\frac{1}{\frac{2}{3} + \frac{4}{5}}$.

13. Oct 26, 2017

### patrickmoloney

$$\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p = -\Bigg{(}\dfrac{2ab}{V^3} +\dfrac{a}{V^2}- P\Bigg{)} \cdot \dfrac{1}{R}$$

Letting

$$p = \dfrac{RT}{(v-b)}-\dfrac{a}{V^2}$$

and substituting into my equation for $\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p$ gives

$$\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p= -\Bigg{(}\dfrac{2ab}{V^3} +\dfrac{2a}{V^2}- \dfrac{RT}{(V-b)}\Bigg{)} \cdot \dfrac{1}{R}$$

But this still looks hideous even though I have everything in terms of $V, a, b, R$ and $T$

14. Oct 26, 2017

### TSny

It looks like maybe the sign of the first term is wrong. (I should have caught that earlier.)

It won't be that bad when you put it all together.

15. Oct 26, 2017

### TSny

Somehow I overlooked @Chestermiller 's post #2. His suggestion is a nice way to get $\left(\frac{\partial V}{\partial T} \right)_p$ without needing to use the reciprocal rule. It's a method that is well worth keeping in mind since it can be used in many different circumstances.

At this point, you are almost done with the problem. After finishing it off, if you would like some guidance in this alternate method, let us know.

16. Oct 26, 2017

### patrickmoloney

I was thinking cause ]
Yeah I would love that in the end I got

$$\dfrac{T^2(2a-1)}{V^3(V-b)}$$ which looks respectable. It's not a homework problem anyway but it was from an exam paper which means it could be something that could come up in the exam. Would you be able to teach the method chester was talking about?

Thanks very much for all the help. You've been more than helpful.

17. Oct 26, 2017

### TSny

I don't think this is correct.

You have
$$c_p - c_v = \Bigg{[}T\Bigg{(}\dfrac{R}{V-b}\Bigg{)}\Bigg{]}\Bigg{(}\dfrac{\partial V}{\partial T}\Bigg{)}_p$$
This can be written
$$c_p - c_v = \frac{T\Bigg{(}\dfrac{R}{V-b}\Bigg{)}}{\Bigg{(}\dfrac{\partial T}{\partial V}\Bigg{)}_p}$$
After substituting for the denominator, I don't see how you would get your result.

Yes, certainly.

18. Oct 26, 2017

### patrickmoloney

Ok I've tried it using your fraction and got

$$\dfrac{RV(V-b)}{4a(\frac{b}{V}-1)}$$
I've just realised how bad I am at algebra.

19. Oct 26, 2017

### Staff: Mentor

I got $$\frac{R}{1-\frac{2a}{V^3}\frac{(V-b)^2}{RT}}$$

20. Oct 26, 2017

### TSny

I get the same result.

21. Oct 27, 2017

### patrickmoloney

I got $$\dfrac{RT}{1-\frac{2a}{V^3}\frac{(V-b)^2}{RT}}$$. But I'm happy enough that I know how to solve the problem should it come up. Thanks guys.

22. Oct 27, 2017

### Staff: Mentor

There should not be a T in the numerator.

23. Oct 27, 2017

### patrickmoloney

I got it from $$T \Bigg{(}\dfrac{\partial p}{\partial T}\Bigg{)}_V$$

I couldn't get an $R$ on its own

24. Oct 27, 2017

### Staff: Mentor

Too bad. Check the units on your expression for the difference between the heat capacities. It doesn't have the correct units.