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### Cv vs. Cp

Posted: Fri Feb 15, 2019 3:06 am
When is it appropriate to use Cv = 3/2 *R versus Cp = 5/2*R?
I know you are supposed to use Cv when the problem is dealing with a monoatomic gas, but do they have any relation to pressure and volume?

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 8:33 am
You use Cv when the volume is constant and Cp when the pressure is constant.

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 8:47 am
Cv is used when the problem states the volume doesn't change and Cp is for a constant pressure. In some cases, it's really tricky like the multi change one. But think about what value is changing at independent steps.

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 11:45 am
When calculating change in entropy for systems that have a change in both volume and temperature:

S = nRln(V2/V1) + nCvln(T2/T1)
Because entropy is a state function: you can add change in volume to change in temperature.
For change in temperature, you use Cv.

Cv and Cp can also be found for linear and nonlinear molecules.

LINEAR: Cv = 5/2R Cp = 7/2R
NONLINEAR: Cv = 3R Cp = 4R

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 2:31 pm
You use the value for Cv when the volume in the problem is constant, and you use the value for Cp when the pressure in the problem is constant.

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 4:22 pm
The professor also stated on a previous chem community post that " a monatomic ideal has 3 degrees of freedom and each contributes (1/2)nRT towards the internal energy. So the total internal energy would be (3/2)nR. And since equating both we get Cv = 3/2R. And you know Cp-Cv = R so Cp = Cv + R = (5/2)R".

### Re: Cv vs. Cp

Posted: Fri Feb 15, 2019 4:51 pm
Cv means constant volume and Cp means constant pressure.