Cv vs. Cp

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Janice Park 1E
Posts: 36
Joined: Fri Sep 28, 2018 12:22 am

Cv vs. Cp

Postby Janice Park 1E » 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?

Kate Chow 4H
Posts: 61
Joined: Fri Sep 28, 2018 12:28 am

Re: Cv vs. Cp

Postby Kate Chow 4H » Fri Feb 15, 2019 8:33 am

You use Cv when the volume is constant and Cp when the pressure is constant.

Ray Huang 1G
Posts: 30
Joined: Fri Sep 28, 2018 12:20 am

Re: Cv vs. Cp

Postby Ray Huang 1G » 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.

Tarika Gujral 1K
Posts: 62
Joined: Fri Sep 28, 2018 12:27 am

Re: Cv vs. Cp

Postby Tarika Gujral 1K » 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

Nicholas Le 4H
Posts: 74
Joined: Fri Sep 28, 2018 12:24 am

Re: Cv vs. Cp

Postby Nicholas Le 4H » 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.

MadelynNguyen1F
Posts: 59
Joined: Fri Apr 06, 2018 11:04 am

Re: Cv vs. Cp

Postby MadelynNguyen1F » 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".

Lopez_Melissa-Dis4E
Posts: 66
Joined: Fri Sep 28, 2018 12:20 am

Re: Cv vs. Cp

Postby Lopez_Melissa-Dis4E » Fri Feb 15, 2019 4:51 pm

Cv means constant volume and Cp means constant pressure.


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