Cv vs. Cp

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Janice Park 1E
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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
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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
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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.

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".

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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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