## 9.13

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Merzia Subhan 1L
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Joined: Thu Jul 13, 2017 3:00 am

### 9.13

I was wondering about when to use Cp or Cv or whether to just use R. In 9.13, we have to assume ideal behavior:
During the test of an internal combustion engine,3.00 L of nitrogen gas at 18.5 degrees Celsius was compressed suddenly and irreversibly to 0.500 L by driving in a piston. in the process, the temperature of the gas increased to 28.1 degrees celsius. Assume ideal behavior. what is the change in entropy of gas?

I know that you have to add entropies but for delta S for temperature, the formula is nRlnT2/T1 instead of CvlnT2/T1. why is that? when it says assume ideal behavior, do we always use nR instead of Cv?

CameronJohari1J
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### Re: 9.13

9.13 is one of the problems listed in the errors in solution manual. Definitely check out the link here: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Solution_Manual_Errors_6Ed.pdf

Christy Lee 2H
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### Re: 9.13

How do we know whether to use Cv or Cp though? I thought the volume and pressure were both changing???

rkusampudi
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Joined: Fri Sep 29, 2017 7:04 am

### Re: 9.13

I think using nR allows us to cancel out the moles in the denominator but I am not sure.

Christy Lee 2H
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### Re: 9.13

I think for the first part of the problem we have to use nR but the second part is the one where we have to use C. And on the solution errors page it says to use Cv but it also says 5/2R

Wenting Hu 2H
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Joined: Fri Sep 29, 2017 7:06 am

### Re: 9.13

I have the same question. Why is it that for the change in volume, we use R, but then for the change in temperature, we use 5/2R?

Christy Lee 2H
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### Re: 9.13

Lavelle's formula sheet gives the approximation for monatomic ideal gases only, which is why it says 3/2R. For diatomic ideal gases, the approximation is 5/2R.

Kayla Ikemiya 1E
Posts: 56
Joined: Fri Sep 29, 2017 7:04 am

### Re: 9.13

We use Cv because N2 is an ideal gas, so we used 5/2R because N2 is diatomic. My TA said that the answer in the solutions manual was wrong. Hope this helps!