### The value of Cp

Posted:

**Wed Jan 31, 2018 8:40 pm**In 9.7 I believe we have to use the equation S = nCp lnT1/T2. Are we always given the value of Cp?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=133&t=26660

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Posted: **Wed Jan 31, 2018 8:40 pm**

In 9.7 I believe we have to use the equation S = nCp lnT1/T2. Are we always given the value of Cp?

Posted: **Wed Jan 31, 2018 8:42 pm**

on the test we should be given the value of Cp and you only use Cp if it explicitly says constants pressure.

Posted: **Wed Jan 31, 2018 9:25 pm**

Cp for ideal gases is 4R. Cv for ideal gases is 3R. When in doubt, remember that Cp = Cv + R

Posted: **Wed Jan 31, 2018 10:22 pm**

Cp of ideal gases is 5/2(R) and Cv is 3/2(R)

Posted: **Wed Jan 31, 2018 10:29 pm**

kaushalrao2H wrote:Cp for ideal gases is 4R. Cv for ideal gases is 3R. When in doubt, remember that Cp = Cv + R

I thought for an ideal gas Cp is (5/2)R and Cv is (3/2)R?

Posted: **Thu Feb 01, 2018 12:37 am**

Yup--for a monatomic ideal gas, Cv = 3/2R and Cp = Cv + R = 5/2R

Note that for diatomic gases, it gets a little tricker (e.g. for N2, Cv = 5/2R) due to more rotational degrees of freedom

Note that for diatomic gases, it gets a little tricker (e.g. for N2, Cv = 5/2R) due to more rotational degrees of freedom

Posted: **Thu Feb 01, 2018 8:38 pm**

So we only use the C value when there is a constant pressure value and use the R constant value when the pressure isnt constant?

Posted: **Thu Feb 01, 2018 11:06 pm**

The C_{p} and C_{v} values can be found on page 281 in the textbook. C_{p} is the heat capacity of the system at constant pressure while C_{v} is the heat capacity at constant volume. R is the ideal gas constant, and you would typically use R in the form 8.3145 J*K^{-1}*mol^{-1} for these calculations.

Posted: **Thu Feb 01, 2018 11:10 pm**

On the test, you will be given the table of values that will help you solve for cP