when to use a different C

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Payton Kammerer 2B
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Joined: Tue May 01, 2018 3:00 am

when to use a different C

Postby Payton Kammerer 2B » Tue Mar 10, 2020 9:46 pm

4F.7 Assuming that the heat capacity of an ideal gas is independent of temperature, calculate the entropy change associated with raising the temperature of 1.00 mol of ideal gas atoms reversibly from 37.6 8C to 157.9 8C at (a) constant pressure and (b) constant volume.

the solution manual wants me to use C=5/2R at constant P and C=3/2R at constant V

Why isn't it correct to do the break-it-down thing where you first calculate the entropy change due to the temp change and then do the entropy change due to the volume/pressure change? You can use PV=nRT to find the change in either of those things as the other stays constant, right? So why do the heat capacity substitution instead of this method?

Kevin Antony 2B
Posts: 99
Joined: Sat Sep 07, 2019 12:16 am

Re: when to use a different C

Postby Kevin Antony 2B » Tue Mar 10, 2020 9:57 pm

Cp and Cv are specifically meant to be used at a constant pressure or a constant volume. The values C=5/2R and C=3/2R correspond to values specifically for ideal gases.

Posts: 100
Joined: Fri Aug 30, 2019 12:18 am

Re: when to use a different C

Postby ShravanPatel2B » Tue Mar 10, 2020 9:58 pm

This is because in each case one of the values of temperature or pressure are being kept constant. Depending on this you would use the constant heat capacities 5/2 R or 3/2 R in your equation when solving to take into account the constant pressure or temperature

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