## Cv vs. Cp?

claudia_1h
Posts: 111
Joined: Fri Aug 09, 2019 12:16 am

### Cv vs. Cp?

How do we know when to use Cv vs. Cp? I know one is for constant volume and one is for constant pressure, but what about for a sudden irreversible compression of gas where there is a temperature and volume change? For the temperature part of the entropy calculation, how would we know which c value to use?

Chem_Mod
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### Re: Cv vs. Cp?

When there is a change in entropy due to temperature and volume changes you would break up the problem as follows: deltaS total = deltaS due to volume change + deltaS due to temperature change. The idea is to split the problem up and calculate deltaS due to volume change and then deltaS due to temperature change. Because in part 1 we've already calculated the deltaS due to a change in volume when it comes to calculating the deltaS due to temperature change, we assume that the volume isn't changing and therefore use Cp.

claudia_1h
Posts: 111
Joined: Fri Aug 09, 2019 12:16 am

### Re: Cv vs. Cp?

we assume that the volume isn't changing and therefore use Cp.

^How does this make sense because if volume isn't changing, isn't that assuming "constant volume" which points to using Cv, not Cp?

805097738
Posts: 180
Joined: Wed Sep 18, 2019 12:20 am

### Re: Cv vs. Cp?

if there is both a change in volume and temperature you do two separate calculations for each using the proper C respectively and then add the two together