## Cv and Cp

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

Kate_Santoso_4F
Posts: 72
Joined: Fri Sep 28, 2018 12:29 am

### Cv and Cp

Can someone please explain how we know to use Cp or Cv when calculating the entropy change for something that has a change in temperature? I know the problem should indicate constant pressure or constant volume but it doesn't always say so.

Manya Bali 4E
Posts: 66
Joined: Fri Sep 28, 2018 12:23 am

### Re: Cv and Cp

If there's nothing specified, you usually use Cp since you can assume that under normal conditions, the pressure is constant.

However, if you're calculating entropy with a temperature and volume change, you break up the calculations into 2 parts. One is the entropy change when the temperature is constant and the volume changes. The other is when the temperature changes and volume is constant. In the second case, you would use Cv.

That's the main case I've seen where it gets confusing. Of course, if the problem specifies constant pressure/volume then you already know what to do.

gwynlu1L
Posts: 62
Joined: Fri Sep 28, 2018 12:19 am

### Re: Cv and Cp

For Cp and Cv, you also have to pay attention to if the gases you're working with are monoatomic, linear, etc. That also changes that value of Cp and Cv.

Mukil_Pari_2I
Posts: 87
Joined: Fri Sep 28, 2018 12:29 am

### Re: Cv and Cp

Usually you will be given some information as to the conditions of the situation. Based of that, you should be able to figure out if it is constant pressure or constant volume.