## Cv vs. Cp

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

JeremyPark14B
Posts: 29
Joined: Fri Sep 28, 2018 12:19 am

### Cv vs. Cp

So Cv = (3/2)R is used when volume is constant

and Cp = (5/2)R is used when pressure is constant.

In the practice midterm worksheets, when volume is being changed, the answer key says to use Cv. Why is this so?

Becky Belisle 1A
Posts: 81
Joined: Fri Sep 28, 2018 12:18 am

### Re: Cv vs. Cp

Cv can be used when the entropy change is being addressed by multiple steps. For example, the entropy change can be calculated for the change in volume with ΔS=nRln(V2/V1). Then, the entropy change can be calculated for the change in temperature with ΔS=nCln(T2/T1). Since entropy is a state function, the entropy changes due to volume change and temperature change can be treated as separate then added together. So it does not matter exactly what the volume was when the temperature change occurred, thus is can be treated as a constant volume.