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### Using Cp vs just R or C for ΔS

Posted: Tue Feb 11, 2020 1:10 am
When reading a problem how do you know which equation to use for ΔS?
options: ΔS= nRln(V2/V1) vs ΔS= Cln(T2/T1)
I'm having trouble distinguishing which to use based on the information given in problems, any insight will be helpful!

### Re: Using Cp vs just R or C for ΔS

Posted: Tue Feb 11, 2020 10:21 am
ΔS= nRln(V2/V1) is used when we are calculating change in entropy from a change in volume

ΔS= Cln(T2/T1) is used when we are calculating change in entropy from a change in temperature. C is Cv if volume is constant (isochoric), Cp if pressure is constant (isobaric).

Note that occasionally we have a situation in which both parameters change, in which case we find both values separately as two different steps, and add them for the total delta S of the process. Something kind of confusing that Lyndon went over during pizza rolls review: if we are using both equations in which volume did change as well as temperature, we first find delta S of the volume change, then use Cp (assuming pressure isn't changing) because we accounted for the change in volume in the first step.

### Re: Using Cp vs just R or C for ΔS

Posted: Tue Feb 11, 2020 8:23 pm
Which equation to use is dependent on which information you were given in the problem or which information could be solved for and then plugged into another equation. You really have to combine equations and move things around to fit what info you were given.