why is the entropy change at constant pressure greater than the entropy change at constant volume?

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

Harjas Sabharwal 1G
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Joined: Sat Jul 22, 2017 3:01 am

why is the entropy change at constant pressure greater than the entropy change at constant volume?

Why is the entropy change at constant pressure greater than the entropy change at constant volume? I understand mathematically by looking at the formula, but I was curious why conceptually?

Chris Pleman 3E
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Re: why is the entropy change at constant pressure greater than the entropy change at constant volume?

Beyond the fact that C(p,m) > C(v,m), it could be that when pressure remains constant that there's no relief in the force acting on the molecules, as when volume changes pressure will change inversely. That's my best bad guess. Maybe it could have to do with the fact that (delta)S=(delta)H/T at constant pressure, whereas (delta)S=(delta)U/T at constant volume.

Juanalv326
Posts: 49
Joined: Fri Sep 29, 2017 7:04 am

Re: why is the entropy change at constant pressure greater than the entropy change at constant volume?

You also have to take into account that there are different Cp for the different states matter

Angel Chen 2k
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Joined: Fri Sep 28, 2018 12:20 am

Re: why is the entropy change at constant pressure greater than the entropy change at constant volume?

In the isochoric change process, the gas does not do any work; on the other hand, in the isobaric change process, the gas does work to expand the gas. In addition, according to the 2nd law of thermodynamics, entropy in the isolated system always stays the same or increases. Therefore, since gas does work in isobaric change processes, the entropy at constant pressure will be greater than that at constant volume.

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