## Constant Volume and Constant Pressure

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

Kelsey Jug 1J
Posts: 38
Joined: Wed Sep 21, 2016 2:59 pm

### Constant Volume and Constant Pressure

Homework problem 9.7 that wants us to find the entropy change of 1 mol of ideal gas with a temperature change under constant pressure and constant volume. For the final, would we need to know that constant pressure is (5/2)R and that constant volume is (3/2)R? Also, for that same problem, how can something expand reversibly under constant pressure? I thought that constant pressure implied that an expansion was irreversible?

Joseph_Ghaly_2L
Posts: 22
Joined: Fri Jul 22, 2016 3:00 am

### Re: Constant Volume and Constant Pressure

Those equations are on the equation sheet, but you will probably have to know which one is for constant pressure or constant temperature. Also, a reversible heat transfer simply implies a change in volume under constant temperature; pressure can decrease or stay the same, but as you can see, the equation for work during a reversible heat transfer is
w=-nRT$ln(\frac{V_{2}}{V_{1}})$; there is no mention of pressure, although it could change during a reversible heat transfer. For an irreversible process, pressure must be constant. The equation for work during an irreversible heat transfer is
w=-P$\Delta T$.

Wayland Leung
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### Re: Constant Volume and Constant Pressure

What equation would we use if the volume or pressure was not constant to find entropy?

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### Re: Constant Volume and Constant Pressure

If the volume is changing you would use: n*R*ln(V2 / V1)

If the pressure is changing you would use: n*R*ln(P1/P2)

where n = moles, V = volume, and P= pressure.

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