Constant Volume and Constant Pressure

Volume:
Temperature:

Moderators: Chem_Mod, Chem_Admin

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

Constant Volume and Constant Pressure

Postby Kelsey Jug 1J » Thu Mar 16, 2017 11:33 am

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

Postby Joseph_Ghaly_2L » Thu Mar 16, 2017 11:31 pm

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; 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.

Wayland Leung
Posts: 51
Joined: Sat Jul 22, 2017 3:00 am
Been upvoted: 1 time

Re: Constant Volume and Constant Pressure

Postby Wayland Leung » Sat Jan 27, 2018 2:18 pm

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

Farah Ahmad 2A
Posts: 30
Joined: Thu Jul 27, 2017 3:00 am
Been upvoted: 1 time

Re: Constant Volume and Constant Pressure

Postby Farah Ahmad 2A » Sat Jan 27, 2018 10:49 pm

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.


Return to “Entropy Changes Due to Changes in Volume and Temperature”

Who is online

Users browsing this forum: No registered users and 1 guest