Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: coffeecat424 on September 23, 2004, 09:46:19 PM
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I am struggling with this problem and I need some *delete me*!!!!!!
An ideal gas is expanded isothermally at 22degrees C from 22.4L to 22.8L three different ways (A, B, and C). For each process, calculate the q, w and delta U.
A. Assume the gas expands freely against zero external pressure.
B. Assume the gas expands against a constant pressure which is equal to the final pressure of the gas.
C. Assume the gas expands reversibly.
I thought A meant it was a vacuum so P=0 which would make Wext=0 also, but I don't know how to get delta U or q.
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please read the sticky thread before posting.
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First of all, A B and C are all isothermal ideal gas scenarios. For such situations delta U is always 0.
A) you're right, P is 0 so w is 0. So calculate q.
B) This is an irreversible process because the gas is expanding against a fixed constant pressure. I
w = -[integral(P dV)]
Now, since P is a contant external pressure, the integration is simple.
w = -P(delta V)
The simple trick here is to substitute for the final Pressure from the ideal gas equation.
C) This is a reversible process. It is the same form as above, but with a difference:
w = -[integral(P dV)] = -[integral(nRT/V dV)]
= -(nRT)ln(V2/V1)
notice that here, P is not constant. It is substituted for and integrated because the external pressure is slowly changing as a gas slowly expands in a reversible process.