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coffeecat424

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« on: September 23, 2004, 09:46:19 PM »
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.

Mitch

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« Reply #1 on: September 23, 2004, 11:46:55 PM »
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
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Demotivator

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« Reply #2 on: September 24, 2004, 06:30:51 PM »
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.

« Last Edit: September 24, 2004, 06:40:47 PM by Demotivator »