P-Chem problem, need help please

[vixen]

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I am struggling with this problem and I need some help!!!!!!!

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.

In B, its a constant Pressure, and I have equations for that but don't know how to get all the things to plug in...

if I get a and b, im sure i could get c...

can anyone help? thanks

 

[vixen]

can a moderator please put this post in the MCAT forums instead because it might be more relevant there...thanks.

 

[hanselthemedic]

ARGGHHHH!!! THE FLASHBACKS!!

 

[Severus]

I am struggling with this problem and I need some help!!!!!!!

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.

In B, its a constant Pressure, and I have equations for that but don't know how to get all the things to plug in...

if I get a and b, im sure i could get c...

can anyone help? thanks

Hmmm... that is tough. Well, no doubt you'd have to apply deltaU=q +w to each situation. So let's see...for situation A, I think you're on the right path. What gets me about the last two sections of the problem is what's happening to the temperature of the system as the gas expands, etc. I mean if we're talking about a reversible constant-pressure expansion of an ideal gas for situation B, you're going to need the temperature differential in order to calculate the integral: deltaU= [integral T1-T2]Cv(T)dT. Well, maybe not. If your prof's just looking for an equation as an answer, then you could just solve it with the integral and get something like: qrev= [integral T1-T2]Cv(T)dT + P1(V2-V1). I think after that... section C is really just sort of the same, since your wrev is going to be negative (work done by the gas), and qrev probably isn't going to matter if it's adiabatic, so... dU=deltaw.

That is a really crappy answer - I'm sorry - but it would so totally help (at least for me) if they gave you more information. Of course, maybe I'm making the problem more difficult than it needs to be.

Good Luck!

Sev

Geez, I forgot the stuff at the beginning of your message... but, I think it should still work given the specific #'s. I have to go to work now, but, I'll think it over during the day.

 

[Wiggy73]

Okay, it's been almost two years since p-chem for me, but IIRC...

deltaU = q+w like Sev said

You don't need any other temperature information since the problem said each expansion was isothermal.

I'm thinking A and B are irreversible expansions since C was specifically mentioned as reversible, and I only remember one type of problem with reversible isothermal expansions. There's a formula involving p(ext) that escapes me at the moment... but it's a very easy plug-in.

C is actually not bad. U=U(T), U is a function of T, so in a reversible isothermal expansion deltaU=0. Then w=-integral of P dV (or whatever the formula is), and q=-w and you're done!

 

[Peterock]

ARGGHHHH!!! THE FLASHBACKS!!

*white hot pain shoots through Peterock's skull*

IT BURNS IT BURNS!!!!!!!!

 

[adamj61]

can a moderator please put this post in the MCAT forums instead because it might be more relevant there...thanks.

Can a moderator suck my balls? This has nothing to do with the mcat