Help Please - Chemistry Final tomorrow and I get a lot of it except thisquestion on Piston, V, q, U, and Temp?

Question

2 Cases:

(-1000J energy lost in each case)= q

1 - Piston can move, change in volume occurs
...... w= P*deltaV... Work is positive. (WHY?)
2 - Piston can not move, no change in volume.
...... w=P*0 = 0

What can you say about U?

U = q + w

(1) - Change in U is less negative. (WHY?)
(2) - Since w = 0, Change in U = q = -1000J

______

In which case does the gas have a higher temperature after the removal of the energy?

Answer: deltaT and deltaU are related (HOW?) ... if deltaU is smaller, deltaT is smaller. ... So case 1 has lower temperature. (WHY?)


These are the parts of the question i dont get.


Note: The answers I gave are correct... The Why and Hows aren't asked of us on the test, they are just me personally not understanding why those answers are correct.

Answer 1

This must be an upper level chemistry class like Physical Chem or somewhere along those lines. Here are the best explanations i can offer to you sorry if they are not good enough to help you.


Work is positive because the piston is puching on the gases in side. If the gases makes the piston stop and go inthe opposite direction then it'll have a negative work on the piston.
--while studying specific heat the equation q=MCdeltaT
now you can replace the q with the equivalent. Then you can deduct what the delta T actually has to do with the delta U.
---Now for why there is less negative for case one. The reason why is that there is some positive value for the delta U, and the value for delta U is -1000. Okay so if the positive value for delta U is added to the -1000 then you are left with a higher number or less negative value.


-----The part about hte deltat U is smaller and delta T is smaller really bugs me. I couldn't help you out on that one.


We'll ask stated previously i hope i help you a little bit, and you should stop worring about the final and have a nice little sleep so you mind can work on what i've said. That'll give you time to rest and what every you do don't dream about it. You'll be exhausted.