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Midterm 2011 2B

Posted: Sat Feb 06, 2016 5:38 pm
by Julia Kim 1D
The question states that
Container A: 1 mol of ideal monatomic gas
Container B: 1 mol of diatomic gas, not vibrationally active
Container C: 1 mol of diatomic gas, vibrationally active.

The answer states that the containers, in order of increasing change in entropy is B<C<A

Why is A the greatest. I could understand that maybe since A is smaller and lighter, it would move around more creating more entropy, but the answer states that it is because it has a larger number of particles than B or C. But if they are all one mole, don't they have the same number of particles?

Can someone explain why A is the greatest?

Re: Midterm 2011 2B

Posted: Sat Feb 06, 2016 8:05 pm
by Jonathan Shih 3H
Hey! My TA actually warned me about this earlier and I think I can help!

Basically, the problem is being tricky here, where it is stating that there is 1mol of the element, and that element is in the form of a monatomic or diatomic gas. This means that for the diatomic gas, there are essentially half the number of particles in the container and thus much less entropy.

As for the difference between B and C, I think the entropy difference is pretty obvious based on the difference between one that is vibrationally active versus one that is not vibrationally active.

I personally don't think it's a greate question and should probably be written in a way that's more clear, but I'm pretty sure that's what the answer is supposed to be. Hope this helps!

Re: Midterm 2011 2B

Posted: Sat Feb 06, 2016 8:07 pm
by Viktor2E
The question specifies that the one mol refers to the amount of atoms not molecules. In the diatomic gas, these atoms are bound together to make half a mol of gas molecules. In the monoatomic gas, the atoms are free. That is why the monoatomic gas has more particles.