Midterm Winter 2013 problem 5B

Boltzmann Equation for Entropy:

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Ryan Cerny 3I
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Midterm Winter 2013 problem 5B

Postby Ryan Cerny 3I » Thu Feb 09, 2017 9:44 pm

We are supposed to rank CHF3, CF4, CH3F, and CH2F2 in order of increasing standard molar entropy.
The answer is CH3F<CH2F2<CHF3<CF4.

Why wouldn't it be CF4<CH3F<CHF3<CH2F2?

Thank you.

Laura Rabichow 1J
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Re: Midterm Winter 2013 problem 5B

Postby Laura Rabichow 1J » Fri Feb 10, 2017 3:39 pm

CF4 is symmetrical, whereas the other three are asymmetrical. Asymmetry gives the other elements different possible structures, which gives them higher standard molar entropy. Furthermore, CH3F has the greatest # of possible arrangements, followed by CH2F2, then CHF3, which is why they're ranked in that order.

Ryan Cerny 3I
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Re: Midterm Winter 2013 problem 5B

Postby Ryan Cerny 3I » Sat Feb 11, 2017 12:39 pm

If CF4 is symmetric then why would it be ranked the highest for molar entropy? Shouldn't it be ranked the lowest?

Diane_Zhao_3I
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Re: Midterm Winter 2013 problem 5B

Postby Diane_Zhao_3I » Tue Feb 14, 2017 2:50 pm

I think the official explanation is that because F is a more complex atom than H, the more F atoms a molecule has, the more standard molar entropy the molecule possesses. However, I don't understand that explanation either, because wouldn't molecular complexity be more important than atomic complexity?

Chiao-Wei Cheng 3e
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Re: Midterm Winter 2013 problem 5B

Postby Chiao-Wei Cheng 3e » Tue Feb 14, 2017 3:22 pm

Check out the molar mass. CF4 is the heaviest therefore it has the most entropy. Hope that helps!

NinaSheridan
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Re: Midterm Winter 2013 problem 5B

Postby NinaSheridan » Tue Feb 14, 2017 5:05 pm

If anyone can explain why we look at molar mass rather than the possible positions of the molecule that would be great :)

Coco Hailey 2E
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Re: Midterm Winter 2013 problem 5B

Postby Coco Hailey 2E » Tue Feb 14, 2017 6:21 pm

I'm also very confused why that is the correct order!! What is the explanation?

Chiao-Wei Cheng 3e
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Re: Midterm Winter 2013 problem 5B

Postby Chiao-Wei Cheng 3e » Tue Feb 14, 2017 9:56 pm

Correct if I'm wrong, I think molar mass is related to molecules quantity, related to the W in S=klnW. Therefore, entropy will increase with the increase of the molar mass.

Iris Feng 2I
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Re: Midterm Winter 2013 problem 5B

Postby Iris Feng 2I » Tue Feb 14, 2017 10:10 pm

Chiao-Wei Cheng 3e wrote:Correct if I'm wrong, I think molar mass is related to molecules quantity, related to the W in S=klnW. Therefore, entropy will increase with the increase of the molar mass.


W is just (# of orientations)^(# of particles), so I don't think W has to do with the molar mass of one single molecule

Angela_Pu_3C
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Re: Midterm Winter 2013 problem 5B

Postby Angela_Pu_3C » Wed Feb 15, 2017 2:08 am

CH4 has highest STANDARD molar entropy bc it has highest mass. When the molecule is heavier, its vibrational energy is higher, so it has more entropy.
You look at atom positions when you're looking at RESIDUAL entropy. This is when the molecules are given at T=0K. In this case, for ex CH4 and CF4 would have the same residual entropy bc they have the same symmetry. However, if you compared CI4 and CH3I, CH3I has more residual entropy even though it has less mass bc it has more positions than CI4. Hope this helps :)

Josephine_Tang_1N
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Re: Midterm Winter 2013 problem 5B

Postby Josephine_Tang_1N » Wed Feb 15, 2017 11:27 am

Chem Mod also posted a response on another post saying that larger molecules with more electrons contribute more entropy because (basically) there are more total particles to be knocked around and cause disorder.


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