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9.75 positional disorder in cis vs trans molecules

Posted: Thu Jan 26, 2017 12:21 am
by Yiling Liu 1N
"Considering positional disorder, would you expect a crystal of octahedral cis-MX2Y4 to have the same, higher, or lower residual entropy than the corresponding trans isomer? Explain your conclusion."

The answer key says that for the cis compound there will be 12 different orientations.

Besides for using trial and error and drawing out each configuration, is there some other way to arrive at the answer?

Re: 9.75 positional disorder in cis vs trans molecules

Posted: Tue Jan 31, 2017 12:11 am
by Chem_Mod
There are not much shortcuts for these type of questions, but a way to reason the orientation is to recognize placing a X in the vertical position (z) will result 4 possible placements of the cis-X. Thus for opposite (z) direction should have 4 as well. The last one to consider is in the x-y plane, which has 4 edges if you draw out the plane, and due to the cis positioning, there are only 4 orientations. Net of 12.