Standard residual entropy problem

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Joanne Reyes 1L
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Joined: Fri Sep 25, 2015 3:00 am

Standard residual entropy problem

Postby Joanne Reyes 1L » Sun Jan 24, 2016 4:09 pm

This question says "Assume COF2 adopted disordered arrangements in its crystal form, what would its residual molar entropy be?" and the answer is W = 3NA, S = k ln 3 NA = R ln 3 = 9.1 J/K. How would I go about solving this problem? Thanks.

Jeannie Huang 3B
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Joined: Fri Sep 25, 2015 3:00 am

Re: Standard residual entropy problem

Postby Jeannie Huang 3B » Sun Jan 24, 2016 4:58 pm

To answer the question, you need to draw the lewis structure for COF2. You should get a trigonal planar shape with C in the center. You could get 3 different orientations of the molecule since the O can be in one of the three corners. Following the equation,

S = k ln W (refer to pg. 32 of course reader)
and since W = 3^(NA) for the three different orientations, and assuming 1 mole of COF2,

You get S = k x (avogadro's # x 1 mol) x ln(3) = R ln 3 = 9.1 J/K.

Hope this helps! (and someone verify that I've explained correctly).

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Re: Standard residual entropy problem

Postby Chem_Mod » Sun Jan 24, 2016 5:09 pm

k = boltzmann constant is J/(particle*K)

NA= Avogadro's number = 6.022 x 1023 = particles/mole

k*NA= J/(mol*K) = R

If you're given a certain number of moles then

S=nRln(a) for the units to work out J/K, where a is the number of ways the molecule can be positioned (W=aNA)

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