Saturnine

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I have a question about one of the practice exam questions. Question 22 (p.10) on AAMC 3R asks about geometry and dipole moments. While I got the question correct, I want to make certain I understand it. Using the EK method, the geometry of PCl3 is sp2 with a corresponding bond angle of 120 degrees. The image has one of the angles at 107 degrees. Is it because of this difference in angles that creates the dipole moment? Also, would someone please explain why PCl3 is pyramidal and not bent/trigonal planar? Is it simply because of the picture in the question?

Thanks
 

gujuDoc

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Saturnine said:
I have a question about one of the practice exam questions. Question 22 (p.10) on AAMC 3R asks about geometry and dipole moments. While I got the question correct, I want to make certain I understand it. Using the EK method, the geometry of PCl3 is sp2 with a corresponding bond angle of 120 degrees. The image has one of the angles at 107 degrees. Is it because of this difference in angles that creates the dipole moment? Also, would someone please explain why PCl3 is pyramidal and not bent/trigonal planar? Is it simply because of the picture in the question?

Thanks
I believe the answer to your question is yes. If memory serves me correct, when I took organic chemistry, I remember learning that based on the angles, you look to see if the dipole moments can cancel each other out.

For instance, in a tetrahedral molecule, the 4 bond moments cancel out because they directly oppose each other.

Also, another example is to look at CO2. Looking at CO2, we see that the 180 degree bond angle allows for the two dipole moments to be directly opposing each other causing them to cancel out. On the other hand with something such as Carbonate, where there are 3 entities, the substituents are not directly opposing each other. So none of the bond moments cancel out.

Also in the case of the question which you are referring to, the reason PCl5 is not the answer, I believe the smaller angles also contribute to the smaller dipole moment. But perhaps shrike or someone else may be able to help you on this one.
 
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Saturnine

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Blast! That's it. Thanks!