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Topic: How do I know which molecules are polar?  (Read 19744 times)

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s3a

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How do I know which molecules are polar?
« on: November 18, 2009, 01:41:04 PM »
Question: The following molecules contain polar covalent bonds. Which of them are polar molecules and which are nonpolar? (a) CCl4; (b) CHCl3; (c) CO2; (d) H2S; (e) SO2

How do I do this? My solution book shows drawings and the cancellation of vectors. I'm good at math so I can deal with vectors but what I'm having trouble with (that is IF I actually have to know how to draw these figures) is with drawing the figures that show the directions of the vectors.

If someone knows an easier method; please explain it to me otherwise I'd still appreciate the explanation of how to determine what these drawings are.

vanklik

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Re: How do I know which molecules are polar?
« Reply #1 on: November 18, 2009, 02:38:08 PM »
You need to draw the structure of the molecules and see if they are symmetrical. The symmetrical molecules like H2S, CO2 will be unpolar because the charges cancell out. The molecule like SO2 will be polar because it contain polarised bonds arranged unsymmetrically in the molecule.
Does it make sense?

cth

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Re: How do I know which molecules are polar?
« Reply #2 on: November 18, 2009, 02:52:51 PM »
Yes, you need to determine the 3-dimensional conformation of molecules and see if the vectors cancel each other out by symmetry.

It is not always as simple as it seams. For example, H2S is polar because it is bended http://en.wikipedia.org/wiki/Hydrogen_sulfide (dipole moment of 0.97D). The polarized bonds do not fully cancel each others in this case. The same is indeed true for SO2, it is polar because it is bended http://en.wikipedia.org/wiki/Sulfur_dioxide.

As for CO2, it is indeed linear: O=C=O. So, the polar vector from one bond is exactly cancelled by the one from the other bond.

vanklik

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Re: How do I know which molecules are polar?
« Reply #3 on: November 18, 2009, 03:25:18 PM »
It is not always as simple as it seams. For example, H2S is polar because it is bended http://en.wikipedia.org/wiki/Hydrogen_sulfide (dipole moment of 0.97D). The polarized bonds do not fully cancel each others in this case.

Thanks for correcting me! I know the theory but didn't have enough practice yet.