Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Geoquest on 18/06/2011 22:12:38

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: Geoquest on 18/06/2011 22:12:38

Hi all...hoping you can provide some direction on a HW question. I've been pulling my hair out trying to figure it out and I'm starting to wonder if I really understand what the question is asking.

The question is:

Which molecule contains the most polar bonds?

(Is this asking which molecule has the most bonds, or which molecule is the most polar based on electronegativity???)

Possible answers:

CF4    CO2    CN-    CH4

Even more then just getting the answer I'm really interested in how one arrives at the best answer. I'm sure there will be an exam question like this and for now this topic is completely eluding me.

I guess I should add that I know there are 4 single bonds in CF4, 4 single bonds in CH4, 2 double bonds in CO2 and one triple bond for CN.

Thanks in advance folks!!

edit--after some further research I am going to with CN- since there is not a balanced element in the center of the molecule with equally balanced bonds surrounding it. Am I right and is my explanation correct?

Title: Re: What are polar bonds, and why are some molecules more polar than others?
Post by: CliffordK on 19/06/2011 01:44:20

Whew...
This has been too long ago!!!!

In the case of Cyanide.

Carbon is fighting for 4 bonds.
Nitrogen prefers 3.

So you get:

  ^-C≡N

With the Carbon picking up an extra electron / ionic bond.

I suppose if someone was asking what the most polar bond bond and there was an ion listed, I'd probably look at the ion first.  I suppose they could obscure the question by asking about HCN or NaCN.

Title: Re: What are polar bonds, and why are some molecules more polar than others?
Post by: damocles on 19/06/2011 06:36:23

I would venture CF4 as having the most polar bonds, simply because of electronegativity difference. C is approx 2.1, F is 4.0.

Double (and triple) bonds tend to reduce the polarity of bonds because the second bond usually counters the polarity of the first (look up 'back bonding' or 'back donation')

But as against a 1.9 electronegativity difference for a CF bond, the difference is only 1.4 for C-O single bond, less for C=O, and 0.5 for C-H.

The polarity of the bonds in a polyatomic ion is not well-defined, because you get a different answer depending on which exact location you assume the charge to be at. But the C-N electronegativity difference is only about 1 anyway.

I was taught that electronegativity difference is probably the best measure of bond polarity.

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: chris on 19/06/2011 22:06:05

CO2 is there to catch you out though, isn't it, because the symmetry of the molecule means that the electronegative effects of one oxygen counteract those of the other, rendering the molecule non-polar?

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: CliffordK on 19/06/2011 22:17:28

Good Point.
There are two questions.
Which molecule is the most polar.
Which molecule has the most polar bonds.

CF₄ has the most polar bonds, but the molecule is symmetric, and thus non polar.
^-C≡N (or HCN or NaCN) would be the most polar molecule.

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: Geoquest on 20/06/2011 03:08:34

Quote from: CliffordK on 19/06/2011 22:17:28

Good Point.
There are two questions.
Which molecule is the most polar.
Which molecule has the most polar bonds.

CF₄ has the most polar bonds, but the molecule is symmetric, and thus non polar.
^-C≡N (or HCN or NaCN) would be the most polar molecule.

I think that sums it up pretty well. That is pretty much what I was thinking too depending on what the question is asking.

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: CliffordK on 20/06/2011 03:41:27

I'm not sure if the unshared electrons on the fluorine will affect molecular interactions.  But, here are the notes on Wikipedia about Carbon Tetrachloride (which is similar to CF₄)

Quote from: http://en.wikipedia.org/wiki/Carbon_tetrachloride#Properties

In the carbon tetrachloride molecule, four chlorine atoms are positioned symmetrically as corners in a tetrahedral configuration joined to a central carbon atom by single covalent bonds. Because of this symmetrical geometry, CCl₄ is non-polar. Methane gas has the same structure, making carbon tetrachloride a halomethane. As a solvent, it is well suited to dissolving other non-polar compounds, fats and oils.

I don't think we used any carbon tetrachloride in the lab, methylene chloride (CH₂Cl₂) would separate from water and could be used to extract oil soluble compounds.

Title: What are polar bonds, and why are some molecules more polar than others?
Post by: lightarrow on 20/06/2011 11:05:09

Quote from: Geoquest on 18/06/2011 22:12:38

The question is:
Which molecule contains the most polar bonds?
(Is this asking which molecule has the most bonds, or which molecule is the most polar based on electronegativity???)

Neither. As already answered, it means which molecule has "the most polar" bonds, and it's CF₄ because every C-F bond in it is more polar than any other bond in the other molecules you have written, because of electronegativity difference.