Another Dumb ME question: What is a "polar" component?

[KernOily]

OK, here's another dumb ME question for you smart ChemE's. What is meant by the term "polar"? As in, a mixture of gases of polar and non-polar components?

Thanks!
Pete
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

[TD2K]

I'll have to go back to chemistry to best explain this. If you looked at a molecule of sodium chloride, sodium 'gives' its extra electon to the chlorine which is short one electron. While the molecule is electrically neutral, the electron tends to stick with the chlorine making that end of the molecule 'negative' in charge while the sodium end is 'positive' in charge. This is a gross simplication mark you.

The same principle makes water as a polar molecule. The two hydrogen atoms sit off at a angle (sort of a broad triangle). The central oxygen atom hogs the electrons the two hydrogen atoms contribute (oxygen wants 2 electons) and so the oxygen is negative and the hydrogens are positive. Of course, being positive, the hydrogen attacts nearly oxygen atoms in other molecules. This bond is actually quite strong which is why water, with a molecular weight of only 18, boils at such a high temperature compared to methane (molecular weight of 18), ethane (30) or propane (40). Same thing causes molecules like alcohols or H2S or be considered polar.

 

[jproj]

PeteChandler:

Polar and non-polar bonds refer to the way electrons are shared between atoms in a molecule. Polar molecules have a dipole moment (and are asymmetrical). Nonpolar molecules do not have a dipole moment because they share electrons equally (due to the symmetry of the molecule).

Check these out:

http://gemini.tntech.edu/~snorthrup/chem111/polar.html

http://www.ndchemistry.org/courses/honors_chem/Chapter12/Chapter12classnotes.html

http://www.chem.uidaho.edu/~chem103/dipole.html

That should clear it up!

Good Luck

jproj

 

[TD2K]

And those sites almost certainly do a far better job of explaining it than I did

 

[DeltaCascade]

Here is a neat test you can do at home to help understand and appreciate polarity ...

Is H2O polar?
(sorry if it doesn't directly address polar gases ... although disccussions of polarity are usually applied to liquids in my experience)

Oxygen attracts electrons moreso than does hydrogen, as shown in Pete's recemmended sites, above, so if water's molecular structure was straight chain or line would be

H-O-H

with net partial positive charge on the ends and net partial negative charge in the middle. The straight line structure is symetric and the water would not be polar. If it is a bent line, then it would be polar, with a partial negative charge on one side, and partial positive charge on the other (have a dipole moment).

O
/ H H

Now, make a small stream of water come out of a tap, get a hair comb and pass it through your hair to get a static charge on it ... hold the charged comb near the water stream . .. watch the charged comb pull the water stream out of its vertical drop! Like a magnet. This proves that water has a dipole moment and hence is polar.... (and also that water molecurles are not symettrical in shape). Trying this on non-polar sovents does nothing.

Like solvents dissolve like solvents; polar water dissolves polar ethanol, polar salt, etc. Non-polar oil does not dissolve non-polar water. etc..,. a great general rule...
(PS: soap is funny as it is a long fatty acid chain ... polar at one end and non-polar at the other .. the non-polar end dissolves oils and the like, the polar end makes the whole thing dissolve in water.. hence the grease cutting action in water .. where would we be without that eh?)

 

[KernOily]

My wife sez I have a magnetic personality. So, in light of the above discussion, that explains why I'm hydrophobic... 8-}

Guys thanks for the great info. As a follow-up to my initial question - how do I determine if a particular substance is polar? Is that something I can look up in the CRC handbook? Thanks!
Pete
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

[DeltaCascade]

Chuckle!!
(and oops, above i should have said, non-polar oil does not dissolve POLAR water ... of course...)

And Pete, you are right! CRC handbook does have dipole moments, determined on molecules in the gas phase (units of Debye, 1 D = 3.33564 x 10E-30 couloumb meters). Water is 1.85, silver chloride is 5.73, sodium chloride is 9, methane, ethane, ethylene and symmytrical organics are 0, ammonia is 1.47, cyclopentene is 0.2, propane is 0.084 (its not symetrical cause, like the water, its a bent chain?), butane <0.05, ethanol 1.65, etc... Some results are also based on molecules not in the gas phase.

Interesting, as i have typically related polarity in terms of solubility: i would have gone to the CRC's &quot;physical constants of compounds&quot; section to see solubility in different (polar and non-polar) solvents to get idea of the compound's polarity. Alas, the dipole moment data in the CRC corresponds to what one would expect for a qualitative assessment of a compound's polarity based on its solubility in other solvents (an organic chemists viewpoint??)... Thanks for the quantitative tip...

Was that a debye taunt?

Your work sounds interesting ...
... is it chromatography related?

Cheers

Wayne//

http://www.DeltaCascade.com,

Calgary, Alberta