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Topic: Why can't a polar substance dissolve a non-polar?  (Read 21168 times)

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Offline Gargamel

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Why can't a polar substance dissolve a non-polar?
« on: February 23, 2010, 03:58:21 PM »
Hi

I wonder why for example water can't dissolve methan or dichlor.

I mean water is polar and therefore contains a partiel positive charge and a partiel negative charge. This charge must be able to disturb the electron cloud in methan or dichlor so that methan or dichlor becomes temporary polar? Correct?

Maybe this is only correct for a small amount of non-polar molecules and therefore we say in generel that polar substances can only disolve in polar solvents and vice versa.

Can someone please confirm or correct me.

Thanks

Offline renge ishyo

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #1 on: February 23, 2010, 07:06:50 PM »
In the case of a substance such as water dissolving methane, the problem is that in order for water to bond to methane it must break bonds to other water molecules first. Since a polar water molecule binds to another polar water molecule more strongly than it binds to non-polar methane, the water molecules will stay bound to each other and "push out" the methane for the most part. In the case of polar molecules such as NaCl, water can bind very strongly to either Na+ or Cl- and this strong interaction allows some water molecules to overcome the bonds to other water molecules to form new bonds surrounding these polar ions.

Offline tumitran

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #2 on: February 24, 2010, 01:06:54 PM »
I think this link http://www.visionlearning.com/library/module_viewer.php?mid=57 might be helpful for you.
Hydrogen bonding is very helpful in explaining dissolution. In water, O has negative charge and H has positive charge because of the big difference in electronegativity, so we have hydrogen bond between H---O and O---H. On the other hand, Ethan is not very polarized, since C and H have relatively same electronegativity. Thus, O and H will not be attracted by C and H. Same thing to Cl-Cl, they are non polar substance.
Tumi Tran
De Anza College, CA.

Offline Gargamel

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #3 on: February 24, 2010, 04:06:48 PM »
Okay thanks for the answers. I understand what you wrote. I dont think I expressed my first topic too clear...I try again

In my databook it says that Br2 can be dissolved in water 3,6g/100mL - not very much but still. Why can that happend, when water has a higher affinity to it self due to the H-bondings?
My explanation:
Dibrom is non-polar so dibrom's electroncloud must be temporary polar due to waters high polarity which to a minor extent will disturb dibroms electrons in one direction causing London-forces. What I mean is that dibroms small solubility can be explained by London-forces. Correct me if Im wrong.

Thx again

Offline Gargamel

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #4 on: February 25, 2010, 02:20:50 AM »
Some thoughts while sleeping (yes this is killing me :) )

Maybe water can make a "kind of H-bonds" with dibroms lonepairs? (I know true H-bonds forms from H to N, O or F's lonepairs)

Offline renge ishyo

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #5 on: February 25, 2010, 01:00:09 PM »
The situation with Br2 dissolving in water (if we assume it stays Br2 and doesn't ionize) has more to do with an entropic effect than it does with the bonding. Even methane will dissolve in water to some extent (3.5 mg/100 mL) despite the very weak bonding between methane and water.

In order to understand why this is, imagine a container separated in the middle by a removable barrier. In one half of the container you fill it with only blue spheres and in the other half of the container fill it with brown spheres. Once you remove the barrier and shake up the container for awhile, you might suspect that the two sphere colors become less and less separate from each other and more and more mixed together. This is due to an entropic effect. The probability of the two colors separating from each other on each side after shaking is much lower than the probability that they will be all mixed up.

Now we do the same experiment, but this time we attach the blue spheres to each other using heavy duty tape (this tape will symbolize "strong bonding" between blue spheres for one another). When we pull out the barrier and shake it up, we might expect that all the blues will now stay together due to the tape and that this would keep them separated from the brown balls. And for the most part (if we did a good job taping), we might be right. But you wouldn't be surprised if at least *some* brown balls got mixed in with our taped blue mass after shaking perhaps because there were parts where the tape weakened long enough to let them leak through or for some other reason. The point is that without the tape they would tend to mix completely but with the tape they will still "want" to mix but will be inhibited for the *most part* (some will still leak through). This is how even some methane gets dissolved in water despite the weak bonding for water and methane. Not a perfect analogy, but for now it will do.

For Br2, this is a factor that allows it to dissolve as well. Br2 is a bad example to use though, because it does not stay Br2 in water forever, but rather it is ionized by water to H+ and Br- in solution. Hence, it's solubility is higher than you would expect from other non-ionizable compounds with poor solubility in water such as N2, O2, and CH4 that are far less reactive towards water.

Offline Dan

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Re: Why can't a polar substance dissolve a non-polar?
« Reply #6 on: February 25, 2010, 01:10:23 PM »
it does not stay Br2 in water forever, but rather it is ionized by water to H+ and Br- in solution.

You'll get hypobromous acid as well as HBr:

Br2 + H2O <----> HOBr + HBr
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