June 05, 2020, 04:15:03 PM
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Topic: Why does non-polar molecules stick to each other in presence of polar molecules?  (Read 8737 times)

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Offline mahrap

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The attraction between polar molecules is obvious due to the opposite electrical charges spread around the molecules. However, why do non-polar molecules attract one another? I know that all molecules attract each other due to van der waal forces. But then by that logic shouldn't polar molecules be also attracted to non-polar molecules due to the instantaneous dipoles in both cases? Or is it because the dipole-dipole attraction is so much more powerful than van der waal forces that polar molecules tend to just stick to each other anyways because the van der waal forces of the non-polar molecules are negligible compared to the dipoles so non-polars are essentially forced to stick to each other.

Offline Sunil Simha

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When you mix polar and non polar molecules, polar molecules do stick to non polar molecules. This is because when a polar molecule A goes close to a non polar molecule B, B's electron cloud gets polarized and it forms a temporary dipole. This dipole exists as long as A is in the vicinity and they attract each other.

Of course, this attraction is weaker compared to that between two permanently polar molecules but there is considerable attraction between B and A.

To help you understand better, think about this experiment. You have rubbed a comb in you hair and tried to  attract pieces of paper with it right (If you haven't, try it now! :)). What happen here is there is charge transfer from your hair to the comb. Now when you bring this comb towards the paper piece, the neutral paper piece gets polarized and thus gets attracted to the comb. This paper when brought close to another piece induces a dipole moment in the new one too and thus they are attracted.

This is similar to what happen on the molecular basis. The induced dipole is capable inducing a dipole in another molecule (and hence attracting) of the same kind.

Offline aHerraez

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This involves a concept that is often misunderstood. Try searching for "hydrophobic effect".
Indeed, nonpolar molecules associate in the presence of polar molecules (mostly and maybe better phrased, in a polar solvent). This is not due to a tendency to associate, but pushed by the tendency of the polar (solvent) molecules to associate between themselves, so excluding the nonpolar ones.

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