Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: BIOKID123 on May 02, 2021, 12:48:34 AM
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Hello,
I understand that water has intermolecular forces such as hydrogen bonding, so it's really hard to separate the molecules. And, ethanol also has hydrogen bonding, but london dispersion force as well. Why is water more polar than ethanol?
is it because ethanol also has a london dispersion force, so part of the bond is weak in attraction?
edit: water also has London dispersion force (LDF), but I think since ethanol is mostly comprised of LDF, it is weak in attraction with other ethanol molecules?
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You got it reversed.
It is not dominating interactions between molecules that tells us whether the molecule is polar, or not, it is whether the molecule is polar or nor not that tells us what kind of intermolecular interaction to expect.
In general polar molecules are either dipoles, or contain several dipoles: https://en.wikipedia.org/wiki/Chemical_polarity
And this statement:
ethanol is mostly comprised of LDF
is simply off. Molecules don't "comprise of forces", they comprise of atoms. Forces are how they interact with other molecules, and these interaction are always combinations of forces we classify as different types.
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Oh okay, thank you. By the way, sorry about the wording. I am severely tired at the moment, I meant ethanol mainly uses LDP, not that is created by it. Anyways thanks for the help.
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Why does water have a greater polarity than ethanol?
Did you consider the difference in polarity of O-CH3 and O-H bond ?
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For problems like this, it is often a very good idea to consult an electronegativity table.
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When I think about polarity, I divide up a molecule into a polar and a non polar portion. For the series CH3(CH2)nOH, where n = 0, 1, 2, as n becomes larger, the overall polarity becomes smaller. Water lacks any non polar portion. Methanol, ethanol, and propanol are miscible with water, but butanol is not, because the non polar portion is large, relative to the polar portion. I just looked up the strengths of these solvents in chromatography, and the Trent is for eluant strength to be inversely related to n.