Evaporation v. Boiling Point of alcohol

[dwc929]

I get that alcohols have higher BP b/c of H-bonding...but how come they evaporate fast?

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[cornpops21]

What are you comparing them to?

 

[dwc929]

What are you comparing them to?

Um, let's say water...does alcohol have higher boiling point than water and have a higher rate of evaporation? Seems contradictory...alcohol makes H-bonds w/ water so takes more heat to get it to boil but then wouldn't it also take longer to evaporate, if we're not boiling?

 

[xIcewind]

I'd say that you're slightly confused.

For example, either methanol or ethanol, I can't remember, has a boiling point of about 78C. Water tends to (unless you add a lot of stuff to it) have a boiling point of 100.

Generally, the larger (and more branched) the molecule is, the higher the boiling point, as hydrogen bonding from the -OH is replaced by a much stronger force -- london forces. With large molecules comes a mass of electrons, and small branched molecules allow them to pack closer together.

 

[Danny289]

when you are comparing water and alcohol, both have hydrogyn bond and high boiling point (relatively) water has higher boiling point because can have more hydrogyn bond compare to ethanol(H20 has two hydogyn ready for hydrogyn bond but ethanol C2H5OH has one hydrogyn).

 

[Kaustikos]

when you are comparing water and alcohol, both have hydrogyn bond and high boiling point (relatively) water has higher boiling point because can have more hydrogyn bond compare to ethanol(H20 has two hydogyn ready for hydrogyn bond but ethanol C2H5OH has one hydrogyn).

Exactly! The thing to remember is that if a molecule/compound can exhibit more hydrogen bonds PER molecule, it will tend to have a higher boiling point.

This is why alcohols evaporate relatively fast, compared to water.