garage chemist - 24-11-2004 at 13:04
Hello,
a mate of mine needs to know why some azeotropes have a lower boiling point than both of their components (for example, the azeotrope ethanol- water).
We asked one of our chemistry teachers but she didn't know!
She said that she should have known it, though. 
I just need a simple explanation, nothing too in-depth.
It's difficult to find information about this with google, and I also searched the board.
I know what azeotropes are, but I never asked myself about those with a lower bp than both of their components.
Mr. Wizard - 24-11-2004 at 13:47
This might be too simple, but here goes:
The boiling point of any liquid, is when it's Vapor Pressure equals atmospheric pressure (760 mm Hg). Vapor pressure is determined by how much
the molecules like to stick to each other as a liquid rather then get kicked loose as a vapor. Imagine molecules as marbles with sticky tape on them,
or velcro hooks, or intricate shapes. You put the marbles in a blender and they all stay stuck together, then as you turn up the heat, by turning on
the blender, the marbles start to bounce around, sometimes coming loose and escaping, sometimes coming loose and re-sticking. The higher the blender
speed, or temperature, the faster the marbles move and the less likely they are to stay stuck together, which corresponds to a higher vapor pressure.
Azeotropes form new marbles with less stickiness to each other, like sand on the tape, which allows them to 'boil off' first.
The analogy is meant to convey the idea only. There is no tape or velcro on the molecules, just the interaction of electrons which accounts for all
the stickiness. Anything that makes the molecules stick together, will lower the vapor pressure, anything which keeps them from sticking will raise
it.
neutrino - 24-11-2004 at 14:50
Not all azeotropes boil at lower temperatures than their components. For example:
HNO<sub>3</sub> = 83*c
H<sub>2</sub>O = 100*c
69% HNO<sub>3</sub> + H<sub>2</sub>O = 120.5*c
Ref: Azeotropes
solo - 24-11-2004 at 17:03
Here is a nice slide show on solutions and azeotropes to gain a better understanding.......solo
..........see what they are azeotropes
[Edited on 25-11-2004 by solo]
Marvin - 24-11-2004 at 17:21
The classic explanation is that the molecules link to form bigger molecules. This explains why most azeotropes seem to be for simple ratios of molar
amounts. The bigger molecules can be harder to boil off or easier depending mainly on intermolecular forces between the new groups.
Many of thiese compounds arnt as stable at high temperatures, so you tend to have more azeotropes in the solid/liquid than in the liquid/vapour.
JohnWW - 26-11-2004 at 16:43
In most cases, azeotropy depends on how hydrogen bonding occurs between the molecules involved in the mixture, because most commonly encountered
examples involve compounds with polar O, N, or F-containing groups like alcohols, carboxylic acids, and amines, with a hydrogen on the O or N in at
least one of the two compounds involved. It can be either less strong than between molecules of the pure substances, in which case the azeotropic
mixture has a boiling point lower than either; or stronger than between molecules of the pure substances, in which case the azeotropic mixture has a
boiling point higher than either.