Could you please explain Azeotropes in detail?

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Poster has been reminded to always show their efforts and links to their reading when asking for help understanding a concept.
Summary:
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Could you please explain Azeotropes in detail? I find it really difficult to wrap my head around it.
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The only thing I know is Azeotropes are the mixtures whose composition is the same in both vapor and liquid state.
This might be a stupid question but why and how is the composition the same? If this question doesn't make sense, please just share your views on Azeotropes. It would mean a lot to me.
Also, if you know some good sources in which they explain azeotropes, please do share.
Thanks
 
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Answers and Replies

  • #3
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Something which is important to note is that binary azeotropes can be low boiling or high boiling. (Azeotropes with more components have more options.)
 
  • #4
jim mcnamara
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Example:

[story]
If you were a bootlegger (somebody who illegally distills ethanol from fermented grain solutions) in early 1900's in the US, and you want your product to contain more than ~95% ethanol and less than 5% water, you were out of luck. The N2/ethanol binary azeotrope "blocks" higher concentrations of ethanol via distillation under normal conditions. Apparently monks distilling brandy hundreds of years ago hit this limit as well. They were all baffled.
[/story]

I used to tell a slightly longer story to Chemistry students. And you can figure out why 95% ethanol is the limit easily. My students got the concept, so you can too. Wade and Merriman coined the term azeotrope in the early 1900's.

PS: google "moonshine"
 
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  • #5
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I think an easy azeotrope to explain is simple low boiling azeotrope. Like water/heptane.
Heptane boils at 98 Celsius. And it is very nearly insoluble in water and vice versa. I think the solubility of heptane in water is quoted as 3 mg/l. Cannot find solubility of water in heptane but this is likewise small.
Therefore water/heptane mixture is pure, unaltered water at the bottom and pure, unaltered hexane on top. Heat it, and each phase has exact same vapour pressure as a pure phase alone.
Now, somewhere around 80 Celsius, water and heptane have somewhere around 500 mbar vapour pressure. Far from enough to boil.
But the point is, about 500 mbar vapour pressure of each adds up to 1013 mbar at the phase surface. Which is boiling.
Now consider a different, more polar substance. 3-pentanone.
Boils at 102 Celsius.
Also not miscible with water. But unlike hexane, appreciably soluble. Saturated solution at 20 Celsius has about 35 g/l in water.
Since it is a pair of immiscible substances, it also has to boil when the vapour pressures of both phases combined add up to external pressure. But now, the phases are not unaltered. Water which contains undersaturated solution of pentanone must have lower water vapour pressure than pure water in equilibrium with hexane. Likewise, pentanone dissolves some water and its vapour pressure is lower than that of pure phase.
Note that saturated solution of pentanone in water has same vapour pressure and composition as saturated solution of water in pentanone.
Water and pentanone repel each other. Or rather, pentanone is not attracted to water as strongly as water is, so pentanone dissolved in water breaks apart water molecules. Thermal motion forces pentanone into water despite the repulsion, but the repulsion still shows, by causing pentanone/water mixtures to have higher vapour pressure than either liquid alone (the molecules are repelled from solution to vapour) and by causing the mixture to separate (the molecules are repelled from solution to precipitate).

And now compare 1-propanol. Again more polar. Boils at 97,3 degrees.
Fully miscible with water. And yet, has a low boiling azeotrope that boils at 87,7 degrees. Almost 10 degrees lower.
Propanol molecules also repel from water. The repulsion is weak enough that thermal motion forces propanol and water to mix at all compositions. Yet the repulsion also repels them from solution to vapour.
So you can see that low boiling azeotropes have a common cause: repulsion between solution components.
 
  • #6
Borek
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The only thing I know is Azeotropes are the mixtures whose composition is the same in both vapor and liquid state.
This might be a stupid question but why and how is the composition the same?

I think you should start from non-azeotropes and ask yourself - why and how the compositions of the vapor and liquid states are different?

Once you got that, it will be obvious that azeotropes are just specific cases of a much general reality of mixtures, something like a minimum of a continuous function.
 
  • #7
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I think you should start from non-azeotropes and ask yourself - why and how the compositions of the vapor and liquid states are different?
Considering a binary solution, does it have something to do with the A-A, B-B and A-B interactions?
When A and B are not similar the interactions are also different. Hence, one is more volatile than the other.
So they have diff. compositions. Is this correct? If not, could you please give a hint?
 
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  • #8
Borek
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Definitely the interactions are behind what is happening. For ideal solutions (all interactions identical) vapor composition follows Raoult's law. For non-ideal solutions it deviates.
 
  • #9
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Considering a binary solution, does it have something to do with the A-A, B-B and A-B interactions?
When A and B are not similar the interactions are also different. Hence, one is more volatile than the other.
So they have diff. compositions. Is this correct? If not, could you please give a hint?
Well, look at my examples. I deliberately picked pairs of A and B which are not similar and have different interactions, but which are equally volatile. Water, and different organic compounds - heptane, pentanone and propanol, all boiling within 2 degrees of water.
For more examples:
Formic acid boils at 100,8 Celsius
Water and formic acid form azeotrope that boils at 107,3 Celsius.
Now, with more differences in volatility:
Perchloric acid boils at 110 degrees (don´ t try that at home - easily decomposes and explodes), but azeotrope with water boils at 203 degrees.
Easier to handle - nitric acid boils at 83 degrees, azeotrope with water at 120 degrees
Hydrogen bromide boils at -67 degrees, azeotrope with water at +126 degrees.
Generally most strong acids form high boiling azeotropes unless they are themselves high boiling or have a large hydrophobic tail.
What IS a high-boiling azeotrope of a strong acid?
Look at nitric acid. Molar mass 63. (Water is 18).
Azeotropic nitric acid is 68 % nitric acid by weight, therefore 32 % water.
Meaning that for one mole of nitric acid (63 g) azeotropic nitric acid contains about 30 g water... which is about 1,7 moles.
Azeotropic nitric acid is a liquid hydrate which thanks to the strong A-B attraction boils higher than either component.
It is important to note that as a liquid hydrate, the azeotropic nitric acid does not have a rigid crystal structure like solid hydrates do - and does not have an integer composition.
High boiling azeotrope as a liquid hydrate is the composition where the A-B attraction is maximized. It boils at a constant composition when the heat finally breaks the A-B bonds at their strongest. If you boil acid of a different composition then the component left over from the hydrate formation - whether excess water or excess acid - boils preferentially until you are left with the azeotrope/hydrate that boils at constant composition.
 
  • #10
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Moonshine. Ethanol and water. Ethanol is more volatile. But the key is that ethanol and water are less attracted to each other than ethanol is to itself, or water is to itself. So the volatility of both substances is increased in each other's presence.

When you have a high concentration of ethanol, lots of ethanol molecules and much fewer water molecules, most of the ethanol molecules aren't close to water molecules. But the vast majority of water molecules are near ethanol molecules. So the overall volatility of the water is increased by more relative to the volatility of the ethanol. When the volatilities match you have an azeotrope.

Or at least that's the cartoon-like explanation that I have in my head. I've never taken any chemistry other than a first year intro class.
 
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  • #11
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When you have a high concentration of ethanol, lots of ethanol molecules and much fewer water molecules, most of the ethanol molecules aren't close to water molecules. But the vast majority of water molecules are near ethanol molecules. So the overall volatility of the ethanol is increased by more relative to the volatility of the water. When the volatilaties match you have an azeotrope.
From your first paragraph:
  • Ethanol-Ethanol/Water-Water Interactions > Ethanol-Water interactions
So when we have a high concentration of ethanol, then most of the ethanol molecules are surrounded by ethanol molecules. So it is strongly bonded. This should make it less volatile and not more volatile if I'm not wrong. Water molecules are mostly surrounded by ethanol molecules and hence loosely bonded (comparatively). So water becomes more volatile.

Now water is already less in amount in the solution. If it vaporizes quickly there is no way the composition in liquid and vapor phase will become the same.

What am I misinterpreting?
 
  • #12
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Mole fraction. Water vaporizes more easily than before but there is a lot less of it. So the vast majority of the vapor is still ethanol.
 

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