azeotropes confuse me

[thebillsfan]

to my dismay, after much searching on the internet I still dont' really understand azeotropes. While I know its not something we need to KNOW, and I realize that it will be in a passage if its on the test, they still confuse me even when they're in a passage. So here's two questions about them:

1. why do they form--for example, why can an ethanol-water mixture only by distilled to 95% ethanol?
2. why are the percent compositions of the liquid phase different than the percent compositions of the vapor phase?

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[RogueUnicorn]

1. after a certain concentration, because of the intermolecular forces between ethanol and water, it's not possible (conventionally speaking) to separate the two out as they now essentially boil at the same temperature (think boiling point changes caused by solutes). the precise physical reason is best explained with a graph but that's not possible via this forum.

2. the compositions are different because the two liquids have different volatilities. take for example a 50-50 ethanol water solution. though the liquid forms are equimolar, ethanol is much more volatile, and thus wants to evaporate more. nothing stops it, so the vapor pressure of ethanol is higher than that of water, even though the mixture is 50-50. this is how distillation works - if water and ethanol had equal vapor pressure as a 50-50 mixture, no matter how much you boiled it, you would still have a 50-50 solution in your beaker. as it so happens, as ethanol has a higher vapor pressure, if you heat it for long enough, more ethanol will leave the solution to become gas than water, making the water purer.

 

[thebillsfan]

okay, but as you increase the ethanol content, say to a 75-25, the composition of vapor changes. is it because since there's more ethanol content in the solution, there are increased IM forces in the ethanol, so its vapor pressure goes down leading to a slightly higher water vapor content?

and what's with the two curves on an azeotrope curve. I know the upper curve is supposed to be vapor composition, but why is this on a temperature graph? vapor is already boiled so I don't understand why it would be putting on a graph of azeotrope composition vs boiling point

thanks for your help

 

[RogueUnicorn]

okay, but as you increase the ethanol content, say to a 75-25, the composition of vapor changes. is it because since there's more ethanol content in the solution, there are increased IM forces in the ethanol, so its vapor pressure goes down leading to a slightly higher water vapor content?

The precise interplay needs to be verified experimentally as to what happens at different concentration, but what you're saying is essentially correct.

and what's with the two curves on an azeotrope curve. I know the upper curve is supposed to be vapor composition, but why is this on a temperature graph? vapor is already boiled so I don't understand why it would be putting on a graph of azeotrope composition vs boiling point

thanks for your help

The azeotrope graph has the BP of the liquids in the bottom curve. The x-axis shows the relative concentrations. At whatever relative concentration (let's say 50-50), you can find the BP by looking at the bottom curve. By drawing a line to the LEFT from that point (provided the more volatile component is to the left), the intersection with that line and the upper curve is the relative VAPOR composition. The Temp is not relevant with the upper curve but it's placed on the same graph to give this very important info.

 

[thebillsfan]

The precise interplay needs to be verified experimentally as to what happens at different concentration, but what you're saying is essentially correct.



The azeotrope graph has the BP of the liquids in the bottom curve. The x-axis shows the relative concentrations. At whatever relative concentration (let's say 50-50), you can find the BP by looking at the bottom curve. By drawing a line to the LEFT from that point (provided the more volatile component is to the left), the intersection with that line and the upper curve is the relative VAPOR composition. The Temp is not relevant with the upper curve but it's placed on the same graph to give this very important info.

nice, but if that's the case, then what is the point at the same temperature but to the right of your line on the upper curve. often you see an upper curve that'll hit the same temp twice

 

[RogueUnicorn]

sorry, i messed up here. draw a line whichever direction in such a way you don't go through the shape drawn by the bottom curve. sorry for the confusion!

 

[thebillsfan]

sorry, i messed up here. draw a line whichever direction in such a way you don't go through the shape drawn by the bottom curve. sorry for the confusion!

perfect! that was so helpful. one last question--this is the distinction btw minimum boiling and maximum boiling azeotropes. minimum boiling is just an azeotrope that, due to the nature of IM forces, decreases in bp until a certain composition is reached and then increases after that.
maximum boiling azeotrope increases until a certain comp and then decreases after that.

is that correct? i assume i dont have to know WHY or how the IM forces cause it to be a minimum or maximum boiling az.

 

[RogueUnicorn]

perfect! that was so helpful. one last question--this is the distinction btw minimum boiling and maximum boiling azeotropes. minimum boiling is just an azeotrope that, due to the nature of IM forces, decreases in bp until a certain composition is reached and then increases after that.
maximum boiling azeotrope increases until a certain comp and then decreases after that.

is that correct? i assume i dont have to know WHY or how the IM forces cause it to be a minimum or maximum boiling az.

essentially correct. minimum boiling azeotropes (also called positive azeotropes, to add to the confusion..) are mixtures where ANY mixture of ANY composition decreases the bp when compared to a pure solution. the opposite for maximum b azeotropes.