Cracking the MCAT: The phase diagram for a mixture containing liquids A and B

[ezsanche]

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Can someone please tell me how I should read a phase diagram for a mixture containing liquids A and B.
I have a scan of a page from the princeton review and I just dont understand how this phase diagram graph works.

http://i646.photobucket.com/albums/uu181/ezsanche/scan0001.jpg

make sure to zoom in so that you can see it clearly.

Thanks!!

 

[bacotell]

All it is saying is that at a certain temp. you will have this much of B that is vapor along with A due to the bioling points being closer together. L2 is the key here as it shows the actual compisition of the liquid if it were allowed to condense. Thus, when you boil the solution at L1, the actual composition is different for the vapor, and actually contains alot more A than B. Thats why the horizontal line is drawn, becuase this tells us the actual composition of the vapor.

this is wikipedia and should help
"A simple example diagram with hypothetical components 1 and 2 in a non-azeotropic mixture is shown at right. The fact that there are two separate curved lines joining the boiling points of the pure components means that the vapor composition is usually not the same as the liquid composition the vapor is in equilibrium with. See Vapor-Liquid Equilibrium for a fuller discussion."

 

[Tokspor]

My interpretation is that let's say you have a composition L1 of liquids A and B (look at the "composition" axis. It should look like 40% A and 60% B if you're reading it right). The text here has you suppose the boiling point of the mixture is already known, and that it's T1. This mixture will evaporate at T1. Now suppose that everything that's been evaporating is allowed to condensed. Just go up until you hit second curve (which looks to me like the temperature at which composition of the gas equals the composition of the liquid?), and then run over to the right, where you are back at the liquid curve at L2. L2 is the composition of the condensed liquid--that portion of the original mixture which evaporated at T1. You can see that a lot more of liquid B is present (it looks to be about 5% A and 95% B). This is probably because the boiling point of pure liquid B (T sub B on the graph) has not been reached while the boiling point of pure liquid A has clearly been surpassed (T sub A). So you would expect more of liquid B to condense than liquid A at the temperature where L2 is at, since it's less than the boiling point of B and much greater than the boiling point of A.

Please correct any places where I might be wrong.

 

[ChemEngSoonMD]

Can someone please tell me how I should read a phase diagram for a mixture containing liquids A and B.
I have a scan of a page from the princeton review and I just dont understand how this phase diagram graph works.

http://i646.photobucket.com/albums/uu181/ezsanche/scan0001.jpg

make sure to zoom in so that you can see it clearly.

Thanks!!

Um. The liquid with the lower boiling point likes to go into the air. When you heat A and B, the more volatile stuff goes into the air at a higher concentration. Thats how you make hard alcohol. If you heat 50% H2O and 50% H20, is the vapor in the air still 50/50? No its not, there is more alcohol because it has a lower boiling point. The lower boiling point solution in the gas phase is the upper curve because it is more volatile.


The liquid concentration is linear at the bottom. The gas concentration is curved at the top.

 

[Tokspor]

Um. The liquid with the lower boiling point likes to go into the air. When you heat A and B, the more volatile stuff goes into the air at a higher concentration. Thats how you make hard alcohol. If you heat 50% H2O and 50% H20, is the vapor in the air still 50/50? No its not, there is more alcohol because it has a lower boiling point. The lower boiling point solution in the gas phase is the upper curve because it is more volatile.


The liquid concentration is linear at the bottom. The gas concentration is curved at the top.

Does the lower curve represents the boiling points of the mixture at different compositions? Or is it that the lower curve represents just the boiling points of A while the upper curve represents the boiling points of B?

 

[ezsanche]

Um. The liquid with the lower boiling point likes to go into the air. When you heat A and B, the more volatile stuff goes into the air at a higher concentration. Thats how you make hard alcohol. If you heat 50% H2O and 50% H20, is the vapor in the air still 50/50? No its not, there is more alcohol because it has a lower boiling point. The lower boiling point solution in the gas phase is the upper curve because it is more volatile.


The liquid concentration is linear at the bottom. The gas concentration is curved at the top.

LIGHT BULB!!!!!!!!!!!! Thanks I finally got it.

The way that I was looking at it was up to down. But i realize you need to look at it from left to right so you can determine the percent composition of each the vapors and liquid phase.
?

 

[ChemEngSoonMD]

Does the lower curve represents the boiling points of the mixture at different compositions? Or is it that the lower curve represents just the boiling points of A while the upper curve represents the boiling points of B?

When, you mix two solutions, you don't have two different boiling points anymore. You only have one boiling point. That boiling point depends on the mixture ratios. If you have 99% A, then obviously the boiling point is going to be near A. If you have 99% B mixture, then the boiling point is going to be near B. This figure is not about boiling point. It is assumed you reach boiling point, whatever that may be.


Given a specific liquid mixture (refer to x-axis) the two liquids go into the gas phase at different concentrations during boiling point. They both boil, just A goes into the air faster than B.

At the ends they merge, why? Well if you have 99% A and 1% B, even if A is more volatile, there is so much A, that still some of B is going to evaporate. Plus the moles fractions have to add up to 1.

You ever wonder why the highest alcohol is 190 proof? Thats because at 95% alcohol, you cant distill it any higher. There is so much alcohol, and so little water content that even thought the alcohol is more volatile, some water is going to vaporize too. Now you reach the azeotrope. But you dont need to know the MCAT because you don't need Chemicizle Engineering skillz.