Liquids A and B form a maximum-boiling azeotrope at a mole fraction of A of 0.35…
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Liquids A and B form a maximum-boiling azeotrope at a mole fraction of A of . Flask 1 holds a mixture with a mole fraction of A of . Flask 2 holds one with . Each mixture is distilled through a very long column. What collects at the top and what stays at the bottom in each case? What would change if the azeotrope were minimum-boiling?
Need a reminder? The key ideaShow
An azeotrope boils at one temperature and has the same composition in the liquid and in the vapour, so distillation cannot separate it. Positive deviation gives a minimum-boiling azeotrope, which comes off at the top of a column. Negative deviation gives a maximum-boiling azeotrope, which stays at the bottom.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The rule. A maximum-boiling azeotrope is less volatile than either pure liquid, so it stays at the bottom.
Step 2. Flask 1. A mole fraction of is on the B-rich side of the azeotrope. The top product is pure B and the bottom is the azeotrope.
Step 3. Flask 2. A mole fraction of is on the A-rich side. The top product is pure A and the bottom is the azeotrope.
Step 4. A minimum-boiling azeotrope. It would be the most volatile part, so it would come off at the top, and the pure liquid in excess would stay in the flask.
Answer: top pure B and bottom azeotrope for flask 1; top pure A and bottom azeotrope for flask 2. For a minimum-boiling azeotrope the azeotrope goes to the top.
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