Course 21.2Lesson 3 of 3
Boil a mixture of two liquids and the vapour is richer in the more volatile one. Repeat the boiling, and you can separate an ideal pair. Some pairs refuse. At one composition the vapour has the same make-up as the liquid, and the mixture boils at one fixed temperature like a pure liquid. This is an azeotrope.
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Take a mixture of A and B in which A is the more volatile. Boil it. The vapour is richer in A than the liquid, as you saw in the first lesson. Cool the vapour and it becomes a liquid that is richer in A. Boil that liquid. The new vapour is richer in A still.
Each boil and condense is one step. In a fractionating column, the steps happen again and again on its plates. With enough steps, A comes out nearly pure at the top, and B stays at the bottom.
The left graph shows the steps for an ideal pair. Each step moves the composition towards pure A. There is no limit, and the more steps there are, the purer A becomes.
Now look at the right graph. The steps get smaller and smaller, and they stop at one composition. No further boiling changes it. This special mixture is an azeotrope. The word comes from Greek and means "boiling without change".
An azeotrope is a liquid mixture of fixed composition that boils at one constant temperature. Its vapour has the same composition as its liquid. Boiling cannot change the composition, so distillation cannot separate it.
Azeotropes appear in pairs that show a large deviation from Raoult's law.
Take a pair with a strong positive deviation. Its total vapour pressure rises above both pure pressures, and the curve of pressure against composition has a maximum. At the maximum, vapour and liquid have the same composition. A mixture with a high vapour pressure boils at a low temperature. So at this composition the boiling point has a minimum. This is a minimum-boiling azeotrope. It boils at a lower temperature than either pure liquid.
The left graph shows the boiling point of a model pair, with the azeotrope marked as a black dot. The blue curve is the liquid and the red curve is the vapour.
The best known example is ethanol and water. The azeotrope holds about 95.6 % ethanol by mass and boils at about .
Now take a pair with a strong negative deviation. The pressure is low, so the boiling point is high. The curve of pressure has a minimum, and the boiling point has a maximum. This is a maximum-boiling azeotrope. It boils at a higher temperature than either pure liquid. The right graph shows it.
Two examples are:
A way to remember it: positive deviation, low boiling point. Negative deviation, high boiling point.
For a pair with an azeotrope, the column gives two products, not two pure liquids.
So distilling a mixture of ethanol and water cannot give pure ethanol, however tall the column. At best it gives the azeotrope. Chemists remove the last of the water by other means. They add a substance that takes up water, such as quicklime, or they add a third liquid.
The azeotrope behaves like a pure liquid at one pressure, but it is a mixture. Change the external pressure and the composition of the azeotrope changes. A compound would keep a fixed formula.
Start with the strength of the deviation at . The curve stays above the dashed line, so the vapour is always richer in A. Raise the strength. When the curve touches the line and crosses it, an azeotrope has appeared.
The program starts with a liquid that is A by moles and repeats the boil and condense eight times for a pair with a positive deviation.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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