Azeotropes

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. You have ethanol mixed with water, and you want pure ethanol. So you distil it. The vapour is richer in ethanol, so you collect it, and distil it again, and again. It gets richer and richer, up to about ninety five percent ethanol. And then it simply stops. Distil it a hundred more times: still ninety five percent. Never one hundred. Why? What is this wall at ninety five? Strange, right? Today we meet the mixtures that build this wall: azeotropes. This idea comes up often in JEE questions on solutions, so let's begin.

0:41What is an azeotrope?

So, first a quick recap. When a mixture of two liquids boils, the vapour usually has a different composition from the liquid. Which of the two liquids goes into the vapour more? The more volatile one, yes! So the vapour is richer in it, and that is exactly how fractional distillation separates two liquids, step by step. But some mixtures refuse to be separated. We call them azeotropes, or constant boiling mixtures. Okay? Why constant boiling? Because an azeotrope boils at a constant temperature, just like a pure liquid. The temperature does not change as it boils away. Right? And here is the reason. The vapour it gives has exactly the same composition as the liquid. y A, with the small A written below, is the mole fraction of A in the vapour, and x A is its mole fraction in the liquid. At the azeotrope, y A equals x A, okay? Now think. The vapour is the same as the liquid. So you condense it, boil it again, condense it again. What changes? Nothing at all! You get back the same mixture every single time. So an azeotrope cannot be separated into its components by fractional distillation.

1:57Two kinds of azeotrope

Clear? Now, where do azeotropes come from? Remember positive deviation from the last lesson: the molecules escape more easily, so the vapour pressure is higher than Raoult's law predicts. With a large positive deviation, the total vapour pressure, capital P, reaches a maximum at one composition. With me? So here is a question. At that composition, the vapour pressure is the highest. Many students say: positive deviation, highest pressure, so the highest boiling point, right? What do you say? No, no, careful! It is the opposite. The higher the vapour pressure, the lower the boiling point. So maximum vapour pressure means minimum boiling point. The double arrow means this leads to a minimum boiling azeotrope. Okay? Now the mirror case. In a large negative deviation, the attractions in the solution are stronger, the molecules escape less easily, and the total vapour pressure P reaches a minimum. Got it? So minimum vapour pressure means maximum boiling point: a maximum boiling azeotrope. Here is a topper's trick: vapour pressure and boiling point move in opposite directions. The top of one is the bottom of the other. Clear?

3:10Reading the boiling-point graphs

Now let's see both kinds on a graph. The boiling point goes up the side, and the composition runs along the bottom; the arrowheads on the axes show the direction of increase. The axis goes from pure B on the left to pure A on the right. In the left graph, for positive deviation, the boiling point dips to its lowest value at the mark a z, the azeotrope. Okay? And in the right graph, for negative deviation, it rises to its highest value at a z. That is why the titles say minimum boiling and maximum boiling. Right? Now look closely. Each graph has two curves: one for the liquid, and one for the vapour. At a z, they touch. What does that tell you? Yes, exactly! There, the liquid and the vapour have the same composition. That is our definition, right on the graph. So the azeotrope boils without any change at all. Got it?

4:05The azeotropes to remember

Now, the examples you must know. Ethanol and water show a large positive deviation, so they form a minimum boiling azeotrope, at about ninety five percent ethanol by mass. The wavy equals sign means approximately, and the dash between the two names means a mixture of the two. It boils at three hundred fifty one point three kelvin, the capital K on screen. Ninety five percent: our puzzle number! Hold that thought, okay? Next, nitric acid and water. From the last lesson, which deviation do they show? And so, which kind of azeotrope do they form? Negative deviation, yes! So a maximum boiling azeotrope. HNO three, with the small three written below, is nitric acid. The azeotrope has about sixty eight percent nitric acid by mass, and it boils at three hundred ninety three point five kelvin. Right? And hydrochloric acid, the HCL on screen, also forms a maximum boiling azeotrope with water, at about twenty percent of the acid. So both these acids with water give maximum boiling azeotropes. Got it?

5:17Example 1: why ethanol stops at 95% by distillation

Alright, example one, and it is our puzzle. Why can ethanol not be made more than about ninety five percent pure by fractional distillation of its aqueous solution? Aqueous simply means in water. Pause the video and try it first. Done? Okay, let's check. As you distil, the ethanol you collect gets richer and richer, until it reaches about ninety five percent. And right there, ethanol and water form a minimum boiling azeotrope. Okay? Now boil that mixture. Its vapour has the same composition as the liquid: about ninety five percent ethanol again. Right? So distil it once more, or a hundred times more: no enrichment at all. That is the wall from our puzzle. Mystery solved! So fractional distillation gives at most about ninety five percent ethanol. This mixture is called rectified spirit. Remember the name, it comes up often. With me?

6:15Example 2: distilling dilute nitric acid in water

Now a harder one, example two. Read it with me: a dilute solution of nitric acid in water is distilled. What is left in the flask at the end, and what distils over first? Pause the video and try it. Hint: first decide which kind of azeotrope this is. Done? Okay, let's check. Nitric acid and water form a maximum boiling azeotrope, at about sixty eight percent nitric acid. So this azeotrope has the highest boiling point of all their mixtures. Okay? Now, in example one, the ninety five percent azeotrope was what we collected first. So here too, the sixty eight percent azeotrope distils over first, right? No, no, careful! That one was a minimum boiling azeotrope: the lowest boiling point, so it came off first. This one boils at the highest temperature, so it is the last to go. Water distils over first. Got it? As the water leaves, the liquid in the flask gets richer in acid. It comes closer and closer to about sixty eight percent nitric acid. Right? Once it reaches the azeotrope, the vapour has the same composition as the liquid. So the azeotrope then distils unchanged, at a constant temperature. Clear? So the answer: water distils over first, and the sixty eight percent azeotrope is left in the flask, which can then only boil off unchanged. See? Two mixtures, two opposite answers, and the kind of azeotrope decides.

7:48Quick revision: minimum and maximum boiling azeotropes

With me? So, quick revision. An azeotrope boils at a constant temperature, and its vapour has the same composition as the liquid, so distillation cannot separate it. Large positive deviation gives a minimum boiling azeotrope, like ninety five percent ethanol and water. Large negative deviation gives a maximum boiling azeotrope, like sixty eight percent nitric acid and water. Now go and try the practice problems on this page. All the best!