Distillation

Transcript

0:00Introduction

Hello everyone! Let us start with a puzzle. Water boils at one hundred degrees Celsius, and aniline, an organic liquid, at one hundred and eighty four. Now mix the two and heat them. The mixture should boil somewhere in between, right? But it boils at ninety eight point five degrees, below both of them! How is that possible? Today we will meet four kinds of distillation, and solve this puzzle with real numbers. Choosing the right method comes up often in JEE, so let's begin.

0:33Simple distillation: a wide gap

So, the first kind is simple distillation. It is for a volatile liquid, one that turns into vapour easily, with non-volatile impurities, which stay behind. Right? It also separates two liquids whose boiling points differ widely, by more than about twenty to thirty degrees Celsius, written on screen with a small circle and a capital C, okay? For example, take chloroform at sixty one degrees, and aniline at one hundred and eighty four. The gap is one hundred and twenty three degrees, very wide, so simple distillation does the job. Clear?

1:09Fractional distillation: a close gap

When the boiling points are close, as in crude petroleum, we need fractional distillation. Look at the figure. The mixture is heated in the flask, and above it stands a tall column of glass beads, the fractionating column, with a thermometer at the top. Then comes the condenser, with cooling water flowing in and out, and a beaker to collect the liquid. Okay? The rising vapour condenses on the beads and evaporates again, many times. So let me ask you this: as it rises, which liquid does the vapour become richer in? It gets richer in the lower-boiling liquid, yes, exactly! That one is more volatile, so it stays in the vapour, and it collects first. Right? The higher-boiling vapour condenses first and runs back into the flask. So the vapour reaching the condenser is almost pure lower-boiling liquid, as the caption says. Got it?

2:03Careful: an azeotrope defeats fractional distillation

But there is a limit. Some mixtures boil at constant composition, and such a mixture is called an azeotrope. Okay? The famous one is rectified spirit, ninety five point six per cent ethanol, the rest water. Can repeated fractional distillation turn it into pure ethanol? No, it cannot, and this is a common trap. In an azeotrope, the vapour has the same composition as the liquid, so nothing ever separates. Clear?

2:33Distillation under reduced pressure

Next, some liquids decompose at or below their normal boiling point. So how can we distil such a liquid without destroying it? We lower the pressure, exactly! Lower the external pressure, and the boiling point drops, so the liquid boils before it can decompose. This is distillation under reduced pressure. Okay? In the soap industry, glycerol is recovered from spent lye this way, since glycerol decomposes near its boiling point.

3:03Steam distillation

Right? The fourth kind is steam distillation. The liquid must be steam-volatile, so steam carries it over, and immiscible with water, so it does not mix. And the impurities must be non-volatile. With me? Since the two liquids do not mix, each exerts its own full vapour pressure. Small p means vapour pressure, and p atm is the atmospheric pressure. The mixture boils when p water plus p organic equals p atm. So does it boil above or below one hundred degrees? It boils below one hundred, exactly! The organic liquid adds its own pressure, so the sum reaches atmospheric pressure before water alone would boil. That is the secret of our puzzle. Got it? The course examples are aniline, essential oils like rose and eucalyptus, and o-nitrophenol, where the little o means ortho. Keep that last one in mind. Okay?

4:00Steam distillation: how much comes over?

Now the numericals. In the distillate, the liquid we collect, the mole ratio equals the pressure ratio. Here n is the number of moles, subscript o r g means organic, and subscript w means water. So n organic over n water equals p organic over p water. Right? Mass is moles times molar mass. So, with small m for mass and capital M for molar mass, m organic over m water equals p organic times capital M organic, over p water times capital M water. Now, the organic vapour pressure is usually small. So does only a tiny mass come over? No, look at the formula. It also multiplies by capital M organic, and organic molecules are usually much heavier than water. So a high molar mass makes up for a low vapour pressure.

4:54Careful: ortho or para nitrophenol?

Clear? Now, a classic trap. Ortho nitrophenol and para nitrophenol both have an O H group. So which one does the steam carry over? It is the ortho isomer. Its hydrogen bond forms inside the same molecule, an intramolecular H bond, as written on screen. So no free O H is left to bond with water, and that makes it steam-volatile. Okay? In p-nitrophenol, where p means para, not pressure, the hydrogen bonds link different molecules, so they are intermolecular. That gives it a higher boiling point, and it is not steam-volatile. Right? So steam distillation separates the two: the ortho isomer distils over, and the para isomer stays behind. Intra and inter, don't mix them up! With me?

5:42Example 1: acetone and methanol, which distillation?

Now, Example one, an easy one. Acetone boils at fifty six degrees Celsius, and methanol at sixty five, where b p on screen means boiling point. Which distillation separates them? Pause the video and try it first. Done? Okay, let's check. The boiling points are different, so simple distillation will do, right? No, no, careful, look at the gap. Sixty five minus fifty six gives only nine degrees, far less than twenty to thirty. Got it? So the boiling points are close, and the answer is fractional distillation. Always find the gap first. Okay?

6:25Example 2: steam distilling aniline, mass per gram of water

Now Example two, our opening puzzle. Here, a mixture of aniline and water boils at ninety eight point five degrees Celsius, at seven hundred and sixty m m, which means millimetres of mercury. The vapour pressure of water there is seven hundred and eighteen millimetres, and aniline's molar mass, capital M, is ninety three. Find the mass of aniline in the distillate per gram of water. Pause the video and try it first. Done? Okay, let's check. The two vapour pressures add up to seven hundred and sixty. So p aniline is seven hundred and sixty minus seven hundred and eighteen, which gives forty two millimetres. Right? Now, forty two over seven hundred and eighteen is the mole ratio. But is it the mass ratio too? No, it is not, because we must multiply by the molar masses. So m aniline over m water equals forty two times ninety three, over seven hundred and eighteen times eighteen, the molar mass of water. That gives three thousand nine hundred and six on top, and twelve thousand nine hundred and twenty four at the bottom. Clear? Divide, and we get about zero point three zero, where the wavy equals sign means approximately. So about zero point three grams of aniline comes over with every gram of water. Its vapour pressure is low, but its molar mass of ninety three makes up for it. And it all boils at ninety eight point five degrees, below one hundred. That is our puzzle, fully solved.

8:00Quick revision: simple, fractional, reduced pressure, steam

So, quick revision. Simple distillation for a wide gap in boiling points, fractional for a close gap, but fractional distillation cannot separate an azeotrope. Reduced pressure if the liquid decomposes on boiling, and steam distillation for a steam-volatile liquid that does not mix with water. In steam distillation the mixture boils below one hundred degrees, and the mass ratio uses vapour pressure times molar mass. Now go and try the four practice problems on this page. All the best!