Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Okay, here is a statement; tell me, true or false. Water boils at one hundred degrees Celsius, and aniline at one hundred and eighty four. Yet in steam distillation, their mixture boils below one hundred degrees. That sounds impossible, right? Hold your answer; Example one settles it. This is Part C of the unit problem bank: concept checks on purification, chromatography and analysis. Each looks like a one-word answer, and each hides a trap. Statements like these come up often in JEE, so let's begin.

0:38Check 1: what can sublimation separate?

Check number one is on sublimation. Some solids, on heating, turn straight into vapour without melting, and on a cold surface the vapour turns back into solid. The arrows on screen mean turns into. Okay? Camphor, naphthalene and iodine behave like this. Now the statement: sublimation separates any two solids with different melting points. Is that true, or false? Here is the key idea. Sublimation needs the solid to have a high vapour pressure below its melting point, not merely a low melting point. Right? So it separates a sublimable solid, like iodine, from a non-sublimable one. Different melting points alone do not do it, so the statement is false. If you said false, very good! With me?

1:27Check 2: can a column separate an azeotrope?

For check number two, look at the figure. The flask at the bottom holds the mixture, with the heat below it. Above it is the fractionating column, packed with glass beads, with a thermometer at the top. On the right is the condenser, with cooling water in and out, and the beaker that collects the liquid. This set-up is for liquids with close boiling points. Clear? As the note beside the flask says, the rising vapours are condensed and re-vaporised again and again. Each time, the vapour gets richer in the lower-boiling liquid, so that liquid collects first. Got it? Now, an azeotrope is a mixture that boils at constant composition. Check number two says: an azeotrope can be separated by fractional distillation. Is that true, or false? Oh no, careful here, because this is where students slip. When an azeotrope boils, the vapour has exactly the same composition as the liquid. Okay? So the vapour never gets richer in either liquid, and the azeotrope cannot be separated this way. The statement is false. Right?

2:31Checks 3 and 4: what decides the Rf value?

Next comes chromatography, with a developed TLC plate, for thin-layer chromatography. The mixture was spotted on the base line near the bottom, and the solvent climbed up to the dashed line, the solvent front. R f, a capital R with a small f below, is the distance moved by the spot, over the distance moved by the solvent front, both from the base line. The figure marks these distances as d A, d B and d solvent, with small letters below the d. With me? Check number three: R f depends on the solvent used. Is that true, or false? Yes, it is true, well done if you said that. The solvent is the mobile phase, so change the solvent, and the R f value changes. Okay? Now think about polarity. Silica is polar, so it holds polar compounds strongly, and they move less. Spot B is like this; the note beside the plate calls it more polar, with a low R f. Check number four: in TLC on silica, the least polar compound has the lowest R f. Is that true, or false? Careful now, because this is exactly the trap. The least polar compound is adsorbed least, so it moves furthest with the solvent, like spot A in the figure. Clear? So it has the highest R f value, not the lowest, and the statement is false.

3:53Checks 5 and 6: what about oxygen?

Got it? Now let us move to analysis. In Lassaigne's test, the compound is fused with a small piece of sodium metal. Right? Nitrogen, sulphur and the halogens, the N, S and halogens on screen, become ionic sodium salts, soluble in water. They are sodium cyanide, N a C N, sodium sulphide, N a two S, with a small two below, and the sodium halide, N a X, with X for the halogen. Check number five: Lassaigne's test can detect oxygen. Is that true, or false? It is false. Oxygen forms no characteristic ion, so there is nothing to test for. With me? Check number six: the oxygen content of an organic compound is usually found by difference. Is that true, or false? Yes, it is true. We estimate every other element, and the percentage of oxygen, shown with the per cent sign, is one hundred minus the sum of the others, the sum inside the brackets.

4:55Example 1: does steam distillation boil below 100 C?

Okay? Now the two examples. Example one is our opening statement: in steam distillation, the mixture boils below one hundred degrees Celsius; the small circle and C on screen mean degrees Celsius. Is it true or false? Pause the video and try it first. Here is a hint: think about vapour pressures. Done? Okay, let's check. Water alone boils at one hundred degrees. So a mixture containing water cannot boil below that, right? No, no, wait, look at the conditions first. Steam distillation is for a liquid like aniline, which is steam-volatile and immiscible with water; the two liquids do not mix. Okay? So the two vapour pressures simply add up. The mixture boils when p water, plus p organic, equals p atm, the atmospheric pressure. Each p is a vapour pressure, and the small word below says whose. Right? So at the boiling point, p water is less than p atm; that is the less-than sign. Water alone has not reached its own boiling point, so the temperature is still below one hundred degrees. With me? Part B of this chapter puts numbers on it. At the boiling point, water gives seven hundred and twenty millimetres, the liquid gives forty, and together they make seven hundred and sixty millimetres, the full atmospheric pressure; the letters m m stand for millimetres. Clear? So the mixture boils below one hundred degrees Celsius, and our opening statement is true, even though aniline alone boils far higher. Got it?

6:30Example 2: is K2SO4 the catalyst in Kjeldahl's method?

And now Example two, a step harder. In Kjeldahl's method, K two S O four, potassium sulphate, is the catalyst; the small numbers below count the atoms. Is it true or false? Pause the video and try it first. Here is a hint: two sulphates are added, and each has its own job. Done? Okay, let's check. The compound is heated with concentrated sulphuric acid, H two S O four; the short word conc means concentrated. Its nitrogen turns into ammonium sulphate: N H four in brackets, a small two outside, then S O four. Okay? Along with the acid, two salts are added: potassium sulphate, and copper sulphate, C u S O four. They look alike, and students often swap their jobs. So which one is the catalyst? It is not potassium sulphate; its job is to raise the boiling point. Right? Yes, exactly, copper sulphate is the catalyst. So the statement is false, and that was the trap. With me? Here is one more condition to remember. Kjeldahl's method does not work for nitrogen in nitro or azo groups, or in a ring, as in pyridine; the capital N here means nitrogen. That nitrogen is not converted into ammonium sulphate, so we use Dumas' method, which works for all organic nitrogen. Clear?

7:59Quick revision: sublimation, azeotrope, Rf, oxygen, Kjeldahl

So, quick revision. Sublimation separates a sublimable solid from a non-sublimable one, and an azeotrope cannot be separated by fractional distillation. R f changes with the solvent, and on silica the least polar compound has the highest R f. Oxygen is not detected by Lassaigne's test; it is found by difference. In steam distillation, the mixture boils below one hundred degrees, and in Kjeldahl's method, copper sulphate is the catalyst. Now go and try the practice problems on this page. All the best!