Sulphur and Sulphur Dioxide

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Bleach a dye with moist sulphur dioxide, and the colour disappears. Leave it in air for a while, and slowly, the colour comes back! Bleach the same dye with chlorine, and the colour never returns. So what is the difference? By the end of this video, you will know why. This topic comes up often in JEE, so let's begin.

0:26Rhombic and monoclinic sulphur

Solid sulphur has two forms. Rhombic, or alpha sulphur, with the Greek letter alpha, is yellow and stable at room temperature. Its m p, the melting point, is three hundred and eighty five point eight kelvin, the capital K, and it dissolves in C S two, carbon disulphide; the small two counts atoms. Okay? Above three hundred and sixty nine kelvin, monoclinic, or beta sulphur, with the Greek letter beta, is stable. That is the transition temperature, and on slow heating or cooling through it, the change is reversible. Monoclinic melts at three hundred and ninety three kelvin. Right? Now, what are these crystals made of? Oxygen, in the same group, is diatomic, O two. So is solid sulphur made of S two molecules too? No, not at all, because both forms are built from S eight rings, eight atoms in a puckered crown shape. S six rings, with a chair shape, are also known. Clear? So S two is found in hot sulphur vapour. At about one thousand kelvin, the vapour is mainly S two, and S two, just like O two, is paramagnetic. With me?

1:38Heating sulphur: runny, then thick

Now let us melt it. At about one hundred and nineteen degrees Celsius, the small circle and C, sulphur melts to a mobile yellow liquid. Here the S eight rings simply slide past each other. Got it? Now keep heating. Most liquids flow more easily when hotter, right? So what happens to molten sulphur? Does it get runnier, or thicker? Oh no, careful, it gets thicker! Above one hundred and sixty degrees Celsius, the rings break open into long chains, and these chains tangle with each other. Okay? So the liquid turns dark and very viscous, since tangled chains cannot easily slide past each other. Pour it into water, and you get plastic sulphur. Right?

2:23Preparing sulphur dioxide, SO₂

Now, let us make sulphur dioxide. Burn sulphur: S plus O two gives S O two, the arrow meaning gives. Or add acid to a sulphite: S O three, two minus, the raised two minus being its charge, plus two H plus, gives water and S O two. Clear? In industry, it comes from roasting pyrites, F e S two. Four F e S two plus eleven O two gives two F e two O three plus eight S O two. Got it?

2:56Properties and shape of SO₂

Now, the properties of S O two: it is a colourless, pungent gas, very soluble in water. So it is not collected over water. With me? And it is acidic. With water, it gives sulphurous acid, H two S O three; the two half arrows mean reversible. With sodium hydroxide, N a O H, it gives sodium sulphite, N a two S O three, and with more S O two, sodium bisulphite, N a HSO three. Okay? Now, think about its shape. Carbon dioxide, C O two, also has two oxygens on a central atom, and is linear. So is S O two linear too? No, no, wait, S O two is bent! The bond angle is one hundred and nineteen point five degrees, the small circle, and sulphur is s p two hybridised, written s p with a raised two. Its two S O bonds, the dash, are equal because of resonance. Right?

3:56SO₂ as a reducing agent

Now the key property: moist S O two is a reducing agent. Two F e three plus, iron with charge plus three, plus S O two and two water, give two F e two plus, sulphate, S O four, two minus, and four H plus. So iron is reduced, from plus three to plus two. Clear? Permanganate, M n O four minus, is reduced too. Five S O two plus two permanganate plus two water give five sulphate, four H plus, and two M n two plus. Now, permanganate is purple. So what do you see? Yes, exactly, the purple colour disappears; permanganate is decolourised. And acidified dichromate turns green, because C r three plus forms. That is the test for S O two. Got it? And S O two is an even stronger reducing agent in alkali, as the sulphite ion, than in acid. Okay?

4:56Oxidising agent, bleaching, two more reactions

But towards a stronger reducing agent, like H two S, hydrogen sulphide, S O two is an oxidising agent. S O two plus two H two S gives three S plus two H two O, and yellow sulphur is deposited. Right? Now, back to our puzzle. Moist S O two is a reducing agent. So how does it bleach a dye? Yes, exactly, it bleaches by reduction! Moist S O two reduces the coloured matter. But the air slowly oxidises it back, so the colour returns; the bleaching is temporary. With me? Chlorine, C l two, bleaches by oxidation, and that is permanent. So our puzzle is solved. Air undoes the reduction by S O two, but the oxidation by chlorine stays. Clear? Now, two more reactions to remember. Over charcoal, written above the arrow, S O two and chlorine give sulphuryl chloride, S O two C l two. And over the catalyst V two O five, two S O two plus O two gives two S O three.

6:01Example 1: lab test for SO2 with acidified dichromate

Okay? Now example one, an easy one. How is sulphur dioxide identified in the laboratory? Pause the video and try it first. Done? Okay, let's check. Filter paper moistened with acidified potassium dichromate turns green in the gas. Now, where does that green colour come from? Yes, exactly, S O two reduces dichromate to green C r three plus. C r two O seven, two minus, plus three S O two plus two H plus, gives two C r three plus, three sulphate, and water. Right? Always check the charges. On the left, minus two plus two gives zero. On the right, two times plus three, plus three times minus two, gives zero again. So the equation is balanced. Got it?

6:54Example 2: volume of SO2 at STP from burning 1 kg sulphur

Now example two, a numerical one. What volume of S O two, at STP, is produced by burning one point zero zero kilograms of sulphur? STP is standard temperature and pressure, and the molar volume is twenty two point four litres per mole, the L mol to the minus one. Pause the video and try it first. Done? Okay, let's check. S plus O two gives S O two, so one mole of sulphur gives one mole of S O two. Sulphur is thirty two point zero six grams per mole, so the moles are one point zero zero over thirty two point zero six, right? Oh no, careful, that mass is in kilograms! One point zero zero kilograms is one thousand grams. So n, the number of moles, equals one thousand over thirty two point zero six, which gives thirty one point two moles. Clear? Each mole occupies twenty two point four litres. So the volume, V, equals thirty one point two times twenty two point four, which gives about six hundred and ninety nine litres; the wavy equals sign means approximately. With me?

8:10Quick revision: allotropes, SO2 redox, temporary bleaching

So, quick revision. Rhombic sulphur is stable below three hundred and sixty nine kelvin, monoclinic above, and both are S eight crown rings. Moist S O two reduces iron three plus, permanganate and dichromate, but oxidises H two S, hydrogen sulphide. And S O two bleaches by reduction, temporary, while chlorine bleaches by oxidation, permanent. Now go and try the practice problems on this page. All the best!