Combustion and Other Reactions

Transcript

0:00Introduction

Hello everyone! Alkanes have a funny nickname: paraffins, which means little affinity. Under ordinary conditions they ignore acids, bases, oxidising and reducing agents. And yet, alkanes are the fuels we burn every day: CNG, LPG and petrol. So here is a small puzzle for you. You burn ten litres of butane gas completely. How much oxygen does it need? Ten litres? Twenty litres? Keep your guess in mind; the real answer is bigger. Questions on these reactions come up often in JEE papers. So, let's begin.

0:39Combustion: one equation for every alkane

Let us start with combustion, which is simply burning. Alkanes have the general formula C n H two n plus two; the letters and numbers written low count the atoms. The alkane burns in oxygen, O two, and the arrow means gives: C O two, carbon dioxide, and H two O, water. Not balanced yet, as the brackets say. Okay? So let us balance it. Every carbon goes into one C O two, and every two hydrogens make one water. Tell me, how many C O two and how many water molecules do we get? Yes, exactly! There are n carbon atoms, so we get n C O two. And two n plus two hydrogens, taken two at a time, give n plus one water molecules. Right? Now count the oxygen atoms on the right. Two in each C O two gives two n, and one in each water gives n plus one. Together, that is three n plus one oxygen atoms. With me? Oxygen comes in pairs, as O two, so we need three n plus one, over two, oxygen molecules. So the full equation is: C n H two n plus two, plus three n plus one, over two, O two, gives n C O two, plus n plus one H two O, water. Write this down. Clear?

2:02Lots of heat, and the trouble with too little air

Now look at the heat this reaction gives out. For methane, C H four, it is minus eight hundred and ninety kilojoules per mole, and for butane, minus two thousand eight hundred and seventy six. The minus sign means heat is released, and the minus one written high on mol means per mole. Okay? So combustion is strongly exothermic, and that is exactly why alkanes are used as fuels: CNG, LPG and petrol. Right? But let me ask you this. What happens if the flame does not get enough air? Do we still get only carbon dioxide and water? No, not always, and this is important. With insufficient air, we get carbon monoxide, or even carbon black, which is finely divided carbon. Clear? For methane: C H four plus O two gives C, which is carbon, plus two H two O, water. Count it: one carbon, four hydrogens and two oxygens on each side. Got it?

3:03Controlled oxidation: useful products, not just burning

Alright, with the right catalyst and conditions, oxygen gives controlled oxidation instead of burning. The first one is two C H four plus O two, over copper, C u, at five hundred and twenty three kelvin and one hundred atmospheres; the conditions are written over the arrow. It gives two C H three O H, methanol. Okay? Next, C H four plus O two, over M o two O three, an oxide of molybdenum, with heat. This gives HCHO, methanal, plus water. Right? And ethane, C H three C H three. Two molecules of ethane with three O two, over manganese ethanoate with heat, give two C H three COOH, ethanoic acid, plus two water molecules. In the catalyst, the bracket with the two written low means two ethanoate groups on one manganese. With me? Now, ordinary alkanes resist potassium permanganate, K M n O four. So no alkane is ever oxidised by it, right? No, no, careful, there is one exception! An alkane with a tertiary hydrogen, that is, a hydrogen on a carbon joined to three other carbons, is oxidised to a tertiary alcohol. On screen, C H three in brackets, with a three written low, means three methyl groups on one C H group. So two methyl propane gives two methyl propan two ol. Got it?

4:32Aromatisation: chains that become rings

Now, aromatisation, also called reforming. Alkanes with six or more carbons are passed over C r two O three, V two O five, or M o two O three, which are oxides of chromium, vanadium and molybdenum, on alumina. Okay? At seven hundred and seventy three kelvin and ten to twenty atmospheres, the chain cyclises, which means it closes into a ring, and it loses hydrogen. Clear? So hexane gives benzene. Six carbons go in, and the ring keeps all six. With me? Now, toluene is benzene with a methyl group, so it has seven carbons. Which alkane gives toluene? Heptane, yes, exactly! The aromatic product keeps the same number of carbons as the alkane. Right?

5:20Isomerisation, steam and cracking

Now three more, quickly. Isomerisation uses anhydrous, that is completely dry, aluminium chloride, A l C l three, along with H C l. The straight chain changes into its branched isomers: same formula, different arrangement. Okay? Next, methane with steam, over nickel, N i, at one thousand two hundred and seventy three kelvin. It gives C O plus three H two, and this is an industrial source of hydrogen. Clear? And finally, pyrolysis, also called cracking: strong heating in the absence of air. With me? Higher alkanes break into smaller alkanes, alkenes and hydrogen, and they do it by a free radical mechanism. Got it?

6:04Example 1: oxygen and CO2 volumes to burn 10 L of butane

Now, remember our puzzle? Let us finish it. What volume of oxygen is needed to burn ten litres of butane completely, and what volume of C O two is formed, under the same conditions? Pause the video and try it first. Done? Okay, let's check. Butane has four carbons, so n is four. Three times four, plus one, gives thirteen, and thirteen over two gives six point five. Right? So C four H ten, plus six point five O two, gives four C O two, plus five H two O, water. Okay? Now, for gases under the same conditions, the volume ratio is the same as the mole ratio. Clear? So oxygen is six point five times ten, which gives sixty five litres. And carbon dioxide is four times ten, which gives forty litres. So you see, not ten, not twenty, but sixty five litres of oxygen, and that solves our puzzle!

7:03Example 2: hexane to benzene, moles of hydrogen released

Now one more, a little harder. Hexane is passed over C r two O three on alumina, at seven hundred and seventy three kelvin and ten to twenty atmospheres. Name the product. And how many moles of hydrogen are released per mole of hexane? Pause the video and try it first. Done? Let us check. These are aromatisation conditions, and the ring keeps all six carbons. So the product is benzene. Clear? Now the hydrogen. Hexane, C six H fourteen, has fourteen hydrogen atoms, and benzene, C six H six, has six. Fourteen minus six gives eight. So eight moles of hydrogen, right? No, no, careful, those are eight hydrogen atoms! Hydrogen gas is H two, with two atoms in each molecule, so eight over two gives four. C six H fourteen gives C six H six plus four H two: four moles of hydrogen per mole of hexane. Got it?

8:09Quick revision: combustion, oxidation, reforming, cracking

So, quick revision. Combustion: one mole of C n H two n plus two needs three n plus one over two moles of oxygen, and too little air gives carbon monoxide or carbon. Controlled oxidation gives methanol, methanal and ethanoic acid, and K M n O four needs a tertiary hydrogen. And for isomerisation, aromatisation, steam and cracking, learn each catalyst and condition. Now go and try the practice problems on this page. All the best!