Carbon: Allotropes and Oxides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Diamond is the hardest natural substance, and graphite is so soft and slippery that it is used as a dry lubricant. Yet both are nothing but carbon. And the soft one, graphite, conducts electricity, while the hard one, diamond, does not. How can the same atom build such different solids? Today we find out, and then we meet carbon's two oxides. These come up often in JEE, so let's begin.

0:31Diamond: a rigid three-dimensional network

So, first diamond, on the left. Each carbon is s p three, the three written small and raised, bonded tetrahedrally to four other carbons. The C, dash, C is a carbon carbon bond, one hundred and fifty four picometres long, written p m. Okay? These bonds make one rigid three dimensional network, the three D on screen. No wonder diamond is the hardest natural substance. Right? It has a very high melting point, and it is an electrical insulator. Now let me ask you this: an insulator for electricity must be poor with heat too, right? Oh no, careful, that is exactly the trap, because diamond is an excellent conductor of heat. Poor for electricity, excellent for heat; remember the pair. Got it?

1:21Graphite: flat layers of hexagons

Now graphite, on the right. Here each carbon is s p two, and the carbons form flat layers of hexagonal rings. Okay? The fourth electron of each carbon is delocalised, spread over the whole layer. Keep this in mind; it will solve our puzzle. So the C C bonds are shorter than in diamond, one hundred and forty one point five picometres, with bond order one and one third, the mixed fraction on screen. Clear? The layers are three hundred and forty picometres apart, held together only by weak van der Waals forces. With me? So the layers slide over one another, and graphite is soft and slippery. It also conducts electricity, mainly along the layers. Now a quick question. Which form of carbon is the most stable? Surely the hardest one, diamond, right? No, no, wait, the most stable form is actually graphite! Its standard enthalpy of formation, delta f H, with the small f below meaning formation and the small circle meaning standard, is taken as zero. Diamond's is plus one point nine kilojoules per mole, the mol to the power minus one meaning per mole, so diamond sits higher in energy. Okay?

2:36Fullerenes, graphene and amorphous carbon

Next, the fullerenes. The famous one is C sixty, the sixty written small below, called buckminsterfullerene. It is a hollow cage with twenty rings of six carbons and twelve rings of five carbons. So, what is the hybridisation of each carbon here? It is s p two, very good, as each carbon is bonded to three others. The C C single bonds are one hundred and forty three point five picometres, and the C double bond C, the two short lines, one hundred and thirty eight point three picometres. Okay? And students often forget this: fullerenes are the only pure, molecular form of carbon, and they dissolve in organic solvents. Right? Graphene is a single layer of graphite, and there are carbon nanotubes too. The amorphous forms are coke, charcoal and carbon black.

3:30Carbon monoxide: how it is made

Now the oxides, starting with carbon monoxide, C O, okay? In the lab, formic acid, HCOOH, is dehydrated by concentrated sulphuric acid, H two S O four, written above the arrow; the small numbers below count atoms. The arrow means gives, and we get C O plus water. Clear? Carbon with water at four hundred and seventy three to one thousand two hundred and seventy three kelvin, written above the arrow, gives C O plus H two, and this mixture is water gas. With air instead, two C plus O two plus four N two gives two C O plus four N two. The nitrogen stays unchanged, and this mixture is producer gas. Okay? And carbon monoxide with more water gives C O two plus H two, the water gas shift reaction. Got it?

4:21Carbon monoxide: properties and structure

Carbon monoxide is a colourless, odourless gas. Now let me ask you this. Carbon dioxide, coming soon, is an acidic oxide. So carbon monoxide should be acidic too, right? Careful here, because carbon monoxide is not acidic at all; it is a neutral oxide. It is a good reducing agent: F e two O three plus three C O gives two F e plus three C O two. Right? Its structure is C, triple bond, O, with a lone pair, the two dots, on each atom. The triple bond is one sigma and two pi bonds, sigma and pi being Greek letters. The lone pair on carbon binds metals, giving metal carbonyls such as N i, C O in brackets, four. With me? And carbon monoxide binds haemoglobin about three hundred times more strongly than oxygen, so haemoglobin can no longer carry oxygen. That is why it is highly poisonous.

5:20Carbon dioxide and dry ice

Clear? Now carbon dioxide. Calcium carbonate, C a C O three, plus two H C l gives C a C l two, plus C O two, plus water. Okay? Carbon dioxide is linear, with s p carbon, and acidic: with water it gives H two C O three, the double half arrows meaning equilibrium. It is also a greenhouse gas. Right? Solid carbon dioxide is dry ice. At one hundred and ninety five kelvin it sublimes, straight from solid to gas, without melting. So it cools things without wetting them. Nice, isn't it?

5:56Example 1: why graphite conducts but diamond does not

Now, example one, which solves our puzzle. Why is graphite a good conductor of electricity, but diamond an insulator? Pause the video and try it first. Done? Okay, let's check. In graphite, each carbon uses three electrons in sigma bonds. The fourth, a pi electron, is delocalised over the layer and moves freely. Now, in diamond, can any electron move? Not a single one, because in diamond all four electrons are in localised sigma bonds, so it is an insulator. So the answer is the delocalised pi electrons of graphite, and our puzzle is solved, very good!

6:41Example 2: volume of CO at STP from 4.6 g of formic acid

Now example two, a small numerical. What volume of C O at STP, where one mole takes twenty two point four litres, comes from dehydrating four point six grams of formic acid with concentrated H two S O four? The L mol to the power minus one means litres per mole. Pause the video and try it first. Done? Okay, let's check. The equation: HCOOH gives C O plus water. Now tell me, one mole of formic acid gives how many moles of carbon monoxide? One mole, exactly, a one to one ratio. The molar mass, capital M, of formic acid: two for the hydrogens, twelve for carbon, thirty two for the oxygens, total forty six grams per mole. Okay? So moles of acid are four point six over forty six, which gives zero point one zero, and the same zero point one zero moles of carbon monoxide. With me? Finally, the volume, capital V, is zero point one zero times twenty two point four, which gives two point two four litres. Oh no, careful here, as the note in brackets says: multiply the moles, not the grams, by twenty two point four, because that number is litres per mole. Got it?

8:04Quick revision: diamond, graphite, C60, CO and CO2

So, quick revision. Diamond is an s p three network, the hardest, and an insulator; graphite has s p two layers, conducts, and is the most stable. C sixty has twenty rings of six and twelve rings of five, all s p two, and is the only pure form. Carbon monoxide is neutral and reducing, and binds metals and haemoglobin; carbon dioxide is linear and acidic, and its solid is dry ice. Now go and try the practice problems on this page. All the best!