Organic chemistry, Year 11: combustion, cracking and alkenes

It increases

HintBigger molecules attract each other more strongly.

The whyThe forces between larger molecules are stronger, so more energy is needed to separate them. Methane boils far below room temperature, while the largest hydrocarbons in crude oil are solids.

Viscosity

HintTreacle has a lot of it; water has little.

The whyThe more viscous a liquid, the more slowly it pours. In hydrocarbons, viscosity increases as the molecules get bigger.

The one with 20 carbon atoms

HintLong molecules get more tangled and hold on to each other more.

The whyViscosity increases with molecular size, so the liquid with the bigger molecules is thicker and flows more slowly. This is why petrol is runny and lubricating oil is not.

It decreases: bigger molecules are harder to ignite

HintCompare lighting gas on a hob with trying to light thick engine oil.

The whySmall hydrocarbon molecules turn to vapour easily and the vapour catches fire readily. Large ones do not vaporise easily and are hard to set alight.

It catches light more easily and flows more easily

HintThink about getting the fuel to the engine, and then getting it to burn.

The whySmall molecules mean high flammability and low viscosity, so petrol can be pumped through narrow pipes and ignites readily. Heavy fuel oil is thick and hard to light, which suits large ship engines and power stations instead.

carbon dioxide; water

HintOne product comes from each of the two elements in the fuel, joined to oxygen.

The whyWith plenty of oxygen, every carbon atom in the fuel ends up in CO2 and every hydrogen atom ends up in H2O. The reaction also releases energy, which is the reason for burning the fuel.

5

HintEvery water molecule needs two hydrogen atoms.

The whyAll 10 hydrogen atoms in the butane molecule end up in water, H2O, which holds two each: 10 ÷ 2 = 5. In the same way the 4 carbon atoms give 4 molecules of CO2.

C3H8 + 5O2 → 3CO2 + 4H2O

HintBalance carbon first, then hydrogen, and leave oxygen until last.

The whyThree carbon atoms need 3CO2 and eight hydrogen atoms need 4H2O. The products then contain 6 + 4 = 10 oxygen atoms, which is 5 molecules of O2.

oxidised

HintIt is the word for gaining oxygen.

The whyCombustion is an oxidation: the carbon gains oxygen to become carbon dioxide and the hydrogen gains oxygen to become water.

Hydrogen atoms in the fuel combine with oxygen from the air

HintLook at the two elements in H2O and ask where the engine gets each one.

The whyWater is a product of the combustion reaction, not something left over. Hydrocarbon fuels contain hydrogen, and burning oxidises it to H2O, which leaves the hot exhaust as vapour.

Cracking

HintThe same word describes what happens to a dropped plate.

The whyCracking is a chemical reaction: covalent bonds in the long molecules break and new, shorter molecules form. It is different from fractional distillation, which only sorts the molecules already there.

Catalytic cracking

HintThe method is named after the kind of substance the vapour passes over.

The whyThe long-chain hydrocarbons are heated until they vaporise, and the vapour is passed over a hot catalyst. The molecules break apart on its surface.

A high temperature

HintBreaking strong covalent bonds takes a lot of energy.

The whyIn both methods the hydrocarbons are first vaporised and then made very hot. Steam cracking uses no catalyst, so it needs an even higher temperature than catalytic cracking.

2

HintCount the carbon atoms still needed on the right-hand side.

The whyThe left has 12 carbon atoms and C6H14 supplies 6, so 6 more are needed: two molecules of C3H6. Check the hydrogen: 14 + 2 × 6 = 26, which matches.

It converts surplus long molecules to short ones that are in high demand as fuels

HintPicture a shop swapping slow-selling stock for something customers queue up for.

The whySmall molecules make the best fuels, so demand for them is high. Cracking matches supply to demand by breaking the less useful long chains into shorter alkanes for fuels, and into alkenes for making polymers and other chemicals.

Orange to colourless

HintThe starting shade is that of the bromine; the alkene removes it.

The whyBromine water is orange. An alkene reacts with the bromine, using it up, so the colour disappears. This is the test for an alkene.

Q

HintThe more reactive family is the one that uses up the bromine.

The whyAlkenes react with bromine water and remove its colour; alkanes do not react, so the orange colour stays. Q has decolourised the bromine water, so Q is the alkene and P is an alkane.

Alkenes

HintIts name differs from that of the starting family by a single letter.

The whyThere are not enough hydrogen atoms in one long alkane to make two or more smaller alkanes, so at least one product is an alkene. An alkene has two fewer hydrogen atoms than the alkane with the same number of carbon atoms, and a double bond between two of its carbon atoms.

Alkenes are more reactive than alkanes

HintAsk which family actually did something in the test.

The whyAlkenes take part in reactions that alkanes do not. That reactivity is what makes them useful as the starting point for polymers and many other chemicals, while alkanes are mostly just burned.

polymers

HintPoly(ethene), a common plastic, is one of them.

The whyMany small alkene molecules can be joined into very long chains, and those chains are polymers. This is one reason cracking matters: it supplies the alkenes that the plastics industry is built on.