Modern life depends on hydrocarbons for fuels and materials
Definition
Hydrocarbon
A compound containing carbon and hydrogen atoms only.
- Most crude oil is burned as fuel, because hydrocarbons release a lot of energy when they burn.
- Petrol and diesel power cars and lorries, kerosene powers aircraft and fuel oil powers ships.
- Natural gas, which is mostly the hydrocarbon methane, heats most UK homes and generates a large share of the country's electricity.
- The rest of the crude oil is a feedstock for making plastics, synthetic fibres, solvents, detergents, lubricants and medicines.
- Food packaging, polyester clothing, the bitumen in road surfaces and the plastic casing of a phone all come from hydrocarbons.
- Replacing hydrocarbons would mean changing how people travel, heat buildings and make almost every manufactured product.
Example
A single bus journey uses diesel as the fuel, plastics in the seats, synthetic rubber in the tyres and bitumen in the road, all made from crude oil.
Crude oil is finite and forms far too slowly
Definition
Finite resource
A resource that is used up faster than it can be replaced, so supplies will eventually run out.
- Crude oil formed over millions of years from the remains of tiny sea plants and animals buried in mud.
- Heat and high pressure slowly changed these remains into crude oil, which became trapped in porous rock.
- People are using crude oil up in a tiny fraction of the time it took to form, so it is non-renewable.
- As the easily reached reserves run out, oil has to come from deeper or more remote places, which costs more and carries more risk.
- Burning crude oil destroys a feedstock that cannot be replaced, which is one reason to use alternatives such as biofuels, electric vehicles and recycled plastics.
Key Idea
Once a finite resource is used it is gone, so every litre of crude oil burned as fuel is a litre that can no longer be made into materials.
Fractional distillation makes too many long-chain hydrocarbons
- The amount of each fraction depends on the crude oil used, not on what customers want to buy.
- Crude oil usually contains more long-chain hydrocarbons, such as fuel oil, than the market needs.
- Long-chain hydrocarbons are viscous, hard to ignite and burn with smoky flames, so they make poor fuels for cars.
- Demand is highest for the short-chain fractions, such as petrol, which crude oil supplies in smaller amounts.
- Cracking converts the surplus long-chain molecules into shorter, more useful ones, so supply can match demand.
Example
- A refinery makes 303030 tonnes of fuel oil from 100100100 tonnes of crude oil but can only sell 101010 tonnes, while it makes 101010 tonnes of petrol but could sell 252525 tonnes.
- Cracking the surplus fuel oil turns it into extra petrol and alkenes, which closes the gap.
Cracking breaks long-chain alkanes into shorter alkanes and alkenes
Definition
Cracking
The thermal decomposition of long-chain alkanes into shorter, more useful alkanes and alkenes, using heat and often a catalyst.
Definition
Catalyst
A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy without being used up.
- Cracking uses heat to break long molecules apart, so it is a type of thermal decomposition.
- In catalytic cracking, the long-chain alkane is heated to vaporise it and the vapour is passed over a hot catalyst.
- The catalyst is usually a powdered zeolite or aluminium oxide at about 500 ∘C500\ ^{\circ}\text{C}500 ∘C.
- In steam cracking, the vapour is mixed with steam and heated above 800 ∘C800\ ^{\circ}\text{C}800 ∘C without a catalyst.
- The products are usually a mixture of shorter alkanes and alkenes, and there is always at least one alkene.
- Atoms are conserved, so the carbon and hydrogen atoms in the products add up to those in the starting alkane.
- Decane can crack into octane and ethene: C10H22→C8H18+C2H4\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4C10H22→C8H18+C2H4
Practical
Cracking liquid paraffin in the laboratory
- Aim: crack a long-chain alkane in the laboratory and show that an alkene forms.
- Apparatus: a hard-glass boiling tube with a bung and delivery tube, mineral wool, liquid paraffin, broken porcelain or aluminium oxide as the catalyst, a clamp and stand, a Bunsen burner, a trough of water, test tubes with bungs and bromine water.
Method
- Soak some mineral wool in liquid paraffin and push it to the closed end of the boiling tube.
- Put pieces of broken porcelain or aluminium oxide in the middle of the tube.
- Clamp the tube almost horizontally and connect the delivery tube so that it leads to an upturned test tube full of water.
- Heat the catalyst strongly, moving the flame to the mineral wool from time to time so that the paraffin vaporises and passes over the hot catalyst.
- Collect several tubes of gas, throwing away the first one because it is mostly air.
- Lift the delivery tube out of the water before you stop heating.
- Shake a tube of the gas with a few drops of bromine water and record the colour change.
Accuracy and safety
- Lifting the delivery tube out first stops cold water being sucked back into the hot tube, which could crack it.
- Bromine water turns from orange to colourless, which shows that the gas contains an alkene.
- Keep the collected gas away from flames, because it is flammable.
- Wear eye protection, and wear gloves when handling bromine water because it is harmful.
Cracking products are more useful than long-chain alkanes
Definition
Alkene
A hydrocarbon that contains a double covalent bond between two carbon atoms.
- The shorter alkanes are blended into petrol and other fuels that are in high demand.
- The alkenes contain a C=C\text{C}=\text{C}C=C double bond, which makes them more reactive than alkanes.
- Alkenes with one double bond have the general formula CnH2n\text{C}_n\text{H}_{2n}CnH2n, with two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
- This reactivity makes alkenes a valuable feedstock for the petrochemical industry.
- Ethene and propene from cracking are joined into polymers such as poly(ethene) and poly(propene).
- Alkenes are also used to make other chemicals, such as ethanol from ethene.
Example
- Hexadecane, C16H34\text{C}_{16}\text{H}_{34}C16H34, is cracked to give octane, C8H18\text{C}_8\text{H}_{18}C8H18, and one other product; find its formula and name its type of hydrocarbon.
- Subtract the carbon atoms: 16−8=816 - 8 = 816−8=8.
- Subtract the hydrogen atoms: 34−18=1634 - 18 = 1634−18=16.
- The other product is C8H16\text{C}_8\text{H}_{16}C8H16, which fits CnH2n\text{C}_n\text{H}_{2n}CnH2n, so it is an alkene.
Self review
- Give two ways in which modern life depends on hydrocarbons.
- Why is crude oil described as a finite resource?
- Why do oil refineries crack some of the fractions from crude oil?
- What conditions are used for catalytic cracking?
- What is the missing product in C12H26→C10H22+ ?\text{C}_{12}\text{H}_{26} \rightarrow \text{C}_{10}\text{H}_{22} + \ ?C12H26→C10H22+ ?
