Cracking and alkenes: Students should be able to give examples to illustrate the usefulness of cracking. They should also be able to explain how modern life depends on the uses of hydrocarbons.

The idea

Cracking turns long-chain hydrocarbons from crude oil into shorter, more useful molecules, including alkenes. For example, decane can be cracked into octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. The shorter alkanes make fuels such as petrol, which burn more cleanly and are in high demand, while alkenes are reactive starting materials for polymers such as poly(ethene) and for other chemicals. Modern life depends on hydrocarbons because fuels power transport, heating and electricity generation, and petrochemical feedstocks provide plastics, medicines, detergents, lubricants, solvents and synthetic fibres. Without cracking, supplies of short-chain fuels and alkenes would be limited, so petrol, plastics and many everyday products would be scarcer and more expensive.

What earns the marks

• Cracking converts long-chain hydrocarbon molecules into shorter-chain molecules, which are more useful as fuels.
• Cracking produces alkenes, which are reactive and can be used to make polymers such as poly(ethene).
• A valid example is cracking decane to give octane and ethene, showing a shorter alkane fuel and an alkene feedstock.
• Shorter-chain hydrocarbons are in greater demand as fuels because they ignite and burn more easily than long-chain hydrocarbons.
• Modern life depends on hydrocarbons as fuels for transport, heating and electricity generation.
• Hydrocarbon feedstocks are used to make plastics, medicines, detergents, lubricants, solvents and synthetic fibres.
• A consequence of limited cracking would be reduced supplies of petrol and petrochemical products, raising costs.

Common mistakes and how to put them right

• Thinking cracking joins small molecules together.

  Put it right: Cracking breaks long-chain hydrocarbons into shorter, more useful molecules.

• Stating that cracking only makes fuels.

  Put it right: It also makes alkenes, which are used to make polymers and other chemicals.

• Claiming all crude oil fractions are equally useful.

  Put it right: Long-chain fractions are less in demand, so cracking converts them into shorter-chain products that are more useful.

Exam tips

• Link each example of cracking to a named useful product, such as petrol or poly(ethene), rather than describing cracking in general terms.
• When explaining dependence on hydrocarbons, cover both fuels and feedstocks, giving at least one everyday product for each category.
• Use a balanced symbol equation or a labelled flow diagram to show a cracking example clearly and accurately.