AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Cracking and alkenes: Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules.
Cracking is a thermal decomposition reaction in which large hydrocarbon molecules are broken down into smaller, more useful molecules.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Cracking is a thermal decomposition reaction in which large hydrocarbon molecules are broken down into smaller, more useful molecules. It is used because long-chain hydrocarbons from crude oil are less useful as fuels, while short-chain alkanes are in high demand and alkenes are needed to make polymers. Cracking can be done by heating the hydrocarbon with a catalyst (catalytic cracking) or by heating it to a high temperature with steam (steam cracking). The products include smaller alkanes and alkenes. For example, decane can crack to produce octane and ethene. Alkenes are unsaturated hydrocarbons with a C=C double bond and are more reactive than alkanes, so they are useful for making polymers such as poly(ethene).
What earns the marks
- Cracking is a thermal decomposition reaction: large hydrocarbon molecules are broken down into smaller molecules by heating.
- Cracking produces smaller alkanes and alkenes; alkenes are unsaturated and contain a carbon-to-carbon double bond, C=C.
- Cracking is important because it converts less useful long-chain hydrocarbons into shorter-chain alkanes that are better fuels and alkenes that are used to make polymers.
- Catalytic cracking uses a catalyst and heat, while steam cracking uses high temperature and steam; both break large molecules into smaller ones.
- A typical example is decane cracking to form octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄, showing that the products are smaller molecules.
Common mistakes and how to put them right
Saying cracking is the same as combustion.
Put it right: Cracking is thermal decomposition without oxygen, whereas combustion reacts a fuel with oxygen.
Writing that cracking produces only alkenes.
Put it right: Cracking produces a mixture of smaller alkanes and alkenes.
Confusing saturated and unsaturated hydrocarbons.
Put it right: Alkanes are saturated with single C–C bonds; alkenes are unsaturated and contain a C=C double bond.
Exam tips
- When explaining why cracking is used, refer to supply and demand: long-chain hydrocarbons are in less demand, while short-chain fuels and alkenes for polymers are in higher demand.
- If asked to name products, check whether the question wants an alkane, an alkene or both; give the correct molecular formula where possible.
- Use the term 'thermal decomposition' and state that no oxygen is needed, to distinguish cracking from burning.
Try the lesson
The start of the Cracking and alkenes: Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these is the main reason why long-chain hydrocarbon molecules from crude oil are cracked?
Listen
Cracking is a thermal decomposition reaction in which large hydrocarbon molecules are broken down into smaller, more useful molecules. It is used because long-chain hydrocarbons from crude oil are less useful as fuels, while short-chain alkanes are in high demand and alkenes are needed to make polymers. Cracking can be done by heating the hydrocarbon with a catalyst (catalytic cracking) or by heating it to a high temperature with steam (steam cracking). The products include smaller alkanes and alkenes. For example, decane can crack to produce octane and ethene. Alkenes are unsaturated hydrocarbons with a C=C double bond and are more reactive than alkanes, so they are useful for making polymers such as poly(ethene). A strong answer earns marks for these points. Cracking is a thermal decomposition reaction: large hydrocarbon molecules are broken down into smaller molecules by heating. Cracking produces smaller alkanes and alkenes; alkenes are unsaturated and contain a carbon-to-carbon double bond, C=C. Cracking is important because it converts less useful long-chain hydrocarbons into shorter-chain alkanes that are better fuels and alkenes that are used to make polymers. Catalytic cracking uses a catalyst and heat, while steam cracking uses high temperature and steam; both break large molecules into smaller ones. A common slip: saying cracking is the same as combustion. Instead, cracking is thermal decomposition without oxygen, whereas combustion reacts a fuel with oxygen. Exam tip: When explaining why cracking is used, refer to supply and demand: long-chain hydrocarbons are in less demand, while short-chain fuels and alkenes for polymers are in higher demand.
See it
Key words
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Cracking
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Alkenes
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Catalytic cracking
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Steam cracking
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See every definition
- Cracking
- A thermal decomposition reaction in which large hydrocarbon molecules are broken down into smaller, more useful molecules by heating.
- Alkenes
- Unsaturated hydrocarbons with a C=C double bond that are more reactive than alkanes, so they are useful for making polymers.
- Catalytic cracking
- Cracking done by heating the hydrocarbon with a catalyst.
- Steam cracking
- Cracking done by heating the hydrocarbon to a high temperature with steam.
Label the diagram
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