AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Cracking can be done by various methods including catalytic cracking and…
Full specification wording
Cracking and alkenes: Cracking can be done by various methods including catalytic cracking and steam cracking.
Cracking is thermal decomposition of large, less useful hydrocarbon molecules into smaller, more useful ones.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Cracking is thermal decomposition of large, less useful hydrocarbon molecules into smaller, more useful ones. Because long-chain fractions from fractional distillation are in low demand, cracking converts them into shorter alkanes and alkenes. Two named industrial methods are catalytic cracking, which passes vaporised hydrocarbons over a hot catalyst, and steam cracking, which mixes them with steam at high temperature. For example, decane can be cracked into octane and ethene. Both methods break C–C and C–H bonds using energy, so they are endothermic overall. The choice of method depends on the feedstock and the product mixture wanted. Understanding both routes lets you explain why refineries can match supply of fuels and alkenes to demand.
What earns the marks
- Cracking is the breakdown of large hydrocarbon molecules into smaller molecules, usually by heating.
- Catalytic cracking uses a hot catalyst, often a zeolite, to speed up the breakdown of vaporised long-chain hydrocarbons.
- Steam cracking uses high temperature with steam, without a catalyst, to crack hydrocarbons.
- Cracking is economically important because it converts surplus long-chain fractions into shorter fuels and reactive alkenes for polymers.
Common mistakes and how to put them right
Thinking cracking is a separation process like fractional distillation.
Put it right by stressing that new smaller molecules are made by breaking bonds.
Believing cracking needs no energy input.
Put it right by noting that bond breaking makes the process endothermic overall.
Assuming only one product forms.
Put it right by stating that a mixture of alkanes and alkenes is produced.
Exam tips
- Name both methods and give one condition for each, such as a hot catalyst for catalytic cracking and steam with high temperature for steam cracking.
- Use the word hydrocarbon accurately and refer to large molecules being broken into smaller ones.
- Link cracking to supply and demand: long-chain fractions are less useful, short chains and alkenes are more useful.
Try the lesson
The start of the Cracking and alkenes: Cracking can be done by various methods including catalytic cracking and steam cracking. lesson, made from checked teaching notes for this specification point.
Have a go first
Which two methods are used to crack large hydrocarbon molecules?
Listen
Cracking is thermal decomposition of large, less useful hydrocarbon molecules into smaller, more useful ones. Because long-chain fractions from fractional distillation are in low demand, cracking converts them into shorter alkanes and alkenes. Two named industrial methods are catalytic cracking, which passes vaporised hydrocarbons over a hot catalyst, and steam cracking, which mixes them with steam at high temperature. For example, decane can be cracked into octane and ethene. Both methods break C–C and C–H bonds using energy, so they are endothermic overall. The choice of method depends on the feedstock and the product mixture wanted. Understanding both routes lets you explain why refineries can match supply of fuels and alkenes to demand. A strong answer earns marks for these points. Cracking is the breakdown of large hydrocarbon molecules into smaller molecules, usually by heating. Catalytic cracking uses a hot catalyst, often a zeolite, to speed up the breakdown of vaporised long-chain hydrocarbons. Steam cracking uses high temperature with steam, without a catalyst, to crack hydrocarbons. Cracking is economically important because it converts surplus long-chain fractions into shorter fuels and reactive alkenes for polymers. A common slip: Thinking cracking is a separation process like fractional distillation. Instead, stressing that new smaller molecules are made by breaking bonds. Exam tip: Name both methods and give one condition for each, such as a hot catalyst for catalytic cracking and steam with high temperature for steam cracking.
See it
Key words
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Cracking
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Catalytic cracking
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Steam cracking
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See every definition
- Cracking
- Thermal decomposition of large, less useful hydrocarbon molecules into smaller, more useful ones.
- Catalytic cracking
- A method that passes vaporised hydrocarbons over a hot catalyst.
- Steam cracking
- A method that mixes hydrocarbons with steam at high temperature.
Label the diagram
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1Box 1
2Box 2
3Box 3
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