AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Cracking and alkenes: Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking.
To describe cracking conditions in general terms, compare the two methods.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To describe cracking conditions in general terms, compare the two methods. Catalytic cracking passes vaporised long-chain hydrocarbons over a hot catalyst, commonly a zeolite, at a high temperature; the catalyst speeds up the breakdown. Steam cracking mixes the hydrocarbon feedstock with steam and heats it strongly, without a catalyst. Both methods use high temperature because breaking covalent C–C and C–H bonds needs energy. The conditions are chosen to produce shorter-chain alkanes and alkenes, such as ethene, which are more useful as fuels and as monomers for polymers. A good answer names the feedstock state, the presence or absence of a catalyst, the role of steam, and the need for strong heating, without inventing exact industrial temperatures or pressures.
What earns the marks
- Catalytic cracking uses a hot catalyst, often a zeolite, and a high temperature.
- In catalytic cracking, long-chain hydrocarbons are vaporised before passing over the catalyst.
- Steam cracking uses steam and a high temperature, without a catalyst.
- The conditions are chosen to produce shorter-chain alkanes and alkenes, including ethene.
- General descriptions should identify the key difference: catalyst present in catalytic cracking, steam present in steam cracking.
Common mistakes and how to put them right
Saying steam cracking uses a catalyst.
Put it right: Steam cracking uses steam and high temperature, not a catalyst.
Forgetting that the feedstock is vaporised in catalytic cracking.
Put it right: The hydrocarbon must be a gas so it can contact the catalyst surface.
Exam tips
- Use a comparison table or two labelled bullet points to contrast catalytic cracking and steam cracking clearly.
- Include the word 'catalyst' only for catalytic cracking and 'steam' only for steam cracking to avoid mixing the methods.
- Link each condition to its purpose, such as high temperature to break bonds or catalyst to speed up the reaction.
Try the lesson
The start of the Cracking and alkenes: Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking. lesson, made from checked teaching notes for this specification point.
Have a go first
Which set of conditions describes catalytic cracking in general terms?
Listen
To describe cracking conditions in general terms, compare the two methods. Catalytic cracking passes vaporised long-chain hydrocarbons over a hot catalyst, commonly a zeolite, at a high temperature; the catalyst speeds up the breakdown. Steam cracking mixes the hydrocarbon feedstock with steam and heats it strongly, without a catalyst. Both methods use high temperature because breaking covalent C–C and C–H bonds needs energy. The conditions are chosen to produce shorter-chain alkanes and alkenes, such as ethene, which are more useful as fuels and as monomers for polymers. A good answer names the feedstock state, the presence or absence of a catalyst, the role of steam, and the need for strong heating, without inventing exact industrial temperatures or pressures. A strong answer earns marks for these points. Catalytic cracking uses a hot catalyst, often a zeolite, and a high temperature. In catalytic cracking, long-chain hydrocarbons are vaporised before passing over the catalyst. Steam cracking uses steam and a high temperature, without a catalyst. The conditions are chosen to produce shorter-chain alkanes and alkenes, including ethene. A common slip: Saying steam cracking uses a catalyst. Instead, steam cracking uses steam and high temperature, not a catalyst. Exam tip: Use a comparison table or two labelled bullet points to contrast catalytic cracking and steam cracking clearly.
See it
Key words
Say what each word means before you turn the card.
Catalytic cracking
Try answering aloud or in your head before turning the card.
Steam cracking
Try answering aloud or in your head before turning the card.
See every definition
- Catalytic cracking
- Passes vaporised long-chain hydrocarbons over a hot catalyst, commonly a zeolite, at a high temperature, and the catalyst speeds up the breakdown.
- Steam cracking
- Mixes the hydrocarbon feedstock with steam and heats it strongly, without a catalyst.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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