AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
There is a high demand for fuels with small molecules and so some of the…
Full specification wording
Cracking and alkenes: There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels.
Crude oil fractions contain many large hydrocarbon molecules that are not ideal as fuels because they are viscous, hard to ignite and burn less cleanly.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 7 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crude oil fractions contain many large hydrocarbon molecules that are not ideal as fuels because they are viscous, hard to ignite and burn less cleanly. Cracking breaks these large molecules into smaller ones, some of which are alkanes with small molecules such as methane, ethane, propane and butane. These small alkanes ignite easily, burn with a clean flame and can be stored and transported as gases or volatile liquids, so they are in high demand as fuels for homes, cooking, heating and vehicles. The process also produces alkenes, which are valuable as chemical feedstocks. Demand for small-molecule fuels therefore drives cracking, and the products are separated and used according to their properties.
What earns the marks
- Large hydrocarbon molecules in crude oil are less useful as fuels because they are viscous and difficult to ignite.
- Cracking breaks large molecules into smaller molecules.
- Small-molecule alkanes such as methane, ethane, propane and butane burn readily and are useful fuels.
- There is high demand for fuels with small molecules for heating, cooking and transport.
- Cracking produces a mixture of smaller alkanes and alkenes, and the alkanes can be used as fuels.
- The products are separated before use, so useful fuels are obtained from the mixture.
Common mistakes and how to put them right
Thinking all cracking products are alkenes.
Put it right: Cracking produces both smaller alkanes and alkenes.
Assuming large molecules are in higher demand as fuels.
Put it right: Small molecules are in higher demand because they ignite and burn more easily.
Confusing cracking with fractional distillation.
Put it right: Cracking is a chemical breakdown, while fractional distillation separates without chemical change.
Exam tips
- Name at least two small alkanes to show the products are useful fuels.
- Contrast the properties of large and small molecules to justify the demand.
- Mention that cracking gives a mixture, so separation is needed before the fuels are used.
Try the lesson
The start of the Cracking and alkenes: There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels. lesson, made from checked teaching notes for this specification point.
Have a go first
Why is there a high demand for fuels made of small molecules?
Listen
Crude oil fractions contain many large hydrocarbon molecules that are not ideal as fuels because they are viscous, hard to ignite and burn less cleanly. Cracking breaks these large molecules into smaller ones, some of which are alkanes with small molecules such as methane, ethane, propane and butane. These small alkanes ignite easily, burn with a clean flame and can be stored and transported as gases or volatile liquids, so they are in high demand as fuels for homes, cooking, heating and vehicles. The process also produces alkenes, which are valuable as chemical feedstocks. Demand for small-molecule fuels therefore drives cracking, and the products are separated and used according to their properties. A strong answer earns marks for these points. Large hydrocarbon molecules in crude oil are less useful as fuels because they are viscous and difficult to ignite. Cracking breaks large molecules into smaller molecules. Small-molecule alkanes such as methane, ethane, propane and butane burn readily and are useful fuels. There is high demand for fuels with small molecules for heating, cooking and transport. A common slip: Thinking all cracking products are alkenes. Instead, cracking produces both smaller alkanes and alkenes. Exam tip: Name at least two small alkanes to show the products are useful fuels.
Key words
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Cracking
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Small-molecule alkanes
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Large hydrocarbon molecules
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Demand for small-molecule fuels
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See every definition
- Cracking
- A chemical process that breaks large hydrocarbon molecules into smaller molecules, producing a mixture of smaller alkanes and alkenes.
- Small-molecule alkanes
- Alkanes such as methane, ethane, propane and butane that ignite easily, burn with a clean flame and are in high demand as fuels.
- Large hydrocarbon molecules
- Molecules in crude oil fractions that are less useful as fuels because they are viscous, hard to ignite and burn less cleanly.
- Demand for small-molecule fuels
- High demand for fuels with small molecules for heating, cooking and transport, which drives cracking.
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