AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Biodiesel is a mixture of methyl esters of long-chain carboxylic acids.
Full specification wording
Carboxylic acids and esters (A-level only): Biodiesel is a mixture of methyl esters of long-chain carboxylic acids.
Biodiesel molecules have the form RCOOCH₃, where R is a long hydrocarbon chain of typically 15 to 17 carbons and the methyl group sits on the single-bonded oxygen.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 7 of 8
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Biodiesel molecules have the form RCOOCH₃, where R is a long hydrocarbon chain of typically 15 to 17 carbons and the methyl group sits on the single-bonded oxygen. It is a mixture rather than a single compound because the vegetable oil it came from contains several different fatty acid chains, of different lengths and with different numbers of C=C bonds, and each one becomes its own methyl ester. That mixture burns in a diesel engine much like the hydrocarbon fuel it replaces. Biodiesel is often called carbon neutral because the carbon dioxide released on burning was taken from the air by the crop as it grew, but the claim is only partly true: fuel is burned to farm, harvest, transport and process the oil, and land is taken out of food production.
What earns the marks
- The structure written as RCOOCH₃ with the methyl group on the single-bonded oxygen.
- Explaining the mixture as the several different chains present in the parent oil.
- The carbon dioxide released having been absorbed by the growing crop.
- A stated limit on carbon neutrality, such as fuel used in farming, processing or transport.
Common mistakes and how to put them right
Drawing an ethyl ester when the specification names methyl esters.
Saying biodiesel is glycerol, which is the by-product and not the fuel.
Claiming biodiesel is completely carbon neutral with no qualification.
Describing biodiesel as a hydrocarbon, when it contains two oxygen atoms per molecule.
Exam tips
- If asked why biodiesel is a mixture, answer about the parent oil, not about impurities from the process.
- A carbon-neutral argument needs both halves: the carbon dioxide taken in and the carbon dioxide put out.
Try the lesson
The start of the Carboxylic acids and esters (A-level only): Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of compound is present in biodiesel?
Listen
Biodiesel molecules have the form RCOOCH₃, where R is a long hydrocarbon chain of typically 15 to 17 carbons and the methyl group sits on the single-bonded oxygen. It is a mixture rather than a single compound because the vegetable oil it came from contains several different fatty acid chains, of different lengths and with different numbers of C=C bonds, and each one becomes its own methyl ester. That mixture burns in a diesel engine much like the hydrocarbon fuel it replaces. Biodiesel is often called carbon neutral because the carbon dioxide released on burning was taken from the air by the crop as it grew, but the claim is only partly true: fuel is burned to farm, harvest, transport and process the oil, and land is taken out of food production. A strong answer earns marks for these points. the structure written as RCOOCH₃ with the methyl group on the single-bonded oxygen. explaining the mixture as the several different chains present in the parent oil. the carbon dioxide released having been absorbed by the growing crop. a stated limit on carbon neutrality, such as fuel used in farming, processing or transport. Exam tip: If asked why biodiesel is a mixture, answer about the parent oil, not about impurities from the process.
Key words
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methyl esters
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long-chain carboxylic acids
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See every definition
- methyl esters
- Compounds of the form RCOOCH₃, where the methyl group sits on the single-bonded oxygen.
- long-chain carboxylic acids
- Acids whose R group is a long hydrocarbon chain of typically 15 to 17 carbons.
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