AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Biodiesel is produced by reacting vegetable oils with methanol in the…
Full specification wording
Carboxylic acids and esters (A-level only): Biodiesel is produced by reacting vegetable oils with methanol in the presence of a catalyst.
Biodiesel is made by transesterification: the triglyceride is warmed with a large excess of methanol and a catalyst, usually sodium hydroxide or potassium hydroxide, though a strong acid also works.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 8 of 8
Study notes
From the checked teaching notes for this specification point.
The idea
Biodiesel is made by transesterification: the triglyceride is warmed with a large excess of methanol and a catalyst, usually sodium hydroxide or potassium hydroxide, though a strong acid also works. Methanol replaces the glycerol at each of the three ester links, so one mole of triglyceride needs three moles of methanol and gives three moles of methyl ester plus one mole of glycerol. The reaction is reversible, which is why the methanol is in excess: it shifts the position of equilibrium towards the esters. The glycerol is denser and separates as a lower layer, so it is run off and sold, and removing it also pulls the equilibrium forwards. The catalyst is recovered rather than consumed.
What earns the marks
- Three moles of methanol reacting per mole of triglyceride.
- Glycerol identified as the co-product.
- Naming a catalyst such as sodium hydroxide, potassium hydroxide or a strong acid.
- Stating that excess methanol shifts the equilibrium towards the methyl esters.
Common mistakes and how to put them right
Writing a 1:1 equation between the oil and methanol.
Leaving glycerol out of the products.
Saying the catalyst increases the equilibrium yield rather than the rate.
Confusing transesterification with hydrolysis, so giving the carboxylic acid rather than the methyl ester.
Exam tips
- Write the equation with the triglyceride as a block structure and three separate methanol molecules; three arrows of substitution is easier to balance than a full displayed formula.
- If asked why excess methanol is used, answer in equilibrium language, not rate language.
Try the lesson
The start of the Carboxylic acids and esters (A-level only): Biodiesel is produced by reacting vegetable oils with methanol in the presence of a catalyst. lesson, made from checked teaching notes for this specification point.
Have a go first
Which alcohol is reacted with vegetable oils to produce biodiesel?
Listen
Biodiesel is made by transesterification: the triglyceride is warmed with a large excess of methanol and a catalyst, usually sodium hydroxide or potassium hydroxide, though a strong acid also works. Methanol replaces the glycerol at each of the three ester links, so one mole of triglyceride needs three moles of methanol and gives three moles of methyl ester plus one mole of glycerol. The reaction is reversible, which is why the methanol is in excess: it shifts the position of equilibrium towards the esters. The glycerol is denser and separates as a lower layer, so it is run off and sold, and removing it also pulls the equilibrium forwards. The catalyst is recovered rather than consumed. A strong answer earns marks for these points. three moles of methanol reacting per mole of triglyceride. glycerol identified as the co-product. naming a catalyst such as sodium hydroxide, potassium hydroxide or a strong acid. stating that excess methanol shifts the equilibrium towards the methyl esters. Exam tip: Write the equation with the triglyceride as a block structure and three separate methanol molecules; three arrows of substitution is easier to balance than a full displayed formula.
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