Specification: AQA GCSE Chemistry 4.7.1.2 – Fractional Distillation and Petrochemicals
Suitable for: GCSE Combined Science and Separate Chemistry (Foundation and Higher)
Learning objectives
Students should be able to:
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Describe how fractional distillation separates crude oil into fractions.
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Explain the process in terms of evaporation, condensation and boiling points.
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Explain why fractions contain hydrocarbons with similar numbers of carbon atoms.
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Identify important fuels and products made from crude oil.
Suggested teaching sequence
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Inspect crude oil: Begin by examining the mixture of different hydrocarbon molecules. Establish that crude oil is a mixture rather than a single substance.
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Run the column: Use the animated fractionating column to introduce the process. Switch to Step through to follow one batch through heating, evaporation, rising vapours, condensation and collection. Pause between stages for questioning.
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Inspect fractions: Select different outlets and examine the molecules collected. Compare carbon chain lengths and reinforce that each fraction contains a mixture of similar hydrocarbons.
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Molecule journey: Use Track one and Compare two to investigate why hydrocarbons with different boiling points condense at different heights. Use Molecules together to revisit changes of state and the role of intermolecular forces.
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Fuels and feedstock: Explore how separated fractions provide fuels and raw materials for the petrochemical industry, including polymers, solvents, lubricants and detergents.
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Check understanding: Use the explanation-building activity and diagnostic questions to assess students' understanding.
Key teaching points
Emphasise that the fractionating column is hot at the bottom and cooler at the top. Hydrocarbons with higher boiling points condense lower down, while those with lower boiling points travel further up before condensing.
Reinforce that fractional distillation is a physical separation process. Hydrocarbon molecules do not break apart or undergo chemical reactions. During condensation, intermolecular attractions bring molecules closer together, but the covalent bonds within molecules remain unchanged.
Common misconceptions
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Each fraction contains only one hydrocarbon.
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All hydrocarbons evaporate and condense at the same temperature.
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Larger molecules rise further up the column.
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Covalent bonds break when hydrocarbons evaporate.
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Fractional distillation produces new substances rather than separating existing ones.
Classroom tip: Use Projector View and Focus Mode for whole-class demonstrations. Hide explanations initially, ask students to predict what will happen, then reveal the animations and explanations to test their ideas.