Propene (C₃H₆) is an alkene produced during the cracking of long-chain hydrocarbons. Explain why propene is described as unsaturated and outline how bromine water can be used to distinguish propene from propane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propene (C₃H₆) is a short-chain alkene obtained from the cracking of larger hydrocarbon molecules found in crude oil. It is an important feedstock for the chemical industry.
Model answer (5 marks)
Propene is unsaturated because it contains a carbon–carbon double bond (C=C), meaning it does not have the maximum number of hydrogen atoms that a saturated hydrocarbon of the same carbon number would have.
To distinguish it from propane, add a few drops of bromine water to a sample of each gas.
• In propene the double bond reacts with Br₂ in an addition reaction, forming 1,2‑dibromopropane and the bromine water loses its orange colour.
• In propane there is no double bond; it is saturated and does not react with Br₂, so the bromine water remains orange.
Thus the decolourisation of bromine water indicates the presence of an alkene such as propene.
To distinguish it from propane, add a few drops of bromine water to a sample of each gas.
• In propene the double bond reacts with Br₂ in an addition reaction, forming 1,2‑dibromopropane and the bromine water loses its orange colour.
• In propane there is no double bond; it is saturated and does not react with Br₂, so the bromine water remains orange.
Thus the decolourisation of bromine water indicates the presence of an alkene such as propene.
Examiner tips
- Use the term ‘unsaturated’ and mention the C=C bond; link to lack of maximum H atoms. Show the bromine water test step‑by‑step, emphasising colour change. Keep the answer concise and use correct chemical names.
- common_mistakes
- :
- Calling propene ‘saturated’ or forgetting the double bond. Saying bromine water turns colourless for both gases. Not explaining that the colour change is due to an addition reaction.
Mark scheme (5 marks)
- Propene is unsaturated because it contains a carbon-to-carbon double bond (C=C)
- Propene does not contain the maximum possible number of hydrogen atoms (it is not saturated / not fully hydrogenated)
- Add bromine water to each compound
- Propene decolourises (turns from orange/brown to colourless) the bromine water
- Propane does not decolourise the bromine water (it remains orange/brown) because propane is saturated and does not undergo addition reactions
Key terms in this question
unsaturated · bromine water · alkene · saturated
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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