Explain why alkenes are more reactive than alkanes and describe how ethene reacts with bromine to form a single product.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Alkenes contain a C=C double bond, whereas alkanes have only C–C single bonds. The C=C double bond is the reactive site; one of the π‑bonds can break, allowing other atoms to add across the bond. Ethene reacts with bromine by an addition reaction: Br₂ adds across the C=C double bond. The product is 1,2‑dibromoethane – a single dibromoalkane is formed.
Examiner tips
- Mention the double bond and π‑bond as the reactive site; explain addition of Br₂ across the double bond; state the product as 1,2‑dibromoethane; keep answer concise to fit 4 marks.
Common mistakes
- Confusing alkanes with alkenes; not identifying the double bond as the reactive site; saying the reaction gives two products or a mixture; omitting the term ‘addition’ or the specific product name.
Mark scheme (4 marks)
- Alkenes contain a C=C double bond (whereas alkanes only have C–C single bonds)
- The C=C double bond is the reactive site / one of the bonds in the double bond can break to allow other atoms to add on
- Ethene undergoes addition reaction with bromine / bromine adds across the double bond
- The product formed is 1,2-dibromoethane / a dibromoalkane / only one product is formed (no other products)
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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