Explain why alkanes are largely unreactive but do burn readily in excess oxygen, and state the names of the two products formed when a short-chain alkane undergoes complete combustion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Alkanes are largely unreactive because the C–C and C–H bonds are strong and difficult to break. When they do react, they burn readily in excess oxygen because combustion is exothermic and releases a large amount of energy, making alkanes useful fuels. In complete combustion the two products formed are carbon dioxide and water.
Examiner tips
- Use the exact terms ‘C–C and C–H bonds are strong’, ‘exothermic’, ‘carbon dioxide’ and ‘water’.
- Show the cause–effect link: strong bonds → low reactivity; exothermic combustion → fuel use.
- Mention ‘complete combustion’ to justify the two products.
Common mistakes
- Writing ‘reactive’ instead of ‘unreactive’.
- Omitting the word ‘exothermic’ or the energy aspect.
- Listing only one product or confusing incomplete combustion products.
Mark scheme (4 marks)
- Alkanes are largely unreactive because C–C and C–H bonds are strong / difficult to break
- Combustion is exothermic / releases a large amount of energy, making alkanes useful fuels
- Carbon dioxide is produced in complete combustion
- Water is produced in complete combustion
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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