Crude oil is a mixture of hydrocarbons, including alkanes. Describe the structure and bonding of alkanes, and explain why alkanes are described as saturated hydrocarbons. In your answer, state one use of alkanes as fuels and explain why alkanes undergo complete combustion in excess oxygen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkanes are obtained from crude oil by fractional distillation. They are widely used as fuels in homes, vehicles and power stations.
Model answer (5 marks)
Alkanes are hydrocarbons that contain only carbon and hydrogen atoms.
They are bonded exclusively by single covalent bonds – C–C and C–H.
Because they have no C=C double bonds, all available valencies of carbon are satisfied; this is why they are called saturated hydrocarbons.
A common use is methane in natural gas for heating and cooking.
In excess oxygen, alkanes undergo complete combustion to give only CO₂ and H₂O, releasing energy.
They are bonded exclusively by single covalent bonds – C–C and C–H.
Because they have no C=C double bonds, all available valencies of carbon are satisfied; this is why they are called saturated hydrocarbons.
A common use is methane in natural gas for heating and cooking.
In excess oxygen, alkanes undergo complete combustion to give only CO₂ and H₂O, releasing energy.
Examiner tips
- Use the exact terms ‘single covalent bonds’ and ‘saturated’ as defined in the mark scheme.
- Give a specific example of a fuel, e.g. methane, butane or octane.
- Explain that complete combustion produces CO₂ and H₂O and releases energy.
- Keep the answer concise – 5 marks allow a short paragraph, not a long essay.
Common mistakes
- Confusing alkanes with alkenes or alkynes; forgetting that alkanes have only single bonds.
- Failing to state that alkanes are saturated because they lack C=C bonds.
- Giving an incorrect fuel example or not linking it to combustion.
Mark scheme (5 marks)
- Alkanes contain only carbon and hydrogen atoms (are hydrocarbons)
- Alkanes contain only single (C–C and C–H) covalent bonds
- Saturated means alkanes contain no carbon–carbon double bonds (C=C)
- Named use of an alkane as a fuel, e.g. methane used in natural gas for heating/cooking, butane in camping gas, or petrol/diesel (octane) in vehicles
- In complete combustion, alkanes react with excess oxygen to produce only carbon dioxide and water (releasing energy)
Key terms in this question
alkane · saturated · hydrocarbon · complete combustion
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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