Crude oil is a mixture of hydrocarbons. It is separated into fractions by fractional distillation. One fraction contains octane, C₈H₁₈, which is a member of the alkane homologous series. Explain what is meant by the term 'hydrocarbon' and describe the key features that make alkanes a homologous series, including how their properties change as the chain length increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil extracted from beneath the seabed contains hundreds of different hydrocarbon molecules. Refineries separate these molecules to produce fuels and raw materials for the chemical industry.
Model answer (5 marks)
A hydrocarbon is a compound that contains only carbon and hydrogen atoms.
Alkanes are a homologous series because all members have the general formula CₙH₂ₙ₊₂.
Each successive alkane differs from the previous one by the addition of one CH₂ group.
Because of the same general formula, alkanes show very similar chemical properties – they are all relatively unreactive and undergo similar combustion reactions.
As the carbon chain length increases, the boiling point rises, the liquids become more viscous and the compounds become less flammable.
Alkanes are a homologous series because all members have the general formula CₙH₂ₙ₊₂.
Each successive alkane differs from the previous one by the addition of one CH₂ group.
Because of the same general formula, alkanes show very similar chemical properties – they are all relatively unreactive and undergo similar combustion reactions.
As the carbon chain length increases, the boiling point rises, the liquids become more viscous and the compounds become less flammable.
Examiner tips
- Use the exact definition of a hydrocarbon – carbon and hydrogen only. Mention the general formula CₙH₂ₙ₊₂. State the CH₂ increment. Highlight similarity of chemical behaviour. Note the trend: higher boiling point, higher viscosity, lower flammability.
Common mistakes
- Confusing hydrocarbons with other elements present. Forgetting the CH₂ unit difference. Not linking chain length to boiling point/viscosity/flammability.
Mark scheme (5 marks)
- A hydrocarbon contains hydrogen and carbon only / is a compound of hydrogen and carbon only
- Members of a homologous series share the same general formula
- Each successive member differs by one CH₂ unit
- Members of the series have similar chemical properties
- As chain length increases, boiling point increases / they become less flammable / they become more viscous
Key terms in this question
hydrocarbon · homologous series · alkane
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →