Methane, ethane, propane and butane are the first four members of the alkane homologous series. Explain why alkanes are described as 'saturated hydrocarbons' and describe how the boiling points of alkanes change as the number of carbon atoms in the molecule increases, giving a reason for this trend.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkanes are widely used as fuels. Methane (CH₄) is the main component of natural gas, ethane (C₂H₆) is found in LPG alongside propane (C₃H₈), and butane (C₄H₁₀) is used in portable camping stoves.
Model answer (5 marks)
Alkanes are saturated hydrocarbons because they contain only single C–C bonds and no C=C double bonds, and they consist solely of carbon and hydrogen atoms.
As the carbon number increases, the boiling point rises. Longer chains have larger surface areas, which increases London dispersion forces (van der Waals forces) between molecules. Stronger intermolecular forces require more energy to overcome, so the boiling point is higher.
As the carbon number increases, the boiling point rises. Longer chains have larger surface areas, which increases London dispersion forces (van der Waals forces) between molecules. Stronger intermolecular forces require more energy to overcome, so the boiling point is higher.
Examiner tips
- Use the word "saturated" to explain absence of double bonds; mention only C and H. Show the trend: more C → higher BP. Explain the cause: larger surface area → stronger London forces. Keep answer concise and use exam terminology.
Common mistakes
- Confusing saturated with unsaturated; forgetting to mention only C and H. Failing to link chain length to increased London dispersion forces. Giving a general statement about boiling points without explaining the underlying intermolecular forces.
Mark scheme (5 marks)
- Alkanes are described as 'saturated' because they contain only single (C–C) bonds / no double bonds between carbon atoms
- Alkanes are hydrocarbons because they contain hydrogen and carbon only / no other elements
- As the number of carbon atoms increases, the boiling point increases / longer chain alkanes have higher boiling points
- Larger molecules have stronger intermolecular forces / greater van der Waals forces between molecules
- More energy is therefore needed to overcome these stronger intermolecular forces, so the boiling point is higher
Key terms in this question
saturated · hydrocarbon · boiling point · homologous series
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Alkanes questions
- Crude oil is a mixture of hydrocarbons, including alkanes. Describe the structur…
- Octane (C₈H₁₈) is an alkane found in petrol. When octane burns in a limited supp…
- Propane and butane are alkanes used as fuels in camping gas canisters. When prop…
- Alkanes are a homologous series of hydrocarbons. Explain what is meant by the te…