Alkanes
Revise Alkanes for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Alkanes are saturated hydrocarbons with only single bonds; the first four are methane, ethane, propane and butane.
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Topic overview
Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds. Saturated means the molecule contains the maximum possible number of hydrogen atoms.
The general formula is CₙH₂ₙ₊₂, and the first four members are methane, ethane, propane and butane. They form a homologous series, so they have similar chemical properties that change gradually along the series.
Properties depend on the length of the molecule. As the chain gets longer, the boiling point increases, the viscosity increases so the liquid flows less easily, and the flammability decreases so it is harder to ignite.
Revision notes
Saturated hydrocarbons
Alkanes contain only single carbon-carbon bonds, so they are described as saturated.
Saturated means the molecule holds the maximum possible number of hydrogen atoms — no more can be added without breaking a bond.
The homologous series
General formula CₙH₂ₙ₊₂. Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
Members of a homologous series have the same general formula, differ by CH₂ from the next member, and have similar chemical properties.
Trends with chain length
As the molecules get longer: boiling point increases, because intermolecular forces are stronger.
Viscosity increases, so the liquid flows less easily. Flammability decreases, so it is harder to ignite. These three trends are examined together.
Key points
- Alkanes are saturated hydrocarbons.
- Saturated means only single carbon-carbon bonds.
- The general formula is CₙH₂ₙ₊₂.
- Longer chains have higher boiling points.
- Longer chains are more viscous.
- Longer chains are less flammable.
Worked examples
Example 1
Explain what is meant by a saturated hydrocarbon. [2 marks]
Model answer
It contains only single carbon-carbon bonds (1 mark), so the molecule holds the maximum possible number of hydrogen atoms (1 mark).
Example 2
Describe how the properties of alkanes change as the chain length increases. [3 marks]
Model answer
The boiling point increases (1 mark). The viscosity increases, so the liquid flows less easily (1 mark). The flammability decreases, so the alkane is harder to ignite (1 mark).
Example 3
Explain why the boiling point increases as alkane molecules get longer. [2 marks]
Model answer
Longer molecules have stronger intermolecular forces between them (1 mark), so more energy is needed to separate them and the boiling point is higher (1 mark).
Common mistakes
Saying saturated means containing water.
It means only single carbon-carbon bonds, with the maximum hydrogen atoms.
Getting the flammability trend backwards.
Longer chains are LESS flammable.
Explaining boiling point through covalent bonds.
It is the intermolecular forces between molecules that increase.
Forgetting the +2 in the general formula.
CₙH₂ₙ₊₂ is the alkane formula; CₙH₂ₙ is for alkenes.
Exam tips
- Define saturated as only single carbon-carbon bonds.
- Learn all three trends with chain length together.
- Explain boiling point through intermolecular forces.
- Keep the alkane and alkene general formulae distinct.
Key terms
- Saturated
- Containing only single carbon-carbon bonds.
- Alkane
- A saturated hydrocarbon with formula CₙH₂ₙ₊₂.
- Viscosity
- How easily a liquid flows.
- Flammability
- How easily a substance ignites.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.