But-1-ene and but-2-ene are both members of the alkene homologous series and have the same molecular formula, C₄H₈. State the term used to describe this relationship between but-1-ene and but-2-ene, and explain why both compounds belong to the same homologous series despite having different arrangements of atoms.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.4 Alkenes · State and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

But-1-ene has the structural formula CH₂=CHCH₂CH₃. But-2-ene has the structural formula CH₃CH=CHCH₃. Both have the molecular formula C₄H₈.

Model answer (4 marks)

Isomers
Both are alkenes, so they contain the same functional group – a carbon–carbon double bond. They satisfy the general formula for alkenes, CₙH₂ₙ (here C₄H₈). Because they have the same molecular formula and the same functional group, they belong to the same homologous series and therefore share similar chemical properties.

Examiner tips

  • State the word "isomers" first to gain the 1 mark for the term.
  • Show that both have a C=C bond and satisfy CₙH₂ₙ to earn the 2 marks for explanation.
  • Mention that belonging to the same series gives similar properties for the final mark.

Common mistakes

  • Using "congeners" instead of "isomers".
  • Forgetting to note the C=C functional group or the CₙH₂ₙ formula.
  • Claiming they are identical rather than isomers, which loses the explanation marks.

Mark scheme (4 marks)

  1. The term isomers / isomerism is correctly identified
  2. Both compounds have the same functional group (C=C double bond / carbon-carbon double bond)
  3. Members of a homologous series have similar chemical properties
  4. Both compounds fit / satisfy the general formula CₙH₂ₙ for alkenes

Key terms in this question

homologous series

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