Structure and formulae of alkenes (chemistry only)

Practise 20 exam-style questions on Structure and formulae of alkenes (chemistry only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

• 20 questions
• 1–6 marks each
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Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 3

3 marks

The diagram shows the displayed formulae of four compounds, A, B, C and D.

Which two compounds are alkenes? Give a reason for your answer.

Model answer

B and C are alkenes, because they contain only carbon and hydrogen and each has a carbon-carbon double bond. Their formulae, C₂H₄ and C₄H₈, fit CₙH₂ₙ.

Explanation

A has only single bonds, so it is an alkane (ethane). D contains an oxygen atom (an -OH group), so it is not a hydrocarbon and cannot be an alkene.

Mark scheme

1. 1 mark

   B

   if more than two letters are given, deduct 1 mark for each incorrect one

2. 1 mark

   C

3. 1 mark

   they are hydrocarbons with a carbon-carbon double bond

   allow their formulae (C₂H₄ and C₄H₈) fit CₙH₂ₙ

Question 4

4 marks

The table shows information about the first four alkenes.
Relative atomic masses (Ar): H = 1, C = 12

Determine X, Y and Z.

Which of these alkenes are gases at 20 °C?

Model answer

X: propene has 3 carbon atoms, so using CₙH₂ₙ it is C₃H₆.
Y: Mr of C₄H₈ = (4 × 12) + (8 × 1) = 56
Z: C₅H₁₀ is pentene.
Ethene, propene and butene have boiling points below 20 °C, so they are gases at 20 °C. Pentene boils at 30 °C, so it is a liquid at 20 °C.

Explanation

Use the general formula CₙH₂ₙ to find formulae and add up relative atomic masses to find Mr. A substance is a gas at a temperature above its boiling point. Like the alkanes, the boiling points of alkenes increase as the molecules get bigger.

Mark scheme

1. 1 mark

   X = C₃H₆

   allow C3H6; correct answer with no working scores full marks

2. 1 mark

   Y = 56

3. 1 mark

   Z = pentene

4. 1 mark

   ethene, propene and butene

   all three needed and no others