OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify which statements support the delocalised model for the structure of benzene from a given list of three statements.

Practise this question

Question

Multiple-choice question 13 asks which statements support the delocalised model for the structure of benzene. Three numbered statements are given: 1. All carbon-carbon bonds have the same length. 2. The enthalpy change of hydrogenation of benzene is less exothermic than expected. 3. Bromine reacts with benzene less readily than with cyclohexene. Four options A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), and D (Only 1) are provided with an answer box.
Question text

13 Which statement(s) support(s) the delocalised model for the structure of benzene?

1 All carbon–carbon bonds have the same length.

2 The enthalpy change of hydrogenation of benzene is less exothermic than expected.

3 Bromine reacts with benzene less readily than with cyclohexene.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 13 with the correct answer option A.

13 A 1

How to answer it

Evidence for the Delocalised Model of Benzene

OCR A-Level Chemistry — Multiple Choice Question

What this question tests

This question assesses your understanding of the physical and chemical evidence used to compare the Kekulé ( localised pi-bond ) model of benzene with the modern delocalised model. You must know how bond lengths, hydrogenation enthalpy values, and reactivity towards electrophiles differ between alkenes and benzene.

Question 13 Analysis

Evaluating Statements Supporting Delocalisation

✅ Correct Answer: A (1, 2 and 3)

All three statements provide valid experimental evidence supporting the delocalised model over the outdated Kekulé cyclohexatriene structure.

Mark Awarded: 1 / 1 for selecting option A.

💡 Key Knowledge

  • Statement 1: X-ray diffraction shows all C–C bond lengths in benzene are identical (approx. 0.140 nm), intermediate between single (0.154 nm) and double (0.134 nm) bonds.
  • Statement 2: Hydrogenation of benzene is less exothermic by about 152 kJ mol⁻¹ than expected for three isolated double bonds, showing benzene is thermodynamically more stable.
  • Statement 3: Benzene reacts via electrophilic substitution rather than electrophilic addition, requiring a halogen carrier (e.g., FeBr₃), which proves its delocalised ring is stable and resists disruption.

🧠 Exam Technique

In multi-statement questions (1, 2, 3), evaluate each statement independently first. Once you confirm statement 1 is true (bond lengths are equal, unlike Kekulé's alternating bonds), you can immediately eliminate options C. Then, validating statement 2 narrows down your choices further, making efficient use of your time.

❌ Common Errors

  • Assuming benzene readily decolorises bromine water at room temperature like alkenes (cyclohexene) do.
  • Confusing "less exothermic" with "endothermic" when analysing hydrogenation enthalpy data.
  • Failing to recognise that delocalisation of electrons creates a symmetrical electron cloud where all carbon-carbon bond lengths are averaged out.

Topics

Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.