AQA A-Level Chemistry Paper 2, 2021: Question 4

9 marks · Medium difficulty · State/Explain/Numerical

Benzene Structure and Electrophilic Substitution This question explores the thermodynamic stability of benzene through enthalpy calculations and diagrams, comparing the actual delocalised structure to Kekulé's model. It involves calculating enthalpy changes using bond enthalpies and interpreting why benzene is more stable due to electron delocalisation. The latter part focuses on naming and completing the electrophilic substitution mechanism for the nitration of benzene, showing all necessary curly arrows and lone pairs across multiple steps.

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Question

AQA A-Level Chemistry Paper 2, 2021: Question 4
Question text

04 Kekulé suggested this structure for benzene.

Benzene is now represented by this structure.

.

Figure 3 shows the relative stability of compared to

Figure 3

04.1 Use Figure 3 and the data shown in Table 1 to calculate ΔH2

[3 marks]

Table 1

ΔH / kJ mol–1

Enthalpy of atomisation for carbon +715

Enthalpy of atomisation for hydrogen +218

Bond enthalpy (C–C) +348

Bond enthalpy (C=C) +612

Bond enthalpy (C–H) +412

ΔH kJ mol–1

04.2 Explain, in terms of structure and bonding, why

is more thermodynamically stable than

[1 mark]

04.3 A mixture of concentrated nitric acid and concentrated sulfuric acid reacts with

benzene.

*13* Figure 4 shows the incomplete mechanism for this reaction.

Name the mechanism.

Complete the mechanism in Figure 4 by adding

• any lone pairs of electrons involved in each step

• two curly arrows in step 1

• a curly arrow in step 2

• a curly arrow in step 3

• a curly arrow in step 4.

[5 marks]

Name of mechanism

Figure 4

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2021: Question 4

Question Answers Additional Comments/Guidelines Mark

(3 x 612) + (3 x 348) + (6 x 412) = 5352 For LHS M1

04.1 (6 x 715) + (6 x 218) = 5598 For RHS M2

∆H = M2 - M1 - 83 = +163 kJ mol–1 M3

04.2 (𝜋) electrons delocalised 1

– A-LEVEL CHEMISTRY – –

M1 Electrophilic substitution 1

04.3

How to answer it

Study Guide: Benzene Stability and Nitration Mechanism

Q1. Calculating Enthalpy Change (ΔH2)

Step-by-step:

  • LHS (Kekulé): (3 × 612) + (3 × 348) + (6 × 412) = 5352 kJ mol⁻¹
  • RHS (atoms): (6 × 715) + (6 × 218) = 5598 kJ mol⁻¹
  • ΔH2 = RHS – LHS – ΔH1: 5598 – 5352 – 83 = +163 kJ mol⁻¹
Remember: Use 6 × 218 for H atoms — many students wrongly used 3 × 218!
Don’t forget to subtract ΔH1 at the end — it’s crucial for the final value.

Q2. Why Benzene Is More Stable Than Kekulé's Model

Answer: Benzene has delocalised π electrons spread across all six carbon atoms in a ring.

This delocalisation lowers the overall energy, making benzene more thermodynamically stable than the Kekulé structure with alternating single and double bonds.

Just saying “resonance” is not enough — describe what’s actually happening with electrons!

Q3. Nitration of Benzene: Mechanism

Name of mechanism: Electrophilic substitution

Steps to complete:

  1. Step 1: Lone pair from HSO4– to H on HNO3 → Curly arrow from O–H bond to O → Generates NO2+
  2. Step 2: Curly arrow from bond to O in HNO3
  3. Step 3: Curly arrow from benzene ring to the nitrogen of NO2+
  4. Step 4: Curly arrow from C–H bond back into the ring to restore aromaticity, releasing H+
Each curly arrow shows movement of electrons. Always draw from a bond or lone pair towards a positively charged species.
Don’t skip any curly arrows — this question was a good discriminator because it tested full mechanistic understanding.

Topics

Organic Chemistry · Physical Chemistry · 3.3.10 Aromatic Chemistry · 3.1.4 Energetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.