Benzene test with answers for A Level chemistry (free PDF download)

Quick answer: Benzene, C₆H₆, is a planar ring with six delocalised π electrons and six equal C–C bonds of 0.140 nm. This Chemistry 11-12 test on benzene has 8 questions worth 20 marks, to do in 45 minutes: bond lengths, enthalpy of hydrogenation and nitration of benzene. The mark scheme is on a separate page; free printable PDF.

This test lasts 45 minutes and is worth 20 points. Show all working, write balanced equations and give units. Use A_r values: H 1.0, C 12.0, N 14.0, O 16.0.

Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.10

Time: 45 minutes · 8 questions · Total: 20 marks · Calculator allowed

Contents

Question 1 Bond lengths (2 marks)

Benzene has six identical carbon-carbon bonds of length 0.140 nm. Typical C–C and C=C lengths are 0.154 nm and 0.134 nm. Explain what this shows about the Kekulé structure. (2 marks)

Question 2 Enthalpy of hydrogenation (3 marks)

ΔH(hydrogenation) of cyclohexene is −120 kJ/mol and of benzene is −208 kJ/mol.

Calculate the value predicted for Kekulé benzene. (1 mark)
Calculate the stabilisation energy and explain its meaning. (2 marks)

Question 3 Nitration of benzene (4 marks)

State the reagents and the temperature limit. (2 marks)
Write the equation for nitration. (1 mark)
Explain why the limit is needed. (1 mark)

Question 4 The intermediate (3 marks)

The figure shows the intermediate in the bromination of benzene.

State what the dashed arc and the + sign show. (1 mark)
Describe the step that follows to form bromobenzene. (1 mark)
State the role of the halogen carrier. (1 mark)

Intermediate in an electrophilic substitution of benzene.

Question 5 Substitution, not addition (2 marks)

Explain why benzene undergoes substitution with bromine rather than addition. (2 marks)

Question 6 Friedel–Crafts alkylation (2 marks)

Write the equation for the formation of methylbenzene from benzene and chloromethane, and state the catalyst. (2 marks)

Question 7 Percentage yield (3 marks)

3.90 g of benzene (M = 78.0 g/mol) gives 5.18 g of bromobenzene (M = 157.0 g/mol). Calculate the percentage yield. (3 marks)

Question 8 Using methylbenzene in schools (1 mark)

Suggest why schools use methylbenzene or esters instead of benzene in practicals. (1 mark)

Molecular formula and shape of benzene? — C₆H₆, planar hexagon, 120° angles

C–C bond length in benzene? — 0.140 nm, all six equal

Stabilisation energy of benzene? — 152 kJ/mol

Electrophile in nitration? — NO₂⁺, the nitronium ion

Reagents for nitration? — Conc. HNO₃ + conc. H₂SO₄, below 55 °C

Reagents for bromination? — Br₂ with FeBr₃ or iron

Friedel–Crafts catalyst? — Anhydrous AlCl₃

Type of mechanism of benzene reactions? — Electrophilic substitution

Why does the ring restore after attack? — Loss of H⁺ regains delocalisation

Product of reducing nitrobenzene? — Phenylamine, C₆H₅NH₂