The questions are on the benzene test. Take it under real conditions before reading the mark scheme.
Quick answer: Benzene, C₆H₆, is a planar ring with six delocalised π electrons and six equal C–C bonds of 0.140 nm. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on benzene, from bond lengths to using methylbenzene in schools, with the equivalent answers to accept.
The answers give the expected response and the marks. Alternative correct wording earns the mark, and follow-through applies to calculations.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.10
Question 1 Bond lengths (2 marks)
If the Kekulé structure were correct, benzene would have three short C=C bonds (0.134 nm) alternating with three longer C–C bonds (0.154 nm). The bonds are all equal and intermediate, so the electrons are delocalised over the ring.
Marking: 1 for the predicted alternating bonds; 1 for equal intermediate length showing delocalisation.
Question 2 Enthalpy of hydrogenation (3 marks)
- 3 × (−120) = −360 kJ/mol.
- −208 − (−360) = +152 kJ/mol; benzene is 152 kJ/mol more stable than the localised triene because of delocalisation.
Marking: (a) 1; (b) 1 for the value, 1 for the meaning.
Question 3 Nitration of benzene (4 marks)
- Concentrated nitric acid and concentrated sulfuric acid at a temperature below 55 °C.
- C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.
- Higher temperature gives further substitution (dinitrobenzene).
Marking: (a) 1 for the acids, 1 for the temperature; (b) 1; (c) 1.
Question 4 The intermediate (3 marks)
- The positive charge is delocalised over the five carbon atoms, so the ring is no longer fully delocalised.
- The C–H bond breaks, H⁺ is lost and its electrons restore the delocalised ring.
- The halogen carrier (FeBr₃) accepts Br⁻ from Br₂ to make a stronger electrophile, and it is regenerated.
Marking: 1 mark for each part.
Question 5 Substitution, not addition (2 marks)
Addition would destroy the delocalised ring and lose the stabilisation energy of 152 kJ/mol. Substitution keeps the ring intact, so the aromatic stability is retained in the product.
Marking: 1 for the ring being kept; 1 for the stability argument.
Question 6 Friedel–Crafts alkylation (2 marks)
C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl, catalysed by anhydrous AlCl₃.
Marking: 1 for the equation; 1 for the catalyst.
Question 7 Percentage yield (3 marks)
n(C₆H₆) = 3.90 ÷ 78.0 = 0.0500 mol; theoretical mass = 0.0500 × 157.0 = 7.85 g; percentage yield = 5.18 ÷ 7.85 × 100 = 66.0 %.
Marking: 1 for moles; 1 for theoretical mass; 1 for yield.
Question 8 Using methylbenzene in schools (1 mark)
Benzene is carcinogenic and toxic, while methylbenzene or esters are less hazardous and still show the same type of reaction.
Marking: 1 mark for the toxicity of benzene.
