AQA A-Level Chemistry Paper 3, 2024: Question 11

Bond Angles: Spotting when 120° is NOT present

What this question tests

Skills & knowledge

Linking molecular/giant structure to bonding geometry.
Recognising where sp² trigonal planar gives 120° bond angles.
Identifying sp³ tetrahedral arrangements (≈ 109.5° ) and non-planar rings.

Assessment focus

AO1 Recall/understanding of shapes and typical bond angles.
Applying that knowledge quickly to familiar substances (benzene, BF₃, cyclohexane, graphite).

Question type: Multiple choice (1 mark). The mark scheme awards the mark for selecting the single correct option.

Part (a) Multiple choice: “Which substance does not have any bond angles of 120°?” [1 mark]

✅ Correct answer (from mark scheme)

C — cyclohexane

Mark allocation: 1 mark (AO1) for selecting C.

💡 Key knowledge

120° is characteristic of trigonal planar arrangement ( sp² ).
109.5° is characteristic of tetrahedral arrangement ( sp³ ).
Rings can be non-planar to reduce strain (so angles deviate from “ideal”).

🧠 Exam technique: fast elimination

Spot which options are clearly sp² (often planar, 120°).
Leave the one that is sp³ and non-planar (no 120°).

❌ Common errors (why students drop this mark)

Assuming “any ring” must have 120° angles (confusing cyclohexane with benzene).
Forgetting graphite is made of planar hexagonal layers with sp² carbon.
Mixing up “molecular shape” (e.g., BF₃ trigonal planar) with unrelated bond angles.

Why each option does / does not contain 120° bond angles

A) Benzene

Each carbon is sp² and trigonal planar.
Bond angles around each carbon are approximately 120°.
So benzene does have 120° bond angles → not the answer.

B) Boron trifluoride, BF₃

Boron has three bonding pairs and no lone pair → trigonal planar.
F–B–F angles are 120°.
So BF₃ does have 120° bond angles → not the answer.

C) Cyclohexane

Carbons are sp³ with (approximately) tetrahedral geometry.
Bond angles are near 109.5° (and vary slightly due to ring conformation).
Cyclohexane adopts a chair conformation (non-planar), so it avoids 120° angles.
Therefore cyclohexane has no 120° bond angles → correct.

D) Graphite

Each carbon forms 3 σ bonds in planar layers → sp² .
Bond angles in the hexagonal lattice are 120°.
So graphite does have 120° bond angles → not the answer.

🧠 Examiner-style tip: what distinguishes full-mark responses (even in MCQ)

Although only the letter is needed, the quickest reliable method is to link each substance to a known structure/shape: sp² → 120° (benzene, BF₃, graphite) vs sp³ → ~109.5° (cyclohexane). Students who lose the mark typically fail to recognise graphite as sp² planar layers or confuse benzene with cyclohexane because both are rings.

Rapid recall mini-checklist (for future questions)

💡 Shapes → angles

Linear: 180°
Trigonal planar ( sp² ): 120°
Tetrahedral ( sp³ ): 109.5°

❌ Trap to avoid

“Ring” does not automatically mean 120° — check sp² vs sp³ and whether it is planar.

✅ One-line takeaway

If it’s sp² and planar → expect 120°. Cyclohexane is sp³ (chair) → no 120°.