OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 16

✅ Correct Answer

B (ethanol)

Ethanol contains an oxygen atom bonded to a hydrogen and a carbon atom (C-O-H), with two lone pairs of electrons on the oxygen atom. This creates a non-linear (bent) shape with a bond angle of approximately 104.5° , which is smaller than the tetrahedral angles ( 109.5° ) found in alkanes/haloalkanes and the trigonal planar angles ( 120° ) found in alkenes.

💡 Key Knowledge

• Tetrahedral (ethane, bromoethane): 4 bonding pairs, 0 lone pairs = 109.5° bond angle.
• Trigonal Planar (ethene): 3 regions of electron density around each double-bonded carbon = 120° bond angle.
• Non-linear / Bent (ethanol): 2 bonding pairs and 2 lone pairs around the oxygen atom. Lone pair-lone pair repulsion is greater than bonding pair-bonding pair repulsion, compressing the bond angle to 104.5° .

🧠 Exam Technique

Do not just look at the main carbon chain. When oxygen or nitrogen heteroatoms are introduced, always consider functional groups carefully. Identify the shape and bond angle associated with every distinct environment (such as the C-O-H group in alcohols) rather than assuming uniform tetrahedral geometry across the whole molecule.

❌ Common Errors

• Assuming all organic molecules containing sp³ carbons have a uniform 109.5° bond angle everywhere without checking functional groups.
• Confusing the bond angle of water / alcohols ( 104.5° due to two lone pairs) with ammonia ( 107° due to one lone pair).