OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 24

✅ Correct Answers

• Structural isomer definition: Same molecular formula, but different structural formulae.
• Molecular formula: C₅H₁₂ for all three isomers (A, B, and C).
• Boiling point trend: Boiling point decreases as branching increases (or as carbon chains become more branched / shorter main chains).
• Explanation: More branching leads to less surface contact between molecules, resulting in fewer/weaker London forces. Consequently, less energy is required to overcome these intermolecular forces.

💡 Key Knowledge

• Alkanes are non-polar molecules held together solely by weak induced dipole-dipole interactions (London forces).
• Straight-chain isomers (like A, pentane) have a greater surface area of contact, maximizing intermolecular attractions.
• Branched isomers (like C, 2,2-dimethylpropane) are more spherical, reducing contact area and weakening the instantaneous dipole interactions.

❌ Common Errors

• Saying "same molecular formula" without stating "different structural formula" (or vice versa).
• Using vague terms like "space", "spatial", or "3D" which describe stereoisomerism rather than structural isomerism.
• Stating "different arrangement of atoms" without qualifying it with "structural formula".
• Attributing boiling point changes to breaking "covalent bonds" rather than "intermolecular forces / London forces".

🧠 Exam Technique

For trend questions comparing molecules, ensure you make a direct comparative statement using "ORA" (Or Reverse Applies). Always link molecular shape directly to surface contact area, then to the strength of London forces, and finally to the energy required.

✅ Correct Answer

Radical substitution (or Free radical substitution)

🧠 Exam Technique

Learn reaction classifications precisely. Do not confuse substitution with addition mechanisms typical of alkenes.

✅ Correct Answers

• Isomer A (Pentane): 3 structural isomers (1-chloropentane, 2-chloropentane, 3-chloropentane).
• Isomer B (2-methylbutane): 4 structural isomers (1-chloro-2-methylbutane, 2-chloro-2-methylbutane, 3-chloro-2-methylbutane, 1-chloro-3-methylbutane).

❌ Common Errors

Failing to systematically check all possible carbon positions for chlorine substitution, leading to missed isomers or double-counted mirror images.

✅ Correct Answers

• Structure D: A trichloro isomer of compound A ( C₅H₉Cl₃ ). Chlorine atoms can be placed on various carbon positions (e.g., 1,2,3-trichloropentane or 2,2,3-trichloropentane).
• Equation: C₅H₁₂ + 3Cl₂ → C₅H₉Cl₃ + 3HCl

📐 Working Out Molar Mass / Formula Check

1. Compound A is pentane ( C₅H₁₂ ), Mr = (5 × 12.0) + (12 × 1.0) = 72.0 g mol⁻¹.
2. Compound D has molar mass 175.5 g mol⁻¹. Difference = 175.5 - 72.0 = 103.5 g mol⁻¹.
3. Each substituted Cl replaces an H atom (Loss of H = 1.0, Gain of Cl = 35.5, Net change per substitution = +34.5 g mol⁻¹).
4. 103.5 / 34.5 = exactly 3 chlorine atoms added, confirming C₅H₉Cl₃ .

❌ Common Errors

• Using structural or displayed formulae within the chemical equation when the question explicitly requests molecular formulae.
• Incorrect stoichiometry in the equation (forgetting balancing coefficients for Cl₂ and HCl ).

🧠 Exam Technique

Always double-check what formula style is requested. If the question asks for "molecular formulae in the equation", structural drawings or expanded representations will lose marks even if balanced correctly.