Edexcel A-Level Chemistry Paper 2, June 2025: Question 2

✅ Correct Answer

A — (CH₃)₂CHCH₂C(CH₃)₃ (2,2,4-trimethylpentane)

💡 Key Knowledge

• All isomers of octane (C₈H₁₈) have the same molecular mass and identical number of electrons (66 electrons).
• Branching reduces the surface area of contact between adjacent molecules.
• Fewer points of contact mean weaker London dispersion forces, which need less thermal energy to overcome, resulting in a lower boiling point.
• Option A has three methyl branches (most branched), while B and C have only two, and D has none.

❌ Common Errors

Selecting D (octane). Students often confuse lowest boiling temperature with highest. Unbranched straight-chain alkanes pack closely and have the highest surface contact area, giving them the highest boiling temperature.

🧠 Exam Technique

Always circle key words like lowest or highest. Quickly count the number of branches in condensed structural formulas: A has 3 branches, B has 2, C has 2, D has 0.

✅ Model Answer & Mark Scheme

• M1: (Only) pentan-1-ol has hydrogen bonds (between molecules), whereas hexane only has London dispersion forces.
• M2: Intermolecular forces in pentan-1-ol are stronger and require more energy to overcome / break (hence it is less volatile).

💡 Key Knowledge

• Pentan-1-ol (C₅H₁₁OH) and hexane (C₆H₁₄) both have 50 electrons, meaning their London dispersion forces are comparable in magnitude.
• Pentan-1-ol has a highly electronegative O atom bonded to H, allowing intermolecular hydrogen bonding.
• Volatility is the tendency of a substance to vaporise. Stronger intermolecular forces = higher boiling point = lower volatility.

❌ Common Misconceptions

• "Covalent O–H bonds break when boiling": A fatal error! Covalent bonds remain completely intact; only intermolecular forces are overcome.
• Vagueness: Saying "pentan-1-ol has hydrogen bonds so it takes more energy" without explicitly comparing strength or stating that hydrogen bonds are stronger than the London forces in hexane.

🧠 Exam Technique

Structure comparison questions in two sentences:

1. Name the specific intermolecular force present in each compound.
2. Compare their relative strengths and explicitly link to the energy required to overcome them.

✅ Correct Answer

C — London forces

💡 Key Knowledge

• Iodine exists as simple diatomic molecules: I₂ .
• In solid iodine, I₂ molecules are held in a regular crystal lattice by London dispersion forces (induced dipole–dipole forces).
• Upon heating, these weak intermolecular forces break, allowing molecules to separate into the gas phase. The covalent I–I bonds do not break.

❌ Common Distractors

• A (Covalent bonds): Only broken during chemical reactions or in giant covalent lattices (like diamond or graphite), never during simple molecular boiling/sublimation.
• D (Permanent dipole–dipole): Homonuclear diatomic molecules like I₂ have identical electronegativities, so the bond is non-polar and has no permanent dipole.

✅ Model Answer

"Boron trichloride has 3 bonding pairs and no lone pairs around B, giving it a trigonal planar shape. Phosphorus trichloride has 3 bonding pairs and 1 lone pair around P, giving it a pyramidal shape.

Due to its symmetrical planar shape, the bond dipoles in BCl₃ cancel out, making it non-polar. In PCl₃, the asymmetrical pyramidal geometry means the dipoles do not cancel, giving the molecule an overall permanent dipole.

Consequently, PCl₃ molecules are held together by permanent dipole–dipole forces (in addition to London forces), which require more energy to break than the forces in BCl₃, giving PCl₃ a much higher boiling temperature."

💡 VSEPR Theory Refresher

❌ Examiner Pitfalls & Lost Marks

• Saying "charges cancel": The mark scheme explicitly states "Do not award: the charges cancel". You must say dipoles cancel or dipole vectors cancel.
• Discussing London forces: The question explicitly says "Do not consider differences in London forces." Any time spent discussing electron count or London forces gains zero credit.
• Missing the lone pair effect: Describing PCl₃ as "planar" or "tetrahedral" instead of pyramidal.
• Stopping short: Stating that PCl₃ is polar, but failing to name the intermolecular force: permanent dipole–dipole forces.

🧠 Diagram Advice (If Drawing)

If you choose to draw 3D diagrams instead of writing descriptions:

• BCl₃: Draw a flat planar structure with 120° bond angles and dipoles pointing towards Cl (δ+ on B, δ− on each Cl).
• PCl₃: Must use wedge-and-dash bonds to show 3D tetrahedral/pyramidal geometry, show the lone pair on P, and draw a clear net dipole arrow pointing downwards.