AQA A-Level Chemistry Paper 3, June 2025: Question 5

✅ Ideal 6-Mark Model Plan

Stage 1: Oxidation

• Reagent: Acidified potassium dichromate(VI) (or K₂Cr₂O₇ / H₂SO₄).
• Propan-1-ol equation:
  CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O
• Propan-2-ol equation:
  CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Stage 2: Chemical Testing of Products

• Reagent: Fehling's solution (or Tollens' reagent).
• With Propanal (from propan-1-ol): Blue solution turns to a brick-red precipitate (or silver mirror with Tollens').
• With Propanone (from propan-2-ol): No visible change / remains blue.

Stage 3: Infrared Spectroscopy

• Both starting alcohols show a broad O−H absorption at 3230–3550 cm⁻¹, so main functional group peaks cannot distinguish them.
• Compare the fingerprint region (below 1500 cm⁻¹) of each alcohol's spectrum to a known database / spectral library.
• The fingerprint region is unique to each specific isomer.

🧠 Exam Technique & Level Ladder

• Level 3 (5–6 marks): All three stages covered; explanations and equations are fully correct; coherent, logical flow.
• Level 2 (3–4 marks): All three stages covered with minor errors/omissions, OR two stages virtually complete.
• Level 1 (1–2 marks): Two stages covered with omissions, OR only one stage virtually complete.
• Stage Breakdown: To get full marks in Stage 1, you must include the reagent and both balanced equations showing structural or displayed formulae.

💡 Key Knowledge

• Primary alcohols oxidise to aldehydes (distillation) and then carboxylic acids (reflux).
• Secondary alcohols oxidise to ketones only.
• Ketones cannot be readily oxidised further, so they give a negative result with mild oxidising agents (Fehling's/Tollens').
• The region below 1500 cm⁻¹ contains complex vibration patterns (C−C, C−O bends/stretches) unique to a given molecule.

❌ Common Mistakes to Avoid

• Molecular Formulae Trap: Writing C₃H₈O + [O] → C₃H₆O + H₂O scores 0 for equations. You must show the structural difference between CH₃CH₂CHO and CH₃COCH₃ .
• Missing By-product: Forgetting + H₂O on the right-hand side of the oxidation equations.
• Missing Acid: Stating just "potassium dichromate" rather than acidified potassium dichromate(VI).
• Vague Observations: Stating "it reacts" or "turns colourless" instead of specifying "brick-red precipitate" or "silver mirror".

✅ Correct Answer & Requirements

Condition (1 mark):

• Heat under reflux (allow: reflux or heat / boil if diagram clearly shows reflux).

Apparatus Diagram & Labels (2 marks):

• M2 (Drawing): Vertical Liebig condenser fitted directly into the neck of the pear-shaped flask with no gaps and the top of the condenser completely open.
• M3 (Labels):
  • Condenser (or Liebig condenser).
  • Water in at the bottom, water out at the top (indicated by arrows or words).

📐 How to Draw Reflux Apparatus

❌ Critical Drawing Errors (Costing M2 & M3)

• Sealing the apparatus: Drawing a stopper at the top creates a closed system (explosion hazard) → loses drawing marks instantly!
• Drawing a thermometer: A thermometer is used for distillation, never in the top of a reflux condenser.
• Reversing water flow: Putting water in at the top will not fill the jacket properly, leading to air pockets and poor cooling.
• Lines blocking openings: Drawing glass lines across the water inlet/outlet tubes or across the joint between flask and condenser.

💡 Why Reflux?

Heating under reflux allows the reaction mixture to be boiled for an extended period without losing volatile reactants or intermediate products (e.g., propanal) via evaporation. Vapours condense on the cool surface of the vertical condenser and drip back into the flask for complete oxidation to propanoic acid.

✅ Mark Scheme Answer

• To produce a steady stream of small bubbles
• OR: To prevent the formation of large bubbles / promote smooth boiling.

❌ The #1 Tautology Trap

DO NOT write: "To prevent bumping"

The mark scheme explicitly states: IGNORE "to prevent bumping". You cannot use the words in the question to explain the phenomenon! You must explain the mechanical action: providing nucleation sites that promote small, uniform bubbles rather than violent large surges of vapour.