AQA A-Level Chemistry Paper 3, 2024: Question 1

Core chemistry

• Fischer esterification with methanol (writing equations + skeletal organic product).
• Acylation using ethanoic anhydride / ethanoyl chloride: why one is preferred.
• Mechanism name for acylation: (nucleophilic) addition–elimination.
• Role of acid catalyst (H₃PO₄).

Practical skills

• Why reflux may not be essential (boiling points / volatility reasoning).
• Vacuum filtration (Büchner/Hirsch setup and labels).
• Recrystallisation method and marking points (hot solvent, minimum volume, hot filtration, cooling, washing, drying).
• Purity check via melting point: what impurity does to m.p. and range.

Where marks are commonly lost

• Calling the mechanism “esterification/acylation” instead of addition–elimination .
• For the methanol reaction: not showing the correct ester + H₂O, or drawing the wrong functional group changed.
• Filtration diagram missing filter paper label or vacuum/reduced pressure indication.
• Purity: saying “impure = higher melting point” (it’s typically lower and over a wider range).

✅ Correct answer (what earns the marks)

✅ Correct answers (any one idea for 1 mark)

• Ethanoic anhydride is less/not corrosive (than ethanoyl chloride).
• Ethanoic anhydride does not form strong acid HCl; it forms ethanoic acid (weaker).
• Ethanoic anhydride is less/not vulnerable to hydrolysis than ethanoyl chloride.

🧠 Exam technique

• Make it a direct comparison: “Ethanoic anhydride preferred because ethanoyl chloride produces HCl (corrosive).”
• Do not drift into vague safety statements unless linked to the mark scheme point (HCl/corrosive/hydrolysis).

❌ Common errors

• Stating “it’s cheaper” / “higher yield” (not credited here).
• Only saying “more reactive” without explaining the practical drawback (HCl/corrosive/hydrolysis).

✅ Correct answer

(nucleophilic) addition–elimination

💡 Key knowledge

• Acid derivatives (acid chlorides/anhydrides) react via nucleophilic attack on the C=O, forming a tetrahedral intermediate, then elimination.

❌ Common errors

• Writing “nucleophilic substitution” (too vague for this spec marking).
• Writing “electrophilic substitution” (wrong reaction type).

✅ Correct answer

Catalyst (speeds up the reaction / lowers activation energy).

🧠 Exam technique

• Use the word catalyst explicitly, then add “speeds up reaction” for clarity.

❌ Common errors

• Saying it “increases yield” without linking to catalysis (not the credited point here).
• Describing it as a dehydrating agent (not the mark scheme answer for this step).

✅ Correct answer

Because the boiling points of the reactants/product(s) are above 85 °C, so they won’t boil off significantly at 85 °C (no need to condense vapour back).

💡 Key knowledge

• Reflux is used to prevent loss of volatile substances when heating at/near boiling.
• If your temperature is below the boiling points, reflux isn’t necessary for that reason.

❌ Common errors

• Writing “reflux increases rate” (it’s the heating that increases rate; reflux is about preventing loss).
• Talking about “stopping pressure building” (not the assessed point here).

✅ Correct diagram features (what to draw + label)

• Draw a Büchner or Hirsch funnel sitting on a bung/collar in the neck of the conical (side‑arm) flask.
• Show a horizontal filter paper disc in the funnel (solid line or dashed is fine).
• Show the side arm connected to a vacuum pump / water pump (reduced pressure / suction).
• Label at least: Filter paper and Vacuum pump (or “suction/reduced pressure”).

🧠 Exam technique (easy way to secure both marks)

1. Sketch the funnel clearly (wide top, flat perforated plate region).
2. Add the filter paper disc (horizontal line).
3. Draw the side-arm tube and a line to “vacuum/water pump”.
4. Add two labels: “filter paper” and “vacuum pump / suction”.

❌ Common errors

• Drawing gravity filtration (filter funnel into beaker) instead of vacuum filtration.
• Forgetting the filter paper label (explicitly required in MS).
• No indication of reduced pressure/suction (loses the other mark).

✅ Correct answers (any two for 2 marks)

• Ethanoic acid
• Phosphoric acid
• 2‑hydroxybenzenecarboxylic acid (salicylic acid)
• Ethanoic anhydride
• Water

🧠 Exam technique

• Look back at the method: anything added/formed could be present on the crystals or trapped in solution.
• Pick two with confidence: a by-product (ethanoic acid) and a reactant (ethanoic anhydride) is a safe pair.

❌ Common errors

• Writing “aspirin” as an impurity (it’s the product).
• Writing “HCl” (not produced in the anhydride route).

✅ Mark-scheme method (hit these 6 marking points)

1. M1: Dissolve crude product in a hot solvent (here: water + ethanol mixture is acceptable).
2. M2: Use the minimum volume of hot solvent (near-saturated solution).
3. M3: Hot filter to remove insoluble impurities (gravity filtration is fine here; decanting allowed).
4. M4: Allow the filtrate to cool so crystals form (recrystallise).
5. M5: Filter under reduced pressure using Büchner/Hirsch apparatus to collect crystals.
6. M6: Wash crystals with a small amount of cold solvent (water and/or ethanol) and dry the product.

💡 Key knowledge (why each step matters)

• Hot solvent increases solubility so product dissolves.
• Minimum volume maximises yield on cooling.
• Hot filtration removes insoluble impurities before crystals form.
• Cooling decreases solubility so purer crystals form, leaving many impurities in solution.
• Cold wash removes surface mother liquor without dissolving much product.

🧠 Exam technique (how to write a 6‑mark practical answer)

• Write in chronological steps; each step should be a distinct action (easy for the examiner to tick).
• Name key apparatus where relevant: conical flask + heat source, filter funnel/paper for hot filtration, Büchner/Hirsch + side-arm flask + vacuum for reduced pressure.
• Use the exact mark-scheme phrases: hot solvent , minimum volume , hot filter , cool , reduced pressure , wash (cold) and dry .

❌ Common errors (seen in examiner notes)

• Forgetting minimum volume (often loses an easy mark).
• Filtering after cooling but not mentioning a hot filtration step for insoluble impurities.
• Not specifying reduced pressure when collecting crystals.
• Washing with hot solvent (dissolves product → lowers yield).

✅ Correct answer (3 marking points)

• Physical property: melting point
• Sign of impurity (1): melting point is lower than the data book value
• Sign of impurity (2): melts over a wide range of temperatures (not a sharp/narrow range)

🧠 Exam technique

• Give two separate observations for the result: one about the value (lower), one about the range (wider).
• AQA wording: “sharp melting point” indicates purity; “wide melting range” indicates impurity.

❌ Common errors

• Saying “higher melting point” (MS: do not credit; impurity generally lowers m.p.).
• Using “boiling point” (explicitly ignored here).
• Writing “different melting point” without stating lower (MS ignores “different”).

💡 Must-know phrases

• Mechanism: nucleophilic addition–elimination
• H₃PO₄ role: catalyst
• Recrystallisation: hot solvent , minimum volume , hot filter , cool , reduced pressure , wash (cold) + dry
• Purity: melting point lower + wider range

🧠 Diagram recall

• Vacuum filtration must show: Büchner/Hirsch funnel + filter paper disc + side-arm flask + vacuum/water pump label.

✅ Equation recall

Ar–COOH + CH₃OH → Ar–COOCH₃ + H₂O

(Ar also contains the phenolic –OH substituent unchanged.)