AQA A-Level Chemistry Paper 2, 2024: Question 5

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

M1 Misty / white / steamy fumes M1 (immediate) White precipitate forms

05.1

M2 No visible change M2 White precipitate forms slowly (2 x AO3)

M1 Propanal AND (blue solution gives a brick) red precipitate

If M1 incorrect, allow ECF for suitable tests on remaining liquids

(Warm with)

warm with a named (Warm with)

acidified

M2 add Na carboxylic acid with acidified potassium

potassium 3

05.2 conc H2SO4 manganate (VII)

dichromate (VI) (3 x AO3)

Propan-1-ol /

alcohol Propan-1-ol/ Propan-1-ol /

alcohol

AND alcohol alcohol

M3 AND

(orange AND AND

(purple solution)

solution) goes effervescence fruity smell

goes colourless

green

How to answer it

Simple Test-tube Reactions to Identify Organic Liquids

What this question tests

Distinguishing an acid chloride (propanoyl chloride) from a chloroalkane (1-chloropropane) using silver nitrate, and using Fehling’s solution to pick out an aldehyde (propanal), then suggesting a second quick test to separate a primary alcohol (propan-1-ol) from a ketone (propanone).

Examiner tips: many missed the misty fumes (HCl) and immediate AgCl with acid chlorides, and forgot to link their second test to a clear observation.

Part (a) • AgNO₃ test

Propanoyl chloride vs 1-chloropropane

✅ Correct observations

Propanoyl chloride: misty/steamy HCl fumes; an immediate white precipitate of AgCl forms.
1-Chloropropane: no immediate change; a white precipitate forms slowly on standing/warming.

2 marks

💡 Key knowledge

Acid chlorides hydrolyse rapidly in water to give HCl (misty fumes) → Cl⁻ immediately available for AgCl.
Chloroalkanes are far less reactive in water; halide release is slow, so AgCl appears slowly.

❌ Common errors

Forgetting the fumes of HCl with acid chlorides.
Writing the same observation for both samples.

Part (b) • Fehling’s & one more test

Propan-1-ol, Propanal, Propanone

✅ With Fehling’s solution

Liquid that reacts: propanal (the aldehyde).

Observation: blue solution → brick-red precipitate of Cu₂O on warming.

🧠 Further simple test to distinguish the other two

Test: warm with acidified potassium dichromate(VI) , K₂Cr₂O₇/H⁺ .

Expected observation: with propan-1-ol solution goes orange → green; propanone gives no change.

3 marks total for (b)

💡 Alternatives also credited

Na(s) test: effervescence with propan-1-ol (H₂), none with propanone.
Esterification smell: fruity odour with propan-1-ol + carboxylic acid/conc H₂SO₄, no reaction for ketone.
Acidified KMnO₄: purple → colourless with propan-1-ol; no change with propanone.

❌ Common errors

Saying “blue → red solution” (must be a brick-red precipitate).
Giving a valid test but not stating the observation.

Final takeaways

💡 Key knowledge

Acid chloride + H₂O → HCl + carboxylic acid; AgNO₃ picks up Cl⁻ instantly.
Aldehydes reduce Fehling’s/Tollens’; ketones generally do not.
Primary alcohols are oxidised by K₂Cr₂O₇/H⁺ (orange → green).

🧠 Exam technique

Always pair a named test with a clear observation.
Use specific words: “misty/steamy fumes of HCl”, “brick-red precipitate”.

Topics

Required Practicals · Organic Chemistry · Required Practical 6: Tests for alcohol, aldehyde, alkene and carboxylic acid · 3.3.3 Halogenoalkanes · 3.3.6 Organic Analysis · 3.3.8 Aldehydes and Ketones