AQA A-Level Chemistry Paper 3, 2021: Question 28
1 mark · Easy difficulty · Multiple Choice
This question assesses your understanding of identifying functional groups using chemical tests. It focuses on the reaction between silver nitrate and compounds containing a chlorine atom bonded to a carbonyl group. You are expected to know which functional group gives a white precipitate of silver chloride when treated with aqueous silver nitrate.
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Question text
28 Which compound forms a white precipitate when added to aqueous silver nitrate?
[1 mark]
A bromoethane
B ethanal
C ethanoic anhydride
D ethanoyl chloride
Mark scheme
Show the mark scheme
28 D 1 ethanoyl chloride
How to answer it
Multiple Choice Explained: Which compound forms a white precipitate with aqueous silver nitrate?
Correct Answer: D – Ethanoyl chloride
Why? Ethanoyl chloride reacts rapidly with water (including water in aqueous AgNO₃) to form a carboxylic acid and a chloride ion:
CH₃COCl + H₂O → CH₃COOH + HCl
The HCl dissociates to form Cl⁻ ions, which react with Ag⁺ from silver nitrate to form a white precipitate of silver chloride:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Why A is incorrect:
Bromoethane is a haloalkane, but it reacts with AgNO₃ only in the presence of aqueous ethanol and under heating to undergo hydrolysis. It does not produce a white precipitate immediately in aqueous AgNO₃ alone.
Why B is incorrect:
Ethanal is an aldehyde and does not release halide ions. It does not form a precipitate with silver nitrate. Instead, aldehydes can reduce Tollens' reagent (ammoniacal silver nitrate), forming a silver mirror – but that's a different test.
Why C is incorrect:
Ethanoic anhydride reacts with water to form carboxylic acids but does not release chloride ions. No reaction with Ag⁺ ions, so no white precipitate forms.
Topics
Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.