OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify the aqueous reagent used to compare the rates of hydrolysis of 1-chloropropane and 1-bromopropane.

Practise this question

Question

Multiple choice question 1 asking which reagent in aqueous solution should be used to compare the rates of hydrolysis of 1-chloropropane and 1-bromopropane in ethanol. Options are A: Silver chloride, B: Silver nitrate, C: Potassium chloride, D: Potassium nitrate, with an answer box provided.
Question text

1 A chemist compares the rates of hydrolysis of 1-chloropropane and 1-bromopropane in ethanol.

Which reagent in aqueous solution should be used?

A Silver chloride

B Silver nitrate

C Potassium chloride

D Potassium nitrate

Your answer

[1]

Mark scheme

Show the mark scheme A table showing question number 1 with key B and 1 mark.

Question Key Marks Guidance

1 B 1

How to answer it

Rates of Hydrolysis of Halogenoalkanes

What this question tests

This question tests your recall of practical reagents used to identify halide ions formed during the nucleophilic substitution (hydrolysis) of halogenoalkanes, specifically understanding the role of aqueous silver nitrate in detecting halide leaving groups.

Question 1 Multiple Choice

Reagent for Halogenoalkane Hydrolysis

✅ Correct Answer: B (Silver nitrate)

Aqueous silver nitrate is added to halogenoalkanes (usually in an ethanol solvent to mix the organic halogenoalkane with the aqueous reagent) to react with halide ions as they are displaced. The silver ions (Ag⁺) react with halide ions (X⁻) to form distinctive silver halide precipitates (AgCl, AgBr, AgI).

💡 Key Knowledge

  • Hydrolysis reaction: Halogenoalkanes react with water to form an alcohol and a halide ion (R-X + H₂O → R-OH + H⁺ + X⁻).
  • Precipitate formation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) [White precipitate]. Ag⁺(aq) + Br⁻(aq) → AgBr(s) [Cream precipitate].
  • Ethanol role: Acts as a mutual solvent so the halogenoalkane and aqueous silver nitrate can mix into a single solution.

🧠 Exam Technique

For multiple-choice recall questions like this, immediately scan for the ion detection reagent. Since we are measuring the rate at which chloride and bromide ions are released, we need a source of silver ions (Ag⁺) to precipitate them out as they form.

❌ Common Errors

  • Choosing potassium chloride or potassium nitrate (options C and D), which introduce unwanted halide ions or lack silver ions entirely.
  • Mistaking solid silver chloride (option A) for a testing solution; adding solid silver chloride would not provide mobile silver ions to react with newly released halide ions.
Mark Allocation: [1 mark] awarded for selecting option B.

Topics

Module 4: Core organic chemistry · Practical Activity Groups · 4.2 Alcohols, haloalkanes and analysis · PAG 7: Qualitative analysis of organic functional groups

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.