Edexcel A-Level Chemistry AS Paper 2, November 2020: Question 7

12 marks · Hard difficulty · Extended Writing

Investigate the relative rates of substitution reactions of halogenoalkanes using test tube experiments and silver nitrate.

Practise this question

Question

Exam question about halogenoalkanes reacting with water to produce alcohols and halide ions, consisting of parts (a), (b), and (c) detailing experimental procedures involving 1-chlorobutane, 1-bromobutane, 1-iodobutane, ethanol, warm water, silver nitrate, and isomeric bromoalkanes.
Question text

7 Halogenoalkanes react with water to produce alcohols and halide ions.

C H X + H O → C H OH + X− + H+

49 2 4 9

(a) Test tube experiments can be carried out to investigate the relative rates of these

substitution reactions.

The halogenoalkanes 1‑chlorobutane, 1‑bromobutane and 1‑iodobutane can be used.

Some of the steps in these experiments are

● each halogenoalkane is added to a different tube containing 1 cm3 of ethanol

● the test tubes are placed in the same beaker of hot water

● aqueous silver nitrate is added to each tube and the tubes are shaken

● a precipitate forms in each tube.

(i) State the purpose of adding ethanol to each of the test tubes.

(1)

(ii) Give one reason why the test tubes were put in the same beaker of hot water.

(1)

(iii) Give one reason why the test tubes were shaken after the addition of

aqueous silver nitrate.

(1)

(b) (i) State how the halogen atom present in each halogenoalkane can be

identified using observations from this experiment in (a).

(1)

… *P62307A02428*

(ii) Identify further reagents that can be added, including relevant observations,

to confirm the identity of the halogen atom present in each halogenoalkane.

(2)

*(c) Outline the method for a test tube experiment, which expands on the steps in (a),

to investigate how the rate of the substitution reaction depends on whether the

halogenoalkane is primary, secondary or tertiary.

Your experiment should test a series of isomeric bromoalkanes reacting with water.

Your plan should include

● the chemicals you will use

● an outline of how the experiment will be carried out

● the observations or measurements you will make and how you will

interpret them.

(6)

… *P62307A02528*

… 26

… *P62307A02628*

(Total for Question 7 = 12 marks)

Mark scheme

Show the mark scheme Mark scheme detailing expected answers for parts 7(a), 7(b), and 7(c), including purposes of ethanol, maintaining constant temperature, mixing, precipitate observations with dilute and concentrated ammonia, and a 6-mark extended response table for outlining the bromoalkane hydrolysis experiment.

How to answer it

Halogenoalkane Rates and Substitution Reactions

AS Level Chemistry — Edexcel

What this question tests

This question assesses your understanding of nucleophilic substitution reactions of halogenoalkanes with water/aqueous silver nitrate, the identification of halide precipitates using ammonia, and the experimental design required to investigate how carbon skeleton structure (primary, secondary, tertiary) affects reaction rates.

Part (a): Test Tube Experiments Setup

Investigating relative rates of substitution using 1-chloro-, 1-bromo-, and 1-iodobutane

✅ Correct Answers

  • (i) Ethanol acts as a co-solvent to dissolve both the halogenoalkane and water, allowing them to form a homogeneous mixture.
  • (ii) To ensure all test tubes are at the same temperature (or to ensure a fair test / keep temperature constant).
  • (iii) To ensure the reactants are mixed thoroughly.

❌ Common Errors to Avoid

  • In (i): Do not state that ethanol "allows halogens to mix" or that it "dissolves just one reactant". Both must mix.
  • In (iii): Do not mention "increasing kinetic energy of molecules" as the reason for shaking; shaking is purely for physical mixing.
Marks available: 3 marks total (1 mark per subpart)

Part (b): Halide Identification & Confirmatory Tests

Identifying halide ions using silver nitrate and ammonia

✅ Correct Answers

  • (i) Observations: Chloride gives a white precipitate; Bromide gives a cream precipitate; Iodide gives a yellow precipitate.
  • (ii) Further Reagents: Add dilute followed by concentrated aqueous ammonia.
  • (ii) Confirmatory Results: Silver chloride dissolves in dilute ammonia; silver bromide dissolves only in concentrated ammonia; silver iodide is insoluble in both.

💡 Key Knowledge

  • Precipitate colours can look similar under poor lighting. Always use precise descriptors: white, cream, yellow (avoid vague terms like "pale yellow" for bromide).
  • Ammonia solubility is the definitive chemical test to distinguish between ambiguous halide precipitates.
Marks available: 3 marks total (1 for b-i, 2 for b-ii)

Part (c): 6-Mark Extended Response Plan

Investigating the effect of primary, secondary, and tertiary structure on substitution rates

🧠 Exam Technique & Structure (QWC)

This is a 6-mark extended writing question assessed on both your indicative chemical content (up to 4 marks) and the coherence/logical structure of your reasoning (up to 2 marks).

  • Use bullet points or clear sequential steps to ensure logical flow.
  • Explicitly link observations to conclusions (shorter time = faster rate).

🔑 Indicative Content Points (IPs)

  • IP1: Use equal amounts / moles / volumes of each halogenoalkane and silver nitrate.
  • IP2 & IP3: Use a series of isomeric bromoalkanes covering primary, secondary, and tertiary structures (e.g., 1-bromobutane, 2-bromobutane, and 2-bromo-2-methylpropane).
  • IP4: Time how long it takes for a precipitate to form (or observe the order in which they appear).
  • IP5: Shorter time taken indicates a faster rate of reaction ( rate = 1 / time ).
  • IP6: State the correct order of reactivity: tertiary fastest, followed by secondary, with primary being the slowest.
Marks available: 6 marks (4 for chemistry content, 2 for structured reasoning)

Topics

Organic Chemistry · Inorganic Chemistry · Core Practicals · Topic 6: Organic Chemistry I · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 4: Investigate the hydrolysis of halogenoalkanes

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.