OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 23

9 marks · Medium difficulty · Structured Questions

Describe the risks of chlorine, determine oxidation numbers in a redox reaction, write a balanced equation for a firework reaction, and plan a practical to investigate the reactivity trend of halogens.

Practise this question

Question

A four-part question about halogens. Part (a) asks for one risk of using chlorine in drinking water (1 mark). Part (b) presents the reaction of hydrochloric acid with manganese(IV) oxide and asks to identify the element oxidised and reduced with oxidation number changes (2 marks). Part (c) asks for a balanced equation for the reaction between potassium perchlorate and aluminium (1 mark). Part (d) provides a table of halogen colours in cyclohexane and asks to plan an experiment to show the trend in reactivity of Cl2, Br2, and I2, including observations and an ionic equation (5 marks).
Question text

23 This question is about halogens.

(a) Chlorine is used to kill bacteria in drinking water.

State one risk in using chlorine in drinking water.

… [1]

(b) Chlorine can be prepared by reacting concentrated hydrochloric acid with manganese(IV)

oxide, MnO2.

4HCl(aq) + MnO2(s) → Cl2(g) + MnCl2(aq) + 2H2O(l)

Using oxidation numbers, show which element has been oxidised and which has been

reduced in this reaction. State the changes in oxidation numbers, including all signs.

Element oxidised …

Oxidation number change: from … to …

Element reduced …

Oxidation number change: from … to …

[2]

(c) A mixture of potassium perchlorate, KClO4, and aluminium is used in fireworks.

When the firework ignites, KClO4 reacts with aluminium to form potassium chloride, KCl,

and aluminium oxide, Al2O3.

Write the balanced equation for this reaction.

State symbols are not required.

… [1]

(d) A student investigates the trend in reactivity of the halogens Cl2, Br2 and I2.

The student is supplied with:

• solutions of Cl2, Br2 and I2 in cyclohexane (an organic solvent)

• aqueous solutions of the halides: NaCl, NaBr and NaI.

The colours of the halogen solutions in cyclohexane are shown below.

Halogen Cl2 Br2 I2

Colour in cyclohexane Pale green Orange Violet

Plan an experiment on a test tube scale that would show the trend in the reactivity of the

halogens Cl2, Br2 and I2.

Include all the expected observations and an ionic equation for one of the reactions.

… [5]

Mark scheme

Show the mark scheme Mark scheme for question 23 showing acceptable answers for parts (a) through (d), including toxic/poisonous for chlorine, oxidation number changes for Mn and Cl, the balanced equation 3KClO4 + 8Al -> 3KCl + 4Al2O3, and the experimental plan for halogen displacement reactions with expected observations, ionic equations, and reactivity trend.

AO

Question Answer Marks Guidance

element

23 (a) toxic/poisonous 1 AO1.1

OR IGNORE ‘harmful’/’dangerous’

forms chlorinated hydrocarbons

OR IGNORE chlorine is carcinogenic/causes cancer

forms carcinogenic compounds / toxic compounds dangerous for health/causes breathing problems

(b) 2 AO1.2 MAX 1 mark if no ‘+’ sign for oxidation number

×2

Element oxidised: Chlorine/Cl ALLOW Cl2 for chlorine

Change from: –1 to 0 ALLOW 1–

Element reduced: Manganese/Mn

Change from +4 to +2 ALLOW 4+ AND 2+

ALLOW 1 mark for all oxidation numbers correct,

but oxidised and reduced the wrong way around

IGNORE numbers around equation

i.e. treat as rough working

(c) 3KClO4 + 8Al → 3KCl + 4Al2O3 1 AO2.6 ALLOW multiples

AO

element

(d) 5 max IGNORE additions of halogen to same halide

e.g. Chlorine to chloride.

Plan

Mix (solution of) halogen and (solution of) halide AO3.3 ALLOW within text if it is clear that halogen is

added to halide

Observation with chlorine

bromide → orange/yellow AO2.7 Check observations in a presented table.

Observation with bromine

iodide → violet/purple/pink AO2.7

Observation with iodine

No colour change/no reaction AO2.7

Equation

Cl + 2Br– → Br + 2Cl– ALLOW multiples,

e.g. ½Cl + Br– → ½Br + Cl–

OR 2 2

Cl + 2I– → I + 2Cl–

OR

Br + 2I– → I + 2Br–

AO2.6

Reactivity trend

Cl2 > Br2 > I2 /decreases down the group

AO1.1

Total 9

How to answer it

Exam Study Guide: Halogens & Redox Chemistry

What this question tests

This question assesses your understanding of Group 7 (halogens) chemistry, including water treatment risks, redox identification using oxidation numbers, balancing equations, and planning a displacement experiment to determine the reactivity trend of halogens.

Part (a): Risks of Chlorine in Water Treatment

1 Mark

✅ Correct Answer

Any one of the following:

  • Toxic / poisonous
  • Forms chlorinated hydrocarbons
  • Forms carcinogenic compounds / toxic compounds

❌ Common Errors

Vague terms like harmful , dangerous , or causes breathing problems are rejected by examiners. You must be specific about toxicity or the formation of chlorinated/carcinogenic by-products.

Part (b): Oxidation Numbers in Chlorine Preparation

2 Marks

✅ Correct Answers

  • Element oxidised: Chlorine / Cl (or Cl₂)
  • Oxidation number change: From -1 to 0
  • Element reduced: Manganese / Mn
  • Oxidation number change: From +4 to +2

🧠 Exam Technique & Guidance

You must include polarity/sign (+ or -) with your numbers. Examiners will award a maximum of 1 mark if signs are missing (e.g. writing "4 to 2" instead of "+4 to +2"). Ensure you match the element to the correct change.

Part (c): Balancing Equations in Fireworks

1 Mark

✅ Correct Answer

3KClO₄ + 8Al → 3KCl + 4Al₂O₃

💡 Key Knowledge

Balancing complex equations requires methodical checking of oxygen and metal atoms. Fractions are generally not accepted for whole-number molecular equations like this.

Part (d): Halogen Displacement Practical Plan

5 Marks Max

✅ Marking Points Breakdown

  • Plan: Mix halogen solution with halide solution.
  • Observation 1 (Cl₂ + Br⁻): Orange / yellow solution formed.
  • Observation 2 (Br₂ + I⁻): Violet / purple / pink layer/solution formed.
  • Observation 3 (I₂ + Cl⁻/Br⁻): No colour change / no reaction.
  • Ionic Equation: e.g., Cl₂ + 2Br⁻ → Br₂ + 2Cl⁻
  • Trend: Cl₂ > Br₂ > Idash (or reactivity decreases down the group).

🧠 Exam Technique for 5-Mark Plans

To access top marks, structure your answer logically: state the reagents mixed, explicitly list the expected colour changes using organic solvent indicators (cyclohexane), provide a correct ionic equation omitting spectator ions, and conclude with the clear reactivity trend statement.

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Practical Activity Groups · PAG 4: Qualitative analysis of ions · 2.1 Atoms and reactions · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.