Edexcel A-Level Chemistry AS Paper 1, June 2017: Question 4

10 marks · Medium difficulty · Short Open Response

Write ionic half-equations and overall equations for the reaction of chlorine with cold and hot aqueous hydroxide, explain disproportionation, and write equations and safety precautions for the displacement of bromine by chlorine.

Practise this question

Question

Question text

4 This question is about redox chemistry.

(a) (i) Write an ionic half-equation for the reduction of chlorine molecules to

chloride ions.

State symbols are not required.

(1)

(ii) Write an ionic half-equation for the oxidation of chlorine molecules to

chlorate(I) ions in the presence of cold, aqueous hydroxide ions.

State symbols are not required.

(1)

(iii) Combine the two equations in (a)(i) and (ii) to give the ionic equation for the

reaction of chlorine molecules with cold, aqueous hydroxide ions.

(1)

(iv) Use your answer to (a)(iii) to explain why the reaction is described as a

disproportionation reaction.

(2)

(b) A different ion containing chlorine is formed if the solution of aqueous hydroxide ions

is hot.

Give the formula of the chlorine-containing ion and the oxidation number of

chlorine in this ion.

(2)

using chlorine.

Write the ionic equation for this reaction. State symbols are not required.

(1)

(ii) Identify one hazard associated with carrying out this reaction in a school

laboratory and a safety precaution other than wearing a laboratory coat and

eye protection.

(2)

(Total for Question 4 = 10 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

4 (a)(i) Cl + 2e(−) → 2Cl- Allow multiples (1)

Ignore state symbols even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

4 (a)(ii) Cl + 4OH- → 2ClO- + 2H O + 2e(−) Allow multiples (1)

Cl + 2OH- → 2ClO- + 2H+ +

2e(−)

Ignore state symbols even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

4 (a)(iii) Cl + 2OH- → Cl- + ClO- + H O Allow multiples (1)

Ignore state symbols even if incorrect

Do not award mark if electrons are

un-cancelled

Question

Acceptable Answer Additional Guidance Mark

Number

4 (a)(iv) An explanation that makes reference to the following (2)

points:

(disproportionation is simultaneous) oxidation and Allow statement that chlorine is

reduction of an element (in the same species) (1) oxidised and reduced

This can be shown on the equation in

chlorine changes from 0 to -1 and +1 (1) a(iii)

Question

Acceptable Answer Additional Guidance Mark

Number

4 (b) ClO ― (1) Allow NaClO / KClO (2)

33 3

(Cl is) +5 / 5+ (1) Allow (+)V

Do not award 5 unless +5/5+ seen in

the formula or as a label on the

formula

Question

Acceptable Answer Additional Guidance Mark

Number

4 (c)(i) Cl + 2Br- → 2Cl- + Br Allow multiples (1)

Ignore state symbols even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

4 (c)(ii) An answer that makes reference to the following points: 2nd mark dependent on first. (2)

chlorine/bromine toxic /poisonous (1) Do not award harmful, but allow MP2 if

correct for toxic.

(Carry out the experiment in a) fume cupboard (1)

bromine corrosive (1)

wear gloves (1)

(Total for Question 4 = 10 marks)

How to answer it

Redox Chemistry & Group 7 Reactions Study Guide

🔍 What this question tests

This question evaluates core AS Level redox principles: constructing ionic half-equations for oxidation and reduction, combining them into overall ionic equations, defining disproportionation using changes in oxidation numbers, recognizing temperature-dependent halogen-hydroxide reactions, writing displacement reactions, and identifying practical lab safety hazards.

Question 4 (a) (i)

Reduction of Chlorine to Chloride Ions

✅ Correct Answer

Cl₂ + 2e⁻ → 2Cl⁻

Marks: 1

💡 Key Knowledge

Reduction is the gain of electrons.
Chlorine gas exists as a diatomic molecule (Cl₂).

❌ Common Errors

Writing chlorine as a single atom ( Cl ) instead of a molecule ( Cl₂ ).
Incorrect balancing of electrons or chloride ions.

Question 4 (a) (ii)

Oxidation of Chlorine to Chlorate(I) Ions (Cold, Aqueous Conditions)

✅ Correct Answer

Cl₂ + 4OH⁻ → 2ClO⁻ + 2H₂O + 2e⁻

Alternatively accepted: Cl₂ + 2OH⁻ → 2ClO⁻ + 2H⁺ + 2e⁻

Marks: 1

💡 Key Knowledge

Oxidation is the loss of electrons.
In alkaline conditions, hydroxide ions ( OH⁻ ) appear on the reactant side to balance oxygen and hydrogen.

🧠 Exam Technique

Ensure electrons are explicitly shown on the product side as they are being lost during oxidation.

Question 4 (a) (iii)

Combining Half-Equations

✅ Correct Answer

Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O

Marks: 1

❌ Common Errors

Failing to cancel out the 2 electrons from each half-equation. Examiners will not award the mark if un-cancelled electrons remain in the final equation.

Question 4 (a) (iv)

Explaining Disproportionation

✅ Correct Answer

Chlorine is simultaneously oxidized and reduced.
The oxidation number of chlorine changes from 0 (in Cl₂) to -1 (in Cl⁻) and +1 (in ClO⁻).

Marks: 2

🧠 Exam Technique

To secure both marks, you must include both parts: the definition (simultaneous oxidation and reduction of the same element) and specific numerical proof using oxidation numbers.

Question 4 (b)

Hot Aqueous Hydroxide Reaction with Chlorine

✅ Correct Answer

Ion Formula: ClO₃⁻ (or NaClO₃ / KClO₃ )

Oxidation Number: +5 (or 5+ / (+)V )

Marks: 2

❌ Common Errors

Writing the oxidation number without the sign (must be +5 , not just 5).
Confusing the cold product (chlorate(I), ClO⁻ ) with the hot product (chlorate(V), ClO₃⁻ ).

Question 4 (c) (i)

Halogen Displacement Reaction

✅ Correct Answer

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

Marks: 1

💡 Key Knowledge

A more reactive halogen (chlorine) displaces a less reactive halide ion (bromide) from solution.

Question 4 (c) (ii)

Laboratory Safety and Hazards

✅ Correct Answer

Hazard: Chlorine or bromine is toxic / poisonous (or bromine is corrosive).

Precaution: Carry out the experiment in a fume cupboard (if toxic) OR wear gloves (if corrosive).

Marks: 2

❌ Common Errors

Using vague terms like "harmful" instead of specific terms like "toxic" or "corrosive". Note: "Harmful" is rejected, but "toxic" allows the glove/fume cupboard mark.
The second mark is strictly dependent on identifying a valid matching hazard first.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 3: Redox I · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.