AQA A-Level Chemistry Paper 3, June 2018: Question 18
1 mark · Easy difficulty · Multiple Choice
Identify the products of the reaction between chlorine and cold, dilute aqueous sodium hydroxide.
Practise this questionQuestion
Question text
18 Chlorine reacts with cold, dilute, aqueous sodium hydroxide.
Which is a complete list of the products?
[1 mark]
A Sodium chloride, sodium chlorate(I) and water
B Sodium chlorate(I) and water
C Sodium chloride, sodium chlorate(V) and water
D Sodium chloride and sodium chlorate(I)
Mark scheme
Show the mark scheme
18 A
How to answer it
Reaction of Chlorine with Cold, Dilute Sodium Hydroxide
This question assesses your recall and understanding of Group 7 (Halogen) chemistry, specifically:
- The disproportionation reaction of chlorine in cold, dilute, aqueous sodium hydroxide.
- Correct identification of all chemical products (salts and water) formed in this reaction.
- Distinction between the cold, dilute condition and the hot, concentrated condition.
Question 18 Analysis
Multiple Choice (1 Mark)
✅ Correct Answer: A
Sodium chloride, sodium chlorate(I) and water
📐 Balanced Chemical Equation
The reaction taking place is:
Cl₂(aq) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)
Products formed:
- NaCl = Sodium chloride
- NaClO = Sodium chlorate(I) (active ingredient in household bleach)
- H₂O = Water
💡 Key Knowledge: Disproportionation
This is a classic disproportionation reaction (where an element is simultaneously oxidised and reduced):
- Cl in Cl₂: Oxidation state is 0.
- Cl in NaCl: Oxidation state is −1 (reduction).
- Cl in NaClO: Oxidation state is +1 (oxidation).
Notice that for this redox process to be balanced, both chloride and chlorate(I) must be formed alongside water.
🧠 Exam Technique: Temperature Conditions
Always highlight the temperature and concentration specified in the stem:
- Cold, dilute NaOH: Forms chlorate(I) → NaClO (Option A).
- Hot, concentrated NaOH: Chlorine disproportionates further into chlorate(V) → NaClO₃ :
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
Spotting "cold, dilute" instantly rules out Option C!
❌ Common Misconceptions & Distractor Traps
- Confusing Option C (Sodium chlorate(V)): Students often mix up the conditions. Chlorate(V) ( ClO₃⁻ ) only forms in hot, concentrated alkali.
- Forgetting by-products (Options B & D):
- Option B omits NaCl . The reaction is redox; chlorine cannot only be oxidised without also being reduced!
- Option D omits H₂O . Always verify that oxygen and hydrogen atoms from hydroxide ions end up in the products.
Topics
Inorganic Chemistry · 3.2.3 Group 7(17), The Halogens
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.