OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 22
11 marks · Medium difficulty · Structured Questions
Explain disproportionation using oxidation numbers in bromine's reaction with alkali, explain why chlorine is more reactive than bromine, state a risk of water chlorination, and describe qualitative tests to identify halide ions.
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Question text
22 This question is about halogens and their compounds.
(a) An aqueous solution of bromine reacts with cold aqueous sodium hydroxide in a
disproportionation reaction.
The equation is shown below.
Br2(aq) + 2NaOH(aq) NaBr(aq) + NaBrO(aq) + H2O(l)
Explain what is meant by the term disproportionation.
Use the equation above and oxidation numbers.
… [3]
(b) An aqueous solution of chlorine also reacts with cold, aqueous sodium hydroxide.
The reaction is more vigorous than the reaction of Br2(aq) with NaOH(aq) in (a).
(i) Explain why chlorine reacts more vigorously than bromine.
… [3]
(ii) Chlorine is used in water treatment.
State one disadvantage of treating water with chlorine.
… [1]
(c) A group of students is provided with aqueous solutions of sodium chloride, sodium bromide and
sodium iodide in three unlabelled bottles.
Explain how the students can identify which bottle contains each aqueous solution using
precipitation reactions and the solubility of any precipitate formed.
Include an ionic equation in your answer.
… [4]
Mark scheme
Show the mark scheme
Question Answer Mark Guidance
22 (a) Disproportionation ‘Atom’ is insufficient for element
Oxidation and reduction of the same element/bromine 3
OR
Bromine has been oxidised and bromine has been
reduced
ALLOW 1 out of 2 redox marks if NaBrO AND NaBr
Oxidation numbers and redox omitted, i.e.
Br is oxidised from 0 (in Br2) to +1 in NaBrO Br is oxidised from 0 to +1 AND
Br is reduced from 0 to –1
Br is reduced from 0 (in Br2) to –1 in NaBr
ALLOW 1 out of 2 redox marks if oxidation number
IGNORE oxidation numbers shown in equation changes are BOTH correct
(treat as rough working) ...BUT reduction/oxidation is incorrectly assigned, i.e.
BUT Br is reduced from 0 (in Br2) to +1 in NaBrO
If no oxidation numbers in explanation, or if oxidation Br is oxidised from 0 (in Br2) to –1 in NaBr
numbers are not linked to substances, look at equation for
annotations which may be worthy of credit ALLOW 1 out of 2 redox marks for 3 oxidation
numbers correct but no mention of words
oxidation/reduction: e.g.
0 in Br2 AND –1 in NaBr AND +1 in NaBrO
---------------------------------------------------------------
General:
ALLOW number before sign in ox no,
i.e. 1+ for +1 1– for –1
IGNORE ionic charges, e.g. Br1+
IGNORE ‘1’ (signs required)
IGNORE ‘+’ and ‘-‘ (numbers required)
IGNORE references to electron loss/gain
(even if wrong)
Question Answer 13 Mark Guidance
22 (b) (i) Atomic radius 3 Comparison required throughout
Cl has a smaller atomic radius (than Br) ORA throughout
OR Cl has fewer shells ALLOW ‘down the group’ if the group is clearly
OR Cl has less shielding ✓ identified as group 7/halogens
For fewer shells, ALLOW lower energy level
IGNORE fewer orbitals OR fewer sub-shells
IGNORE ‘different shell’
Attraction
Nuclear attraction is more in Cl ALLOW Cl has more nuclear pull
OR (outer) electrons in Cl are more attracted (to
nucleus) IGNORE more effective nuclear charge
OR Decreased distance / shielding in Cl outweighs IGNORE ‘nuclear charge’ for nuclear attraction
decreased nuclear charge ✓
Ease of gaining electron ALLOW easier to reduce chlorine
Easier to gain an electron in Cl ✓
IGNORE comments about energy/electronegativity
22 (b) (ii) toxic/poisonous 1 IGNORE ‘harmful’/’dangerous’
OR
(forms) chlorinated/halogenated hydrocarbons IGNORE chlorine is carcinogenic/causes cancer
OR dangerous for health/causes breathing problems
(forms) carcinogenic compounds / toxic compounds
IGNORE forms HCl
22 (c) Identification of halide 4 ANNOTATE ANSWER WITH TICKS AND
Add (aqueous) silver nitrate OR AgNO3 CROSSES
OR Ag+/silver ions IGNORE addition of HNO but HCl/H SO is a CON
32 4
for AgNO3
IGNORE addition of Ba(NO3)2 but BaCl2 is a CON for
AgNO3
Observations – mark independently ALLOW Cl/chlorine for chloride, etc. (credit is for the
Any 2 precipitate colours from colours and precipitate)
Chloride/Cl– gives white precipitate ALLOW ppt/ptte for precipitate
Bromide/Br– gives cream precipitate
Iodide/I– gives yellow precipitate
Precipitate/solid seen at least once
Correct equation for at least one halide ALLOW equation with Br– OR I–
e.g. Ag+ + Cl– → AgCl e.g. Ag+ + Br– → AgBr
ALLOW Ag+ + X– →AgX
IGNORE state symbols (ppt already assessed)
Solubility ALLOW ‘ammonia’ or NH3 for NH3(aq)
AgCl/precipitate dissolves in dilute NH3(aq)
AND Answer needs to be clear it is the solid/AgX which is
AgBr/precipitate dissolves in concentrated NH3(aq), dissolving
AND
AgI/precipitate does not dissolve (in concentrated NH3(aq))
How to answer it
Group 7 Halogens: Disproportionation, Reactivity & Halide Analysis
This 11-mark question evaluates core Periodicity and Redox chemistry from the OCR Chemistry A specification:
- Disproportionation & Oxidation Numbers: Defining disproportionation precisely in terms of the same element, calculating oxidation states of halogens in elemental form and oxy-anions, and linking them to redox processes.
- Group 7 Reactivity Trends: Explaining oxidising ability and reactivity down Group 7 using atomic radius, electron shielding, and attraction to incoming electrons.
- Water Treatment: Knowing the hazards/risks of chlorine treatment.
- Inorganic Qualitative Analysis: The step-by-step test for aqueous halide ions using silver nitrate followed by dilute and concentrated aqueous ammonia, including ionic equations.
Disproportionation Reaction of Bromine with Cold Alkali
Explaining disproportionation using oxidation numbers and chemical species
✅ Model Answer
Definition: Disproportionation is the simultaneous oxidation and reduction of the same element (bromine).
Oxidation number changes:
- Bromine is oxidised from 0 in Br₂ to +1 in NaBrO
- Bromine is reduced from 0 in Br₂ to -1 in NaBr
🧠 Exam Technique: Securing 3/3 Marks
- Mark 1: Mention that the same element / bromine is both oxidised and reduced. Saying "the same atom" or "molecule" loses this mark.
- Mark 2: State both the initial (0) and final (+1) oxidation number, explicitly named with NaBrO .
- Mark 3: State both the initial (0) and final (-1) oxidation number, explicitly named with NaBr .
- Write oxidation states with the sign first (e.g. +1, not 1+ which is an ionic charge).
❌ Common Errors to Avoid
- Vague definitions: Stating "a reaction where reduction and oxidation happen together" only describes redox in general, not disproportionation.
- Floating numbers: Writing down "0, +1, -1" without explicitly linking which number belongs to which compound (e.g., stating "Br goes from 0 to +1" without mentioning NaBrO limits you to 1/2 for the redox marks).
- Confusing charge with oxidation state: Writing Br¹⁻ or Br¹⁺ instead of -1 and +1.
[1] Oxidation and reduction of the same element / bromine.
[1] Br is oxidised from 0 (in Br₂) to +1 in NaBrO.
[1] Br is reduced from 0 (in Br₂) to -1 in NaBr.
Reactivity Trend: Chlorine vs Bromine
Explaining why chlorine reacts more vigorously than bromine
✅ Model Answer
- Chlorine has a smaller atomic radius / fewer electron shells / less electron shielding than bromine.
- There is a stronger nuclear attraction in chlorine for the incoming electron (the smaller distance/less shielding outweighs the smaller nuclear charge).
- Therefore, it is easier for chlorine to gain an electron (chlorine is a stronger oxidising agent).
💡 Key Knowledge: Halogen Reactivity
Halogens react by gaining an electron to form 1- halide ions (acting as oxidising agents):
X₂ + 2e⁻ → 2X⁻
Down Group 7:
- Atomic radius increases and shielding increases.
- Nuclear attraction to incoming electrons weakens.
- Reactivity and oxidising power decrease down the group.
❌ Common Examiner Traps
- Failing to compare: Saying "chlorine has 3 shells" without contrasting it to bromine (or clearly saying "fewer shells") loses the mark. A direct comparison is required.
- Confusing with Group 2 / Metals: Talking about the ease of losing electrons. Halogens gain electrons!
- Confusing 'nuclear charge' with 'nuclear attraction': Bromine has more protons, so bromine has a higher nuclear charge. Chlorine has stronger nuclear attraction because distance and shielding dominate over charge.
[1] Cl has smaller atomic radius OR fewer shells OR less shielding (comparison required).
[1] Nuclear attraction is greater in Cl OR outer electrons in Cl are more attracted to the nucleus.
[1] Easier to gain an electron in Cl (or easier to reduce Cl).
Chlorine in Water Treatment
State one disadvantage of treating drinking water with chlorine
✅ Acceptable Answers (Give ANY ONE)
- Chlorine gas is toxic / poisonous.
- Forms chlorinated hydrocarbons / haloalkanes / toxic organochlorine compounds (by reaction with organic matter in water).
- Forms carcinogenic compounds / carcinogens.
❌ Insufficient / Rejected Answers
- "Chlorine is harmful / dangerous": Far too vague; must specify toxic or poisonous.
- "Chlorine causes cancer": Chlorine itself is not proven to be carcinogenic; it forms carcinogenic compounds with dissolved organic material.
- "Forms HCl": Acidification/HCl formation alone is rejected.
[1] Chlorine is toxic/poisonous OR forms chlorinated hydrocarbons OR forms carcinogenic/toxic compounds.
Identifying Halide Ions (Cl⁻, Br⁻, I⁻)
Precipitation reactions, solubility in ammonia, and ionic equations
✅ Complete Full-Mark Procedure & Observations
Step 1: Precipitation Test
Add aqueous silver nitrate, AgNO₃(aq) , to each solution:
| Halide Solution | Precipitate Formed | Colour |
|---|---|---|
| Sodium chloride, NaCl(aq) | Silver chloride, AgCl(s) | White precipitate |
| Sodium bromide, NaBr(aq) | Silver bromide, AgBr(s) | Cream precipitate |
| Sodium iodide, NaI(aq) | Silver iodide, AgI(s) | Yellow precipitate |
Step 2: Solubility in Aqueous Ammonia ( NH₃(aq) )
- AgCl (white ppt): Dissolves in dilute NH₃(aq).
- AgBr (cream ppt): Insoluble in dilute NH₃(aq), but dissolves in concentrated NH₃(aq).
- AgI (yellow ppt): Insoluble in both dilute and concentrated NH₃(aq).
Ionic Equation (Give any one):
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
(or Ag⁺ + Br⁻ → AgBr | Ag⁺ + I⁻ → AgI | Ag⁺ + X⁻ → AgX)
🧠 Exam Strategy: The 4 Marks
- Reagent (1 mark): Explicitly name aqueous silver nitrate or write AgNO₃ / Ag⁺ .
- Colours (1 mark): Correctly match at least two precipitate colours (White = Cl⁻, Cream = Br⁻, Yellow = I⁻) and use the word precipitate or solid at least once.
- Ionic Equation (1 mark): Include correct charges and formulas. State symbols are not penalised here, but writing (aq) and (s) is best practice.
- Solubility Confirmatory Test (1 mark): Clearly specify both dilute and concentrated NH₃, and state what dissolves vs what remains insoluble.
❌ Dangerous Contaminant Errors
- Acidifying with HCl or H₂SO₄: In qualitative analysis, nitric acid (HNO₃) is used to acidify. If you mention adding HCl or H₂SO₄ , it is treated as a CONTRADICTION (CON) because chloride/sulfate ions will form their own precipitates with Ag⁺, invalidating the test.
- Vague ammonia solubility: Writing "one dissolves and one doesn't" gets zero. You must mention dilute NH₃ for AgCl and concentrated NH₃ for AgBr.
- Missing the precipitate state: Writing just "turns white, cream, yellow" without stating that a precipitate (solid) is formed.
[1] Add aqueous silver nitrate / AgNO₃ / Ag⁺.
[1] Any 2 precipitate colours: Cl⁻ = white, Br⁻ = cream, I⁻ = yellow (word 'precipitate'/'solid' seen at least once).
[1] Ionic equation: Ag⁺ + Cl⁻ → AgCl (or with Br⁻/I⁻/X⁻).
[1] Solubility: AgCl dissolves in dilute NH₃ AND AgBr dissolves in concentrated NH₃ AND AgI insoluble in concentrated NH₃.
Topics
Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Practical Activity Groups · 2.1 Atoms and reactions · 3.1 The periodic table · PAG 4: Qualitative analysis of ions
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.