AQA A-Level Chemistry AS Paper 1, June 2025: Question 2
14 marks · Medium difficulty · State/Explain/Describe
Explain the trends and reactions of the halogens and halides, including intermolecular forces, reactions of chlorine with water and alkali, redox half-equations, and reactions of halides with concentrated sulfuric acid.
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Question text
02 This question is about the halogens and their compounds.
02.1 Table 1 shows the boiling points of some halogens.
Table 1
Halogen Boiling point / °C
Fluorine –188
Chlorine –35
Bromine 59
Explain the trend in the boiling points of these halogens.
[2 marks]
02.2 Chlorine is toxic but is added to drinking water.
State why chlorine is added to drinking water.
Explain why this is safe to do even though chlorine is toxic.
[2 marks]
Why chlorine is added
Why this is safe
02.3 Give an equation for the reaction of chlorine with water in sunlight.
[1 mark]
02 *.054Give the equation for the reaction between NaOH and C*l2 to form NaClO
[1 mark]
02.5 When gently warmed in aqueous solution, NaClO reacts to form NaCl and NaClO3
Complete the equation to ensure that it is correctly balanced.
Deduce the oxidation state of Cl in NaClO3
[2 marks]
NaClO ⟶ NaCl + NaClO3
Oxidation state of Cl in NaClO3
02.6 Deduce the half-equation, in acidic conditions, for the conversion of ClO – into Cl–
[1 mark]
02.7 A halide ion X– reduces concentrated sulfuric acid to hydrogen sulfide.
Identify the halide ion.
Give an equation for the reaction.
[2 marks]
Halide ion
Equation
02.8 Some solid sodium halides react with concentrated sulfuric acid without any
redox reaction taking place.
Identify a sodium halide that behaves in this way.
Give an equation for the reaction.
State the role of the halide ion in this reaction.
[3 marks]
Sodium halide
Equation
Role of halide ion
Mark scheme
Show the mark scheme
Question Marking guidance Additional Comments/Guidelines Mark
(down the group)
If the trend is stated and is incorrect then cannot
M1: (molecules are) larger / (molecules contain) more electrons / score the next mark gained.
(molecules) have a larger surface area 2
02.1
(2 x AO1)
M2: stronger van der Waals’ forces between molecules / stronger
temporary induced dipole / stronger dispersion forces between
molecules / stronger London forces between molecules
Kill bacteria / to prevent water-borne diseases / to prevent people Accept any named bacterial (water-borne)
getting cholera / to sterilise water infection. 2
02.2
(2 x AO1)
Small enough concentration (to avoid harming humans)
+ –
allow 4H + 4Cl
allow multiples 1
02.3 2Cl2 + 2H2O → 4HCl + O2 ignore state symbols
– AS CHEMISTRY – 7404/1 – (1 x AO1)
allow multiples
ignore state symbols 1
02.4 2NaOH + Cl2 → NaClO + NaCl + H2O
(1 x AO1)
M1: 3NaClO → 2NaCl + NaClO3 allow multiples
allow 1 NaClO3 2
02.5
M2: Oxidation state = (+)5 (2 x AO2)
allow multiples
– + – – ignore state symbols 1
02.6 ClO3 + 6H + 6e → Cl + 3H2O
– AS CHEMISTRY – 7404/1 – (1 x AO2)
I– or iodide allow astatide
allow multiples
02.7 (1 x AO1,
+ – allow 8 HI 1 x AO2)
H2SO4 + 8H + 8I → H2S + 4H2O + 4I2
+ 2– –
allow 10 H + SO4 + 8I → H2S + 4H2O + 4I2
(Sodium) fluoride or (sodium) chloride 2–
allow equations to form SO4 ions
NaCl + H2SO4 → HCl + NaHSO4 2 NaCl + H2SO4 → 2 HCl + Na2SO4
OR OR
(1 x AO1,
02.8
1 x AO2, 1
NaF + H2SO4 → HF + NaHSO4 2 NaF + H2SO4 → 2 HF + Na2SO4
x AO3)
allow ionic equations
Base / proton acceptor
How to answer it
Halogens, Disproportionation & Redox Reactions
This question comprehensively tests your understanding of AQA Group 7 (Halogens) chemistry:
- Intermolecular forces: Explaining trends in boiling points across simple diatomic molecules.
- Chlorine water treatment: Benefits vs. risks of chlorination.
- Disproportionation reactions: Chlorine reacting with water in sunlight vs. cold aqueous NaOH, and thermal decomposition of chlorate(I).
- Redox balancing: Deducing oxidation numbers and constructing half-equations in acidic media.
- Reducing ability of halide ions: Reactions of solid sodium halides with concentrated sulfuric acid (acid-base vs. redox).
Part 02.1: Boiling Point Trend of Halogens
Physical properties and intermolecular forces [2 Marks]
✅ Model Answer
M1: As you go down the group, the molecules become larger / contain more electrons (or have a larger surface area).
M2: This leads to stronger van der Waals' forces (induced dipole-dipole forces) between molecules, requiring more thermal energy to overcome.
🧠 Exam Technique
Always state where the forces act: between molecules. Never say bonds between atoms break!
If you state an incorrect trend initially (e.g. "boiling points decrease"), examiners are instructed not to award M2.
❌ Common Errors
- Claiming that "covalent bonds get stronger" or that "covalent bonds are broken when boiling". Halogens are simple molecular structures!
- Forgetting to explicitly mention electrons when explaining why van der Waals' forces increase down Group 7.
Part 02.2: Chlorination of Drinking Water
Benefits vs. risks of water treatment [2 Marks]
✅ Model Answer
Why chlorine is added: To kill bacteria / microorganisms (or to prevent water-borne diseases like cholera / sterilise water).
Why this is safe: It is used in very small / low concentrations (so it is not harmful to humans).
💡 Key Knowledge
The health benefits of clean water far outweigh the toxic risk posed by chlorine because the dose is tightly controlled at harmless trace levels.
Parts 02.3 & 02.4: Chlorine Reactions in Aqueous Media
Reactions with water in sunlight and cold dilute NaOH [2 Marks total]
✅ 02.3: Reaction with Water in Sunlight [1 Mark]
2Cl₂ + 2H₂O → 4HCl + O₂
Also allowed: 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂
Note: In sunlight, HClO decomposes, so oxygen gas is produced instead of establishing an equilibrium with HClO.
✅ 02.4: Reaction with Cold Aqueous NaOH [1 Mark]
2NaOH + Cl₂ → NaClO + NaCl + H₂O
This forms bleach (sodium chlorate(I)). It is an essential disproportionation reaction where chlorine is both oxidised (to +1 in NaClO) and reduced (to -1 in NaCl).
❌ Common Errors
- Confusing the sunlight reaction with the reversible reaction in the dark ( Cl₂ + H₂O ⇌ HCl + HClO ). Sunlight drives the formation of O₂!
- Forgetting that both NaCl and H₂O are products in the reaction with cold NaOH.
Parts 02.5 & 02.6: Chlorate Decomposition & Redox Half-Equation
Balancing equations and oxidation states [3 Marks total]
📐 02.5: Equation & Oxidation State [2 Marks]
Balanced Equation (M1):
3 NaClO → 2 NaCl + 1 NaClO₃
Oxidation State of Cl in NaClO₃ (M2):
• Na = +1, O = -2 (×3 = -6)
• (+1) + Cl + (-6) = 0 ⇒ Cl = +5 (or 5)
📐 02.6: Acidic Half-Equation for ClO₃⁻ → Cl⁻ [1 Mark]
Follow the standard 4-step redox method:
- Balance Cl atoms: Already balanced (1 on each side).
- Balance O with H₂O: Add 3H₂O to RHS ⇒ ClO₃⁻ → Cl⁻ + 3H₂O
- Balance H with H⁺: Add 6H⁺ to LHS ⇒ ClO₃⁻ + 6H⁺ → Cl⁻ + 3H₂O
- Balance charge with e⁻: LHS is +5, RHS is -1. Add 6e⁻ to LHS:
ClO₃⁻ + 6H⁺ + 6e⁻ → Cl⁻ + 3H₂O
Parts 02.7 & 02.8: Solid Sodium Halides + Conc. H₂SO₄
Trends in reducing ability of halide ions [5 Marks total]
✅ 02.7: Reduction to H₂S [2 Marks]
Halide ion: I⁻ (or iodide / astatide)
Balanced equation:
H₂SO₄ + 8H⁺ + 8I⁻ → H₂S + 4H₂O + 4I₂
Alternative valid forms:
H₂SO₄ + 8HI → H₂S + 4H₂O + 4I₂
SO₄²⁻ + 10H⁺ + 8I⁻ → H₂S + 4H₂O + 4I₂
✅ 02.8: Non-Redox Reaction [3 Marks]
Sodium halide: Sodium fluoride (NaF) OR sodium chloride (NaCl)
Equation:
NaCl + H₂SO₄ → HCl + NaHSO₄
(or: 2NaCl + H₂SO₄ → 2HCl + Na₂SO₄)
(Equivalent equations with NaF are fully accepted)
Role of halide ion: Base / proton acceptor
💡 Key Knowledge: Reducing Power Trend
- F⁻ and Cl⁻: Weak reducing agents. Cannot reduce concentrated H₂SO₄. Only an acid-base reaction occurs, producing steamy fumes of HF or HCl. The halide acts purely as a base (accepting a proton: X⁻ + H⁺ → HX).
- Br⁻: Moderate reducing agent. Reduces H₂SO₄ (S is +6) down to SO₂ (S is +4).
- I⁻: Strongest reducing agent. Reduces H₂SO₄ past SO₂ and S, down to toxic H₂S (S is -2, a total change of -8 in oxidation state!).
❌ Common Errors in 02.7 & 02.8
- Writing "iodine" / I₂ instead of the halide ion ( I⁻ or iodide).
- Miscounting electrons when balancing H₂SO₄ → H₂S. Sulfur goes from +6 to -2, which requires 8 electrons (hence 8I⁻ are needed).
- Stating the role of the halide ion in 02.8 is an "acid" or "reducing agent". In NaCl + H₂SO₄, no oxidation states change; Cl⁻ accepts H⁺, so it is acting as a base!
Mark Scheme 02.8: NaF or NaCl (1) | Correct equation (1) | Base / proton acceptor (1)
Topics
Inorganic Chemistry · Physical Chemistry · 3.2.3 Group 7(17), The Halogens · 3.1.3 Bonding · 3.1.7 Oxidation, Reduction and Redox Equations
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.