11.4 AS Level

The reactions of chlorine

Cambridge A-Level Chemistry (9701)  · Unit 11: Group 17  · 11 flashcards

The reactions of chlorine is topic 11.4 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 11 — Group 17 , alongside The chemical properties of the halogen elements and the hydrogen halides and Some reactions of the halide ions.

Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).

The deck below contains 11 flashcards — 1 definition and 10 key concepts — covering the precise wording mark schemes reward.  Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.

Some cards in this deck are on other syllabus topics: 12.1 Nitrogen and sulfur. Each card is labelled with its topic.

What the Cambridge 9701 syllabus says

2025-2027 syllabus, in our words · AS Level

What the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).

  1. Describe the reactions of chlorine with cold aqueous NaOH and with hot aqueous NaOH, interpret them using changes in oxidation number, and recognise both as disproportionation.
  2. Explain, with an equation, how chlorine is used to purify water, including how it forms the active species HOCl and ClO⁻ that kill bacteria.

Also in this deck: 12.1 Nitrogen and sulfur

  1. Explain why nitrogen is so unreactive, referring to how strong its triple bond is and to its lack of polarity.
  2. Describe and explain: (a) why ammonia is a base, using Brønsted-Lowry theory; (b) the shape and bonding of the ammonium ion, NH₄⁺, and how it forms in an acid-base reaction; (c) how an acid-base reaction displaces ammonia from ammonium salts.
  3. State and explain where oxides of nitrogen come from, both naturally and from human activity, and how catalysts remove them from internal combustion engine exhaust gases.
  4. Understand that atmospheric NO and NO₂ may combine with unburned hydrocarbons, forming peroxyacetyl nitrate (PAN), one of the substances in photochemical smog.
  5. Describe how NO and NO₂ contribute to acid rain, both directly and by catalysing the oxidation of sulfur dioxide in the atmosphere.

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

oxidation number disproportionation water purification active species

Tips to avoid common mistakes in these topics

Key Concept Flip

Topic 11.4 · The reactions of chlorine

Describe the reaction of chlorine with cold, dilute aqueous sodium hydroxide, including oxidation numbers.

Answer Flip

Chlorine disproportionates: Cl₂ + 2NaOH → NaClO + NaCl + H₂O. The oxidation number of Cl changes from 0 in Cl₂ to +1 in NaClO and -1 in NaCl.

Key Concept Flip

Topic 11.4 · The reactions of chlorine

Describe the reaction of chlorine with hot, concentrated aqueous sodium hydroxide, including oxidation numbers.

Answer Flip

Chlorine disproportionates: 3Cl₂ + 6NaOH → NaClO₃ + 5NaCl + 3H₂O. The oxidation number of Cl changes from 0 in Cl₂ to +5 in NaClO₃ and -1 in NaCl.

Key Concept Flip

Topic 11.4 · The reactions of chlorine

Explain how chlorine is used in water purification. Include the relevant equation(s).

Answer Flip

Chlorine kills bacteria in water. Cl₂ + H₂O ⇌ HOCl + HCl. HOCl ⇌ H⁺ + ClO⁻. Both HOCl (hypochlorous acid) and ClO⁻ (hypochlorite ion) are active disinfecting agents.

Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Explain why nitrogen is relatively unreactive.

Answer Flip

Nitrogen gas (N₂) is unreactive due to the strong triple bond (N≡N), which requires a large amount of energy to break. Additionally, the molecule is non-polar.

Definition Flip

Topic 12.1 · Nitrogen and sulfur

Explain the basicity of ammonia using the Brønsted-Lowry theory.

Answer Flip

Ammonia (NH₃) is a Brønsted-Lowry base because it accepts a proton (H⁺) from an acid.

Example: NH₃ + HCl → NH₄⁺ + Cl⁻.
Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Describe the structure of the ammonium ion (NH₄⁺) and its formation.

Answer Flip

The ammonium ion (NH₄⁺) has a tetrahedral shape with the nitrogen atom at the center. It forms when ammonia (NH₃) accepts a proton (H⁺) from an acid, using its lone pair of electrons to form a coordinate (dative covalent) bond.

Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Explain how ammonia can be displaced from ammonium salts.

Answer Flip

Ammonia can be displaced from ammonium salts by a stronger base.

Example: heating an ammonium salt with a strong base like NaOH: NH₄Cl(s) + NaOH(aq) → NH₃(g) + NaCl(aq) + H₂O(l)
Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Give examples of both natural and man-made occurrences of nitrogen oxides.

Answer Flip

Natural occurrences: Lightning strikes causing nitrogen and oxygen in the air to react. Man-made occurrences: Internal combustion engines and industrial processes at high temperatures.

Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Describe the catalytic removal of nitrogen oxides (NOx) from exhaust gases.

Answer Flip

In a catalytic converter, a catalyst such as platinum, palladium or rhodium helps nitrogen monoxide react with carbon monoxide in the exhaust gases: 2NO + 2CO → N₂ + 2CO₂. NO is reduced to nitrogen while CO is oxidised to carbon dioxide.

Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Describe how peroxyacetyl nitrate (PAN) is formed and its significance.

Answer Flip

PAN is formed when atmospheric nitrogen oxides (NO and NO₂) react with unburned hydrocarbons. It is a component of photochemical smog and a secondary pollutant, causing respiratory and eye irritation.

Key Concept Flip

Topic 12.1 · Nitrogen and sulfur

Explain the role of NO and NO₂ in the formation of acid rain.

Answer Flip

Directly: NO is oxidised in air to NO₂, and NO₂ reacts with water and oxygen to form nitric acid: 4NO₂ + O₂ + 2H₂O → 4HNO₃. Catalytically: NO₂ oxidises SO₂ to SO₃ (NO₂ + SO₂ → NO + SO₃), and the NO is reoxidised (NO + ½O₂ → NO₂), so the nitrogen oxides are regenerated. The SO₃ then forms sulfuric acid: SO₃ + H₂O → H₂SO₄.

More Chemistry flashcards

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11.3 Some reactions of the halide ions 13.1 Formulas, functional groups and the naming of organic compounds

More topics in Unit 11 — Group 17

The reactions of chlorine sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

11.2 The chemical properties of the halogen elements and the hydrogen halides

7 flashcards
11.3 Some reactions of the halide ions

10 flashcards

Key terms covered in this The reactions of chlorine deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

Explain the basicity of ammonia using the Brønsted-Lowry theory

How to study this The reactions of chlorine deck

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