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. In one line: Ammonia (NH₃) is a Brønsted-Lowry base because it accepts a proton (H⁺) from an acid.
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.
Explain the basicity of ammonia using the Brønsted-Lowry theory
Ammonia (NH₃) is a Brønsted-Lowry base because it accepts a proton (H⁺) from an acid.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe and interpret, in terms of changes in oxidation number, the reaction of chlorine with cold and with hot aqueous sodium hydroxide and recognise these as disproportionation reactions
- explain, including by use of an equation, the use of chlorine in water purification to include the production of the active species HOCl and Cl O– which kill bacteria
- explain the lack of reactivity of nitrogen, with reference to triple bond strength and lack of polarity
- describe and explain: (a) the basicity of ammonia, using the Brønsted–Lowry theory (b) the structure of the ammonium ion and its formation by an acid–base reaction (c) the displacement of ammonia from ammonium salts by an acid–base reaction
- state and explain the natural and man-made occurrences of oxides of nitrogen and their catalytic removal from the exhaust gases of internal combustion engines
- understand that atmospheric oxides of nitrogen (NO and NO2) can react with unburned hydrocarbons to form peroxyacetyl nitrate, PAN, which is a component of photochemical smog
- describe the role of NO and NO2 in the formation of acid rain both directly and in their catalytic role in the oxidation of atmospheric sulfur dioxide
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.
Tips to avoid common mistakes in The reactions of chlorine
- › Memorize the balanced equation for the decomposition of Group 2 nitrates: 2M(NO3)2 → 2MO + 4NO2 + O2.
- › Practice balancing equations for the thermal decomposition of Group 2 nitrates, ensuring the 2:2:4:1 molar ratio is maintained.
- › Distinguish between reactions that produce gas (like metals with acid) and those that simply dissolve to form a solution.
- › Apply the specific equation: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O to determine the correct molar yield of products.
- › Write out the full balanced equation for precipitation: Al2Cl6 + 6NaOH → 2Al(OH)3 + 6NaCl to find the correct molar ratio y/x.
Describe the reaction of chlorine with cold, dilute aqueous sodium hydroxide, including oxidation numbers.
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.
Describe the reaction of chlorine with hot, concentrated aqueous sodium hydroxide, including oxidation numbers.
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.
Explain how chlorine is used in water purification. Include the relevant equation(s).
Chlorine kills bacteria in water. Cl₂ + H₂O ⇌ HOCl + HCl. HOCl ⇌ H⁺ + ClO⁻. Both HOCl (hypochlorous acid) and ClO⁻ (hypochlorite ion) are active disinfecting agents.
Explain why nitrogen is relatively unreactive.
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.
Explain the basicity of ammonia using the Brønsted-Lowry theory.
Ammonia (NH₃) is a Brønsted-Lowry base because it accepts a proton (H⁺) from an acid.
Describe the structure of the ammonium ion (NH₄⁺) and its formation.
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.
Explain how ammonia can be displaced from ammonium salts.
Ammonia can be displaced from ammonium salts by a stronger base.
Give examples of both natural and man-made occurrences of nitrogen oxides.
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.
Describe the catalytic removal of nitrogen oxides (NOx) from exhaust gases.
Catalytic converters use a catalyst (
Describe how peroxyacetyl nitrate (PAN) is formed and its significance.
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.
Explain the role of NO and NO₂ in the formation of acid rain.
NO and NO₂ dissolve in atmospheric moisture, forming nitrous and nitric acids, contributing directly to acid rain. They also act as catalysts in the oxidation of SO₂ to SO₃, which then forms sulfuric acid (H₂SO₄), a major component of acid rain.
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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.
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