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A-Level Chemistry May/June 2024 Q2(d)(i): When oxides of nitrogen escape into the atmosphere they may be involved in: formation o…
A-Level Chemistry · Paper 9701/23 · May/June 2024 · Question 2(d)(i) · [3 marks]
When oxides of nitrogen escape into the atmosphere they may be involved in: formation of acid rain from sulfur dioxide formation of photochemical smog. Identify the role of NO and NO2 in the formation of H2SO4 from SO2 in the atmosphere to produce acid rain. Use relevant equations to support your answer.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The role of oxides of nitrogen (NO and NO₂) in the formation of sulfuric acid from sulfur dioxide is as a homogeneous catalyst.
They speed up the oxidation of SO₂ to SO₃, which is the slow step in the uncatalysed reaction. The catalytic cycle is shown by the following equations:
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Nitrogen dioxide oxidises sulfur dioxide to sulfur trioxide, and is itself reduced to nitrogen monoxide.
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The nitrogen monoxide produced is then re-oxidised by oxygen in the atmosphere to regenerate the nitrogen dioxide catalyst.
Finally, the sulfur trioxide (SO₃) produced dissolves in atmospheric water droplets (rainwater) to form sulfuric acid (H₂SO₄), which contributes to acid rain.
How the marks are awarded
- M1 — Correctly identifying the role of the nitrogen oxides as a 'catalyst'.
- M2 — Providing two correct, balanced equations to show the catalytic cycle. This includes the oxidation of SO₂ by NO₂ (SO₂ + NO₂ → SO₃ + NO) and the regeneration of NO₂ from NO (2NO + O₂ → 2NO₂).
- M3 — Stating or showing in an equation that sulfur trioxide reacts with water to form sulfuric acid (SO₃ + H₂O → H₂SO₄).
Common mistakes
- Forgetting to state the role of the nitrogen oxides as a 'catalyst', which is the first mark.
- Writing incorrect or unbalanced equations for the catalytic cycle, for example, mixing up the reactants or products (e.g., SO₃ + NO₂ → ...) or using incorrect stoichiometry (e.g., NO + O₂ → NO₃).
- Omitting the final step where sulfur trioxide reacts with water to form sulfuric acid. The question asks for the formation of H₂SO₄, not just SO₃.
- Attempting to combine the steps into one overall equation without showing the individual catalytic steps, which fails to demonstrate how the catalyst functions.
Examiner tip: For environmental chemistry topics, memorise the key catalytic cycles, ensuring you can write the individual balanced equations for each step and identify how the catalyst is regenerated.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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