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A-Level Chemistry May/June 2025 Q2(c): Sulfur dioxide, SO2, reacts very slowly with oxygen in the atmosphere, forming sulfur t…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 2(c) · [3 marks]
Sulfur dioxide, SO2, reacts very slowly with oxygen in the atmosphere, forming sulfur trioxide, SO3. This reaction is much faster in the presence of NO. Explain the role of NO in this process. Include chemical equations in your answer.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
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NO acts as a homogeneous catalyst in this reaction because it is in the same phase (gas) as the reactants and is regenerated at the end of the process. It provides an alternative reaction pathway with a lower activation energy.
The mechanism occurs in two steps:
Step 1: Nitrogen monoxide is oxidised by atmospheric oxygen to form nitrogen dioxide.
Step 2: The nitrogen dioxide intermediate then oxidises sulfur dioxide to sulfur trioxide, regenerating the nitrogen monoxide catalyst.
How the marks are awarded
- M1 — Correctly identifying the role of NO as a homogeneous catalyst, or stating that it is a catalyst because it is regenerated at the end of the reaction.
- M2 — Providing the correct, balanced chemical equation for the oxidation of nitrogen monoxide by oxygen to form nitrogen dioxide.
- M3 — Providing the correct chemical equation for the reaction between nitrogen dioxide and sulfur dioxide, showing the formation of the product (SO3) and regeneration of the catalyst (NO).
Common mistakes
- Writing the overall equation for the formation of sulfur trioxide () instead of the two specific steps of the catalytic cycle involving NO.
- Confusing the roles of NO and NO2, for example by writing that NO reacts directly with SO2 in the first step.
- Incorrectly balancing the equations, or using incorrect species (e.g., N2O instead of NO2).
- Stating that NO is a heterogeneous catalyst, or failing to explain that it is regenerated if only identifying it as a 'catalyst'.
Examiner tip: For catalysis topics, memorise the specific reaction steps and equations for key named processes, such as the role of NOx in acid rain formation or the use of V2O5 in the Contact process.
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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