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A-Level Chemistry May/June 2025 Q3(a)(ii): Describe how photochemical smog is produced during the combustion of petrol in an inter…
A-Level Chemistry · Paper 9701/23 · May/June 2025 · Question 3(a)(ii) · [2 marks]
Describe how photochemical smog is produced during the combustion of petrol in an internal combustion engine.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
During the combustion of petrol in an internal combustion engine, not all of the fuel is burned completely. These unburned hydrocarbons are released into the atmosphere via the exhaust.
In the presence of sunlight, these volatile organic compounds react with oxides of nitrogen (NO and NO₂), which are also formed in the high temperature and pressure conditions of the engine. This complex series of photochemical reactions produces secondary pollutants, including peroxyacetyl nitrate (PAN), a key component of photochemical smog.
How the marks are awarded
- M1 — The first mark is awarded for identifying the role of unburned hydrocarbons, as stated in the first sentence: '...unburned hydrocarbons are released...'
- M2 — The second mark is awarded for stating that these hydrocarbons react with nitrogen oxides, as written in the second paragraph: '...react with oxides of nitrogen (NO and NO₂)...'
- M3 — A mark is also available for naming a specific product of the reaction, which is a component of photochemical smog, as mentioned in the final sentence: '...produces peroxyacetyl nitrate (PAN)...'. Note: Any two of the three available marks (M1, M2, M3) are required to score the full 2 marks for this question.
Common mistakes
- Confusing the pollutants involved, for example, stating that sulfur dioxide (SO₂) or carbon monoxide (CO) react to form smog, instead of unburned hydrocarbons and NOx.
- Only mentioning one of the necessary reactants, e.g., only stating that nitrogen oxides cause smog without mentioning the essential role of unburned hydrocarbons.
- Being too vague, for example, writing 'exhaust fumes react in the air' without naming the specific chemical species required by the mark scheme.
- Forgetting to name a specific product of the reaction, such as PAN, and instead just stating 'smog is formed'.
Examiner tip: For environmental chemistry questions, memorise the specific chemical names of the primary pollutants (e.g., NOx, unburned hydrocarbons) and the secondary pollutants they form (e.g., PAN).
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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