Compare nitrogen and sulfur atmospheric pathways
Distinguish NOₓ sources, PAN formation, direct nitric-acid formation and catalytic sulfuric-acid formation.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Explain the stated molecular, acid–base and atmospheric reaction pathways; balance equations and distinguish direct reactions from catalytic cycles. Written practice is self-checked
Before you begin
- Use covalent bond order, lone pairs, Brønsted–Lowry proton transfer, oxidation numbers and balanced equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which clue points most directly to PAN formation?
What you will learn
- Select the correct pathway from a pollutant scenario
- Distinguish primary sources from secondary products
- Compare direct and catalytic acid-forming routes
Separate source, precursor and secondary product
High-temperature combustion can release NO; oxidation produces NO₂. The NOₓ can contribute to direct nitric-acid formation or participate in sunlight-driven reactions with unburned hydrocarbons to form PAN.
SO₂ can be oxidised through NO₂/NO cycling to SO₃, then hydrated to H₂SO₄. These are connected atmospheric pathways, but PAN, nitric acid and sulfuric acid are different products.
Evidence determines the pathway
Look for the key reactants and conditions: sunlight plus unburned hydrocarbons points to PAN; NO₂ plus water points to direct nitric acid; SO₂ with NO₂ and oxygen points to catalytic sulfur oxidation.
A catalyst is regenerated in the cycle. A direct equation may consume NO₂ in one step, though further atmospheric reactions can return NO to NO₂. Avoid treating all acid rain or smog as a single reaction.
Pause and explain
Which sequence is the catalytic sulfur-oxide route?
Worked example
A city sample contains NO₂, unburned hydrocarbons and SO₂. Name one possible smog product and outline a separate sulfuric-acid route.
Show the worked solution
- Sunlight-driven reactions of NOₓ and hydrocarbons can form PAN.
- SO₂ can react with NO₂ to form SO₃ and NO.
- NO is oxidised back to NO₂; SO₃ reacts with water to form H₂SO₄.
Answer PAN may form in photochemical smog. Separately, NO₂ can oxidise SO₂ to SO₃ and NO; O₂ regenerates NO₂, and SO₃ forms H₂SO₄ with water.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Match sunlight plus hydrocarbons, NO₂ plus water, and SO₂ plus cycling NOₓ to their products.
Give me a hint
Use the distinctive reactants in each pathway.
Compare my reasoning
- Sunlight + NOₓ + hydrocarbons can form PAN.
- NO₂ + H₂O can form HNO₃ and NO.
- SO₂ + NO₂/O₂ cycling forms SO₃, then H₂SO₄.
The products are PAN; nitric acid; and sulfuric acid via SO₃, respectively.
Look for these in your work
- PAN pathway matched
- Direct nitric-acid path matched
- Catalytic sulfur route matched
Explain the difference between a primary exhaust pollutant and PAN.
Give me a hint
Consider where PAN forms and what drives its formation.
Compare my reasoning
- NO can form during high-temperature combustion.
- PAN forms through subsequent atmospheric photochemical reactions.
- PAN is a secondary pollutant and one component of smog.
NO is emitted from combustion; PAN is a secondary atmospheric product formed from NOₓ and hydrocarbons in sunlight.
Look for these in your work
- Source product distinguished
- Atmospheric chemistry included
- Secondary status stated
Evaluate the claim: 'NO₂ is consumed in acid rain and therefore can never be a catalyst.'
Give me a hint
Compare the direct water reaction with its separate SO₂ cycle.
Compare my reasoning
- In a direct nitric-acid step, NO₂ reacts with water and forms acid and NO.
- In the sulfur route, NO₂ oxidises SO₂ to NO, then O₂ regenerates NO₂.
- The catalytic description applies to the complete regenerated cycle, not every individual reaction.
NO₂ can be consumed in a direct step yet regenerated in the SO₂ oxidation cycle; the complete latter pathway is catalytic.
Look for these in your work
- Direct route distinguished
- Regeneration step stated
- Claim qualified
Common mistakes
- Treating PAN as an acid-rain product
- Omitting sunlight from the photochemical context
- Calling one-step direct acid formation catalytic without regeneration
What can you explain now?
Which clue points most directly to PAN formation?
Compare with the explanation
Sunlight with NOₓ and unburned hydrocarbons
Those are the stated photochemical precursors.
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Make it stick
Which reactants distinguish PAN formation from the catalytic oxidation of SO₂?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.