Explain catalytic oxidation of sulfur dioxide by nitrogen oxides
Trace the NO/NO₂ cycle that catalyses atmospheric SO₂ oxidation and leads to sulfuric acid.
Section 12 · Nitrogen and sulfur
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
What sulfur product is formed when NO₂ oxidises SO₂ in the first step?
What you will learn
- Write the SO₂ oxidation step
- Show regeneration of NO₂
- Connect sulfur trioxide to sulfuric acid
Nitrogen dioxide transfers oxygen to sulfur dioxide
Nitrogen dioxide can oxidise sulfur dioxide: SO₂ + NO₂ → SO₃ + NO. Sulfur is oxidised from +4 to +6 while nitrogen is reduced from +4 to +2.
The NO formed can be oxidised by atmospheric oxygen: 2NO + O₂ → 2NO₂. NO₂ is regenerated, so NO and NO₂ can act catalytically in the overall oxidation of SO₂.
Sulfur trioxide can form sulfuric acid
Sulfur trioxide reacts with water: SO₃ + H₂O → H₂SO₄. This connects oxidation of atmospheric sulfur dioxide to acid formation.
When adding the two NOₓ steps, cancel the nitrogen oxide species to show the overall change SO₂ + ½O₂ → SO₃, followed by hydration. The nitrogen oxides are not used up in the catalytic cycle.
Pause and explain
Why can NOₓ act catalytically in SO₂ oxidation?
Worked example
Explain how NO and NO₂ catalyse oxidation of SO₂ to a sulfuric-acid precursor.
Show the worked solution
- SO₂ + NO₂ → SO₃ + NO.
- 2NO + O₂ → 2NO₂ regenerates NO₂.
- SO₃ + H₂O → H₂SO₄; NOₓ cycles rather than being consumed overall.
Answer SO₂ + NO₂ → SO₃ + NO; 2NO + O₂ → 2NO₂; SO₃ + H₂O → H₂SO₄. NO₂ oxidises SO₂ and is regenerated as NO is reoxidised by oxygen, so NOₓ is not consumed overall.
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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.
Write the two equations that show NO₂/NO cycling during SO₂ oxidation.
Give me a hint
First transfer oxygen to SO₂; then reoxidise NO using O₂.
Compare my reasoning
- SO₂ + NO₂ → SO₃ + NO.
- 2NO + O₂ → 2NO₂.
- NO₂ appears again, so the nitrogen-oxide species are regenerated.
SO₂ + NO₂ → SO₃ + NO; 2NO + O₂ → 2NO₂.
Look for these in your work
- First reaction balanced
- Second reaction balanced
- Regeneration identified
Show how sulfur trioxide forms sulfuric acid and state the link to acid rain.
Give me a hint
Add water to SO₃ and identify the acid product.
Compare my reasoning
- SO₃ reacts with H₂O.
- The balanced equation is SO₃ + H₂O → H₂SO₄.
- Sulfuric acid contributes to acid deposition.
SO₃ + H₂O → H₂SO₄; the sulfuric acid can contribute to acid rain.
Look for these in your work
- Reactants stated
- Equation balanced
- Environmental link stated
A diagram shows NO₂ → NO but no route back. What is needed before NOₓ can be called a catalyst?
Give me a hint
A catalyst must be regenerated overall.
Compare my reasoning
- NO is the product when NO₂ oxidises SO₂.
- O₂ can oxidise NO back to NO₂: 2NO + O₂ → 2NO₂.
- The regenerated NO₂ continues the cycle.
Add 2NO + O₂ → 2NO₂; this regenerates NO₂ and completes the catalytic cycle.
Look for these in your work
- Missing step named
- Balanced equation included
- Catalytic role justified
Common mistakes
- Calling NO₂ a reactant consumed overall
- Saying sulfur is reduced from +4 to +6
- Skipping the water step when naming sulfuric acid
What can you explain now?
What sulfur product is formed when NO₂ oxidises SO₂ in the first step?
Compare with the explanation
SO₃
Sulfur dioxide is oxidised to sulfur trioxide.
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Make it stick
Which step regenerates NO₂ after it has oxidised 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.