Interpret catalytic removal of nitrogen oxides from exhaust
Explain how a catalytic converter reduces NOₓ and oxidises carbon monoxide to less harmful products.
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 product contains the nitrogen after NOₓ reduction in a converter?
What you will learn
- Identify NOₓ reduction to N₂
- Explain the role of a catalyst
- Balance a representative converter reaction
A converter removes nitrogen oxides by reduction
In a catalytic converter, nitrogen oxides from exhaust can be reduced to nitrogen. Carbon monoxide is oxidised to carbon dioxide; the redox changes occur together.
One balanced model equation is 2NO + 2CO → N₂ + 2CO₂. Nitrogen goes from +2 in NO to 0 in N₂; carbon goes from +2 in CO to +4 in CO₂.
The catalyst provides a surface pathway
A catalyst gives a lower-activation-energy route and is regenerated overall. Catalytic converter surfaces commonly contain transition-metal catalysts; the syllabus-level point is catalytic removal of NOₓ from exhaust.
The equation is a simplified model of one converter reaction, not a claim that every exhaust mixture follows only this equation. The catalyst changes reaction rate, not the overall atom balance or equilibrium position by itself.
Pause and explain
What is the catalyst's role?
Worked example
Explain how 2NO + 2CO → N₂ + 2CO₂ represents catalytic NOₓ removal.
Show the worked solution
- NO is reduced to N₂, removing nitrogen oxide from the exhaust mixture.
- CO is oxidised to CO₂ and supplies electrons overall.
- The converter catalyst speeds the redox reaction and is regenerated.
Answer 2NO + 2CO → N₂ + 2CO₂. The catalytic converter reduces NO to N₂ while oxidising CO to CO₂; the catalyst provides a faster pathway and is not used up 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.
Identify the oxidation and reduction in 2NO + 2CO → N₂ + 2CO₂.
Give me a hint
Track oxidation numbers for nitrogen and carbon.
Compare my reasoning
- N is +2 in NO and 0 in N₂: reduction.
- C is +2 in CO and +4 in CO₂: oxidation.
- The changes occur in the same redox reaction.
NO is reduced to N₂; CO is oxidised to CO₂.
Look for these in your work
- Nitrogen oxidation numbers correct
- Carbon oxidation numbers correct
- Redox roles identified
Explain why a converter is described as catalytic.
Give me a hint
State how a catalyst affects pathway and whether it is consumed overall.
Compare my reasoning
- The catalyst offers an alternative reaction pathway.
- This pathway has lower activation energy and increases rate.
- The catalyst is regenerated overall.
The catalyst increases the reaction rate by providing a lower-activation-energy pathway and is not consumed overall.
Look for these in your work
- Pathway stated
- Activation energy link
- Regeneration stated
Balance a simplified reaction that converts NO and CO to nitrogen and carbon dioxide.
Give me a hint
Use N₂ as the nitrogen product and conserve N, C and O atoms.
Compare my reasoning
- Two NO molecules provide two nitrogen atoms.
- Two CO molecules balance two carbon atoms.
- The four oxygen atoms form two CO₂ molecules.
2NO + 2CO → N₂ + 2CO₂.
Look for these in your work
- Nitrogen balanced
- Carbon balanced
- Oxygen balanced
Common mistakes
- Saying the catalyst is a reactant consumed in the equation
- Calling NO₂ reduction an oxidation
- Claiming the converter changes atoms into different elements
What can you explain now?
What product contains the nitrogen after NOₓ reduction in a converter?
Compare with the explanation
N₂
Nitrogen oxides are reduced to nitrogen gas in the model.
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Make it stick
Which reactant is reduced and which is oxidised in the converter model?
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