Explain engine pollutants and catalytic removal
Connect incomplete combustion and high-temperature engine conditions to pollutants and their removal in a catalytic converter.
Section 14 · Hydrocarbons
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What can form when nitrogen and oxygen react at engine temperatures?
What you will learn
- Identify CO, NOₓ and unburnt hydrocarbons from an engine
- Connect pollutants to their formation conditions
- Describe catalytic conversion to less harmful products
Different engine conditions form different pollutants
Incomplete combustion of fuel can release poisonous carbon monoxide and unburnt hydrocarbons. High temperatures inside an engine can also allow nitrogen and oxygen from air to react, forming nitrogen oxides (NOₓ).
The pollutant depends on the process: CO indicates incomplete oxidation of carbon; NOₓ is linked to high-temperature reaction of N₂ and O₂; unburnt hydrocarbons pass through without complete combustion.
A converter promotes further reactions
A catalytic converter provides surfaces that help convert CO and unburnt hydrocarbons to carbon dioxide and water, while nitrogen oxides can be reduced to nitrogen. A representative equation is 2CO + 2NO → 2CO₂ + N₂.
Catalysts change reaction rates by providing an alternative pathway; they are not consumed in the overall reaction. The converter reduces some harmful emissions but is not described as removing every pollutant in all conditions.
Pause and explain
Which statement about catalytic converters is accurate?
Worked example
Explain how a catalytic converter can remove carbon monoxide and nitrogen oxides from engine exhaust.
Show the worked solution
- CO can be oxidised to CO₂.
- NO can be reduced to N₂.
- A catalyst promotes the reactions; combine the changes into a balanced representative equation.
Answer 2CO + 2NO → 2CO₂ + N₂; catalytic surfaces promote conversion of carbon monoxide and nitrogen oxides to carbon dioxide and nitrogen.
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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.
Match each pollutant to a plausible formation route: CO, NOₓ and unburnt hydrocarbon.
Give me a hint
Use oxygen supply, engine temperature and unreacted fuel as clues.
Compare my reasoning
- CO forms when carbon is incompletely oxidised.
- NOₓ forms when N₂ and O₂ react at high temperature.
- Unburnt hydrocarbons leave the engine without complete combustion.
CO: incomplete combustion; NOₓ: high-temperature reaction of nitrogen and oxygen; unburnt hydrocarbons: fuel not fully combusted.
Look for these in your work
- CO route correct
- NOₓ route correct
- Unburnt fuel route correct
Balance a representative converter equation 2CO + 2NO → CO₂ + N₂.
Give me a hint
Count carbon, nitrogen and oxygen atoms.
Compare my reasoning
- Two carbon atoms require 2CO₂.
- Two nitrogen atoms form one N₂.
- Check four oxygen atoms on each side.
2CO + 2NO → 2CO₂ + N₂.
Look for these in your work
- Carbon balanced
- Nitrogen balanced
- Oxygen balanced
A student says a converter catalyst is used up because pollutants react on its surface. Correct the explanation.
Give me a hint
Separate temporary surface interaction from net catalyst consumption.
Compare my reasoning
- Reactants can adsorb and react at a catalytic surface.
- The catalyst provides an alternative pathway and is regenerated.
- It is not a stoichiometric reactant in the overall equation.
The catalyst promotes the exhaust reactions but is regenerated and is not consumed in the overall equation.
Look for these in your work
- Surface role described
- Regeneration stated
- Overall role distinguished
Common mistakes
- Saying all NOₓ comes from the fuel carbon
- Describing the catalyst as a reactant consumed in the converter
- Claiming a converter makes exhaust completely harmless
What can you explain now?
What can form when nitrogen and oxygen react at engine temperatures?
Compare with the explanation
Nitrogen oxides
High temperature can enable N₂ and O₂ in air to form NOₓ.
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Make it stick
Which engine conditions form CO, NOₓ and unburnt hydrocarbons, and what does the catalyst do?
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.