Engine nitrogen oxides and catalytic-converter reactions
Explain high-temperature formation from air and balance the required carbon monoxide–nitrogen monoxide removal example.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Recall N₂ and O₂ in air and the required nitrogen-oxide effects.
- Explain catalysts and match atom counts in equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which source supplies nitrogen and oxygen in the stated engine formation explanation?
What you will learn
- Explain formation of oxides of nitrogen from nitrogen and oxygen at high temperatures in car engines.
- Describe catalytic-converter removal using the required balanced example and its pollutant-specific limit.
The nitrogen source can be the air
At the high temperatures in car engines, nitrogen and oxygen from the air can react to form nitrogen oxides. A simple formation example is N₂+O₂→2NO, with NO being nitrogen monoxide. The source need not be a nitrogen-containing fuel; air supplies the elements under the stated hot conditions.
These nitrogen oxides are associated with acid rain, photochemical smog and respiratory problems. High-temperature formation is different from stating that all ordinary cool air immediately turns into NO. The conditions matter, and nitrogen monoxide should not be silently replaced with a differently formulated oxide when checking an equation.
Balance the removal and retain the remaining product
A catalytic converter promotes exhaust reactions that reduce suitable pollutants. The required example is 2CO+2NO→2CO₂+N₂: carbon monoxide becomes carbon dioxide and nitrogen monoxide becomes nitrogen. Carbon and oxygen atoms are rearranged and nitrogen atoms form N₂, with no element transmutation.
The catalyst provides an alternative pathway with lower activation energy and is regenerated overall; it does not supply all the atoms in the products. The example reduces CO and NO, but still forms CO₂. Therefore this useful exhaust treatment is not proof that the engine has become carbon-free or that every environmental effect is removed. Supplied amounts can limit conversion, so any idealised calculation must specify complete reaction and no other changes.
Pause and explain
Why is the converter example not carbon-free?
Worked example
Explain where engine NO comes from and write the required converter reaction with CO.
Show the worked solution
- Air supplies nitrogen and oxygen, which can react at high engine temperatures.
- N₂+O₂→2NO is a balanced formation example.
- 2CO+2NO→2CO₂+N₂ balances the required removal example, still producing CO₂.
Answer Hot air nitrogen/oxygen can form NO; the converter example is 2CO+2NO→2CO₂+N₂.
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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.
Explain why NO can form even when the supplied fuel formula contains no nitrogen.
Give me a hint
The fuel is not the only input to an engine.
Compare my reasoning
- Air contains N₂ and O₂.
- High engine temperatures allow their reaction to form nitrogen oxides; N₂+O₂→2NO is an example.
- A nitrogen-free fuel does not remove the air's nitrogen supply.
N₂ and O₂ from air can react at high temperature; nitrogen need not come from the fuel.
Look for these in your work
- The actual nitrogen source is named.
- The temperature condition and balanced example are present.
Balance CO+NO→CO₂+N₂, count the atoms and explain the catalyst's role.
Give me a hint
Two nitrogen atoms are needed for one N₂ molecule.
Compare my reasoning
- Use 2CO+2NO→2CO₂+N₂.
- Each side has C=2, N=2 and O=4.
- The catalyst lowers the activation barrier through an alternative pathway and is regenerated overall; it does not supply the bulk product atoms.
2CO+2NO→2CO₂+N₂; C2/N2/O4 conserved; the catalyst supplies an alternative lower-barrier pathway.
Look for these in your work
- Coefficients conserve every element.
- Catalyst action is distinct from being a bulk reactant.
A hypothetical converter reaction starts with 0.60 mol CO and 0.40 mol NO. Assuming only the given reaction reaches completion, find CO₂, N₂ and remaining CO, then assess a carbon-free claim.
Give me a hint
Use the 1:1 reacting CO:NO ratio and the 2:1 NO:N₂ ratio.
Compare my reasoning
- NO limits because 0.40 mol needs 0.40 mol CO, leaving 0.20 mol CO.
- Products are 0.40 mol CO₂ and 0.20 mol N₂.
- CO₂ is produced and some CO remains in this idealised starting mixture, so carbon-free and complete-pollutant-removal claims are unsupported.
0.40 mol CO₂, 0.20 mol N₂ and 0.20 mol CO remaining; the result is not carbon-free.
Look for these in your work
- The limiting amount and each ratio are used.
- The idealised reaction assumptions and remaining carbon are stated.
Common mistakes
- Claiming nitrogen oxides require nitrogen atoms in the fuel itself.
- Writing NO₂ in place of NO without rebalancing the required example.
- Making the catalyst the reactant that supplies nitrogen.
- Calling the CO₂-producing reaction carbon-free.
What can you explain now?
State the required catalytic-converter example equation.
Compare with the explanation
2CO+2NO→2CO₂+N₂.
Two carbon, two nitrogen and four oxygen atoms are conserved on each side.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain the hot-air source, balance the converter example, match a supplied amount calculation and identify the remaining CO₂ issue.
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.