Explain natural and human sources of nitrogen oxides
Distinguish natural and human sources of NO and NO₂ and link high-temperature combustion to their formation.
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
Which natural event can provide enough energy for N₂ and O₂ to react?
What you will learn
- Name natural and man-made sources
- Explain thermal formation of NO
- Show further oxidation of NO to NO₂
Nitrogen oxides have natural and human sources
Natural high-energy events such as lightning can make nitrogen and oxygen react. Soil microbial activity also contributes nitrogen oxides to the environment.
Human sources include high-temperature combustion in internal-combustion engines and power stations. At high temperature, normally unreactive N₂ and O₂ can react: N₂ + O₂ → 2NO. This equation represents the main thermal route, not all sources.
Nitric oxide is oxidised in air
The NO formed can react with oxygen: 2NO + O₂ → 2NO₂. Both NO and NO₂ are grouped as NOₓ in emissions and atmospheric chemistry.
A source can emit NO that becomes NO₂ after release. Do not say that all nitrogen oxides are emitted directly as NO₂ or that the atmosphere contains only one NOₓ species.
Pause and explain
Which equation represents oxidation of NO in air?
Worked example
Give one natural and one man-made source of NOₓ, then show how NO₂ can form.
Show the worked solution
- Lightning is a natural high-energy source; soil microbes are another.
- High-temperature engine combustion is a man-made source: N₂ + O₂ → 2NO.
- NO is oxidised in air: 2NO + O₂ → 2NO₂.
Answer Lightning and soil processes are natural sources; high-temperature combustion is a human source. Thermal formation and subsequent oxidation can be represented by N₂ + O₂ → 2NO, followed by 2NO + O₂ → 2NO₂.
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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.
Classify lightning, soil microbes and engine combustion as natural or man-made sources.
Give me a hint
Separate naturally occurring processes from human fuel combustion.
Compare my reasoning
- Lightning is natural.
- Soil microbial activity is natural.
- Engine combustion is man-made.
Lightning and soil microbes are natural sources; engine combustion is a man-made source.
Look for these in your work
- Lightning classified
- Soil classified
- Engine classified
Balance the high-temperature formation of nitric oxide from nitrogen and oxygen.
Give me a hint
Use diatomic N₂ and O₂, then match atoms on both sides.
Compare my reasoning
- Write N₂ + O₂ → NO.
- There are two N and two O atoms on the left.
- Place coefficient 2 before NO.
N₂ + O₂ → 2NO.
Look for these in your work
- Diatomic reactants used
- Nitrogen balanced
- Oxygen balanced
Explain the sequence from atmospheric nitrogen to nitrogen dioxide in an engine plume.
Give me a hint
Show thermal formation first, then oxidation of NO by oxygen.
Compare my reasoning
- High temperature allows N₂ + O₂ → 2NO.
- NO reacts with oxygen in air.
- 2NO + O₂ → 2NO₂.
At high temperature, N₂ and O₂ form NO; the NO can then be oxidised by O₂ to NO₂.
Look for these in your work
- Conditions stated
- Two-step sequence
- Second equation balanced
Common mistakes
- Listing only car exhaust as a source
- Claiming N₂ reacts readily at room temperature
- Forgetting oxygen when converting NO to NO₂
What can you explain now?
Which natural event can provide enough energy for N₂ and O₂ to react?
Compare with the explanation
Lightning
Lightning is a high-energy natural source of NOₓ.
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Make it stick
Why can engines form NOₓ even though N₂ is usually unreactive?
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