Describe nitrogen oxides in direct acid rain formation
Explain how NO₂ contributes directly to acidic precipitation and how NO is oxidised in the atmosphere.
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 nitrogen oxide is directly shown reacting with water to form nitric acid?
What you will learn
- Relate NO₂ to nitric acid formation
- Write a balanced direct acid-forming equation
- Distinguish direct acid formation from a catalytic cycle
Nitrogen dioxide can form nitric acid
NO₂ dissolves and reacts in atmospheric water to form acids. One balanced overall representation is 3NO₂ + H₂O → 2HNO₃ + NO.
The nitric acid contributes acidity to precipitation. The NO product can be oxidised by oxygen back to NO₂, linking this direct step to a continuing atmospheric cycle.
Separate direct and catalytic effects
Direct acid formation consumes NO₂ in the displayed equation and produces nitric acid. The separate catalytic role of NO/NO₂ in oxidising SO₂ is another route to acid rain and is mapped in the next lesson.
Atmospheric acid deposition has multiple sources and processes; this pathway is a syllabus model, not a complete account of every rainwater component.
Pause and explain
Which balanced equation represents direct nitric-acid formation?
Worked example
Use an equation to show a direct route from atmospheric NO₂ to nitric acid.
Show the worked solution
- NO₂ reacts with water.
- Balance N, H and O using three NO₂ molecules.
- 3NO₂ + H₂O → 2HNO₃ + NO.
Answer 3NO₂ + H₂O → 2HNO₃ + NO represents direct nitric-acid formation from nitrogen dioxide in atmospheric water.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Balance an equation showing NO₂ reacting with water to make HNO₃ and NO.
Give me a hint
Balance nitrogen first, then hydrogen and oxygen.
Compare my reasoning
- Use 3NO₂ to give three nitrogen atoms.
- Place 2HNO₃ and 1NO on the right to balance nitrogen.
- One H₂O then balances hydrogen and oxygen.
3NO₂ + H₂O → 2HNO₃ + NO.
Look for these in your work
- Nitrogen balanced
- Hydrogen balanced
- Oxygen balanced
Explain why this equation is a direct acid-forming route.
Give me a hint
Identify the acid product and the reactant that supplies nitrogen.
Compare my reasoning
- NO₂ reacts directly with atmospheric water.
- Nitric acid, HNO₃, is formed.
- The acid can contribute acidity to precipitation.
NO₂ reacts with water to form HNO₃, so it directly creates an acid that can contribute to acid deposition.
Look for these in your work
- Direct reactant stated
- Acid product stated
- Rain link explained
A learner calls 3NO₂ + H₂O → 2HNO₃ + NO 'the catalytic oxidation of SO₂.' Explain the mismatch.
Give me a hint
Check which atoms appear and whether SO₂ is a reactant.
Compare my reasoning
- The equation contains no sulfur dioxide.
- It shows NO₂ and water forming nitric acid and NO.
- SO₂ oxidation is a separate pathway involving a nitrogen-oxide catalyst.
This is direct nitric-acid formation from NO₂; catalytic oxidation of SO₂ requires SO₂ as a reactant and NO/NO₂ cycling.
Look for these in your work
- Missing reactant identified
- Equation classified correctly
- Other pathway distinguished
Common mistakes
- Omitting water from the equation
- Calling the direct step itself a catalytic cycle
- Claiming NO₂ is the only acid-rain source
What can you explain now?
Which nitrogen oxide is directly shown reacting with water to form nitric acid?
Compare with the explanation
NO₂
The displayed direct acid-forming equation starts with nitrogen dioxide.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How does the direct reaction of NO₂ with water differ from its catalytic role in SO₂ oxidation?
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.