---
title: "NO₂ and SO₂ sources; role in acid rain"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:no2-so2-acid-rain"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/no2-so2-acid-rain
site: BCT Engineering
---

# NO₂ and SO₂ sources; role in acid rain

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2068 Magh, 2+2+1 marks
- Video: [4. Sulfur dioxide and nitrogen oxides (HSC chemistry)](https://www.youtube.com/watch?v=eUiyn3yf3do) — Letslearnscience
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/no2-so2-acid-rain
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

What are the sources of NO₂ and SO₂ pollutants in air? How do they play role in acid rain? Give two possible remedies of acid rain.

## Short answer

SO2 from burning coal/oil and smelting; NO2 from vehicle exhaust and high-temperature combustion. They oxidize to H2SO4 and HNO3, making rain acidic (pH<5.6). Remedies: low-sulfur fuels and flue gas desulfurization; catalytic converters on vehicles and shift to clean energy.

## Answer

#### Sources
- **SO₂:** burning of coal and petroleum products, smelting of sulphide ores (ZnS, Cu₂S), brick kilns, thermal power plants; natural source: volcanoes.
- **NO₂:** vehicle exhaust and other high-temperature combustion (N₂ + O₂ → 2NO, then 2NO + O₂ → 2NO₂), thermal power plants, nitric acid and fertiliser industries; natural source: lightning.

#### Role in acid rain
```text
2SO₂ + O₂ → 2SO₃ ;   SO₃ + H₂O → H₂SO₄
4NO₂ + O₂ + 2H₂O → 4HNO₃
```
H₂SO₄ and HNO₃ dissolve in rain water and make its pH fall below 5.6, which damages water bodies, soil, forests, buildings and metals.

#### Two remedies
1. Use low-sulphur fuel and fit flue gas desulphurisation (lime scrubbers) in industry.
2. Fit catalytic converters in vehicles and shift to clean energy such as hydropower and solar.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which are sources of SO2?
   A. Lightning only
   B. Photosynthesis
   C. Burning coal and petroleum, smelting of sulphide ores, brick kilns, thermal plants, volcanoes
   D. Fertiliser use only
   Answer: C. SO2 comes from burning sulphur-containing fuels and smelting. (Key point: Sources)

2. How is NO2 formed?
   A. N2 + H2 -> NO2
   B. CO + O2 -> NO2
   C. NH3 + O2 -> NO2 in vehicles only
   D. N2 + O2 -> 2NO at high temperature, then 2NO + O2 -> 2NO2
   Answer: D. High temperature combustion forms NO then NO2. (Key point: NO2 source)

3. How is sulphuric acid formed in the air from SO2?
   A. 2SO2 + O2 -> 2SO3, then SO3 + H2O -> H2SO4
   B. SO2 + CO2 -> H2SO4
   C. SO3 + O2 -> H2SO4
   D. SO2 reacts with N2 to give H2SO4
   Answer: A. SO2 is oxidised to SO3, which dissolves in water to give H2SO4. (Key point: Acid formation)

4. Which are two remedies for acid rain?
   A. More coal burning and fewer scrubbers
   B. Low-sulphur fuel with flue gas desulphurisation, and catalytic converters / clean energy
   C. Leaded petrol
   D. Increase NOx
   Answer: B. Control SO2 at source and NOx from vehicles. (Key point: Remedies)

## Related in Engineering Chemistry

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