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4.2.3Principles of Chemistry

Nitrogen Oxides and Sulfur Dioxide

5 min read 5 parts Lesson 1 of 50 Unit quiz

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Section 1 of 5Sources and Formation

4.2.3 Nitrogen Oxides and Sulfur Dioxide
1
In engines and furnaces, the high temperature and pressure cause nitrogen and oxygen from the air to react, forming nitrogen monoxide: N₂ + O₂ → 2NO.
2
Nitrogen monoxide is then oxidised by more oxygen in the air to form nitrogen dioxide, NO₂.
3
Fossil fuels contain sulfur impurities, which are oxidised during combustion to form sulfur dioxide: S + O₂ → SO₂.
4
Catalytic converters in car exhausts help to reduce the amount of harmful gases released to the atmosphere.

Section 2 of 5How Acid Rain Forms

How Acid Rain Forms
From sulfur dioxide
  • SO₂ dissolves in rainwater and is oxidised to sulfuric acid: 2SO₂ + O₂ + 2H₂O → 2H₂SO₄.
From nitrogen dioxide
  • NO₂ reacts with rainwater to form a mixture of nitrous and nitric acids: 2NO₂ + H₂O → HNO₂ + HNO₃.
From lightning
  • Lightning can produce nitrogen oxides in air, which form nitric acid with rain and oxygen: 4NO₂ + 2H₂O + O₂ → 4HNO₃.
Falling as rain
  • The acidic droplets grow as clouds rise and cool, and they fall as acid rain when they are heavy enough.

Section 3 of 5Effects on People and the Environment

Corrosion
  • Acid rain attacks metal structures and buildings or statues made of carbonate rocks such as limestone.
Aquatic life
  • Acidified lakes and rivers can harm fish and other aquatic organisms.
Crops and water
  • Acid rain can damage crops and contaminate water supplies.
Smog
  • Nitrogen oxides contribute to photochemical smog, which can cause breathing difficulties.
Breathing
  • Sulfur dioxide irritates the lungs, throat and eyes, and the effects are worse for people with asthma.

Section 4 of 5Worked Example

Question Using the equation 2SO₂ + O₂ + 2H₂O → 2H₂SO₄, calculate how many moles of sulfuric acid form from 4 moles of sulfur dioxide.

  1. The equation shows 2 mol SO₂ gives 2 mol H₂SO₄, a 1:1 ratio
  2. 4 mol SO₂ therefore gives 4 mol H₂SO₄

Answer 4 mol of sulfuric acid.

Section 5 of 5Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Acid rain Rain made acidic by dissolved sulfur dioxide and nitrogen oxides.
  • Nitrogen oxides (NOₓ) Gases such as NO and NO₂ formed when nitrogen and oxygen react at high temperature.
  • Photochemical smog Air pollution containing ozone and other irritants, formed in sunlight from nitrogen oxides.
  • Catalytic converter A device in a car exhaust that converts harmful gases into less harmful substances.
  • Sulfuric acid A strong acid, H₂SO₄, formed when sulfur dioxide dissolves in rainwater and is oxidised.

Common Mistakes

  • Blaming carbon dioxide as the main cause of acid rain; the main acids come from SO₂ and nitrogen oxides.
  • Writing the acid rain equation for SO₂ without the oxygen and water that are needed.
  • Forgetting that nitrogen and oxygen react to form NO only at the high temperatures in engines and furnaces.
  • Describing catalytic converters as removing all pollutants, when they only reduce harmful gases.
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