Nitrogen Oxides & Sulfur Dioxide (Edexcel IGCSE Chemistry: Double Science)

Nitrogen Oxides

• These compounds (NO and NO₂) are formed when nitrogen and oxygen react in the high pressure and temperature conditions of internal combustion engines and blast furnaces
• Exhaust gases also contain unburned hydrocarbons and carbon monoxide
• Cars are fitted with catalytic converters which form a part of their exhaust systems
• Their function is to render these exhaust gases harmless
• The adverse effects of nitrogen oxides include acid rain as well as producing photochemical smog and breathing difficulties, in particular for people suffering from asthma.

Sulfur dioxide

• Fossil fuels are often contaminated with small amounts of sulfur impurities
• When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide

 S (s) + O₂ (g) → SO₂ (g)

• The adverse effects of sulfur dioxide include acid rain:
  • Acid rain causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms
  • It pollutes crops and water supplies, irritates lungs, throats and eyes

How is acid rain formed?

From sulfur dioxide

• The sulfur dioxide produced from the combustion of fossil fuels dissolves in rainwater droplets to form sulfuric acid

2SO₂ (g) + O₂ (g) + 2H₂O (l) → 2H₂SO₄ (aq)

• Sulfuric acid is one of the components of acid rain which has several damaging impacts on the environment

From nitrogen dioxide

• Nitrogen dioxide produced from car engines reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain:

2NO₂ (g) + H₂O (l)  → HNO₂ (aq) + HNO₃ (aq)

• Lightning strikes can also trigger the formation of nitrogen monoxide and nitrogen dioxides in air
• Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid

4NO₂ (g) + 2H₂O (l) + O₂ (g)→ 4HNO₃ (aq)

• When the clouds rise, the temperature decreases, and the droplets get larger
• When the droplets containing these acids are heavy enough, they will fall down as acid rain

The formation of acid rain

Formation of acid rain from nitrogen and sulfur oxides