Nitrogen Oxides & Sulfur Dioxide (Edexcel IGCSE Chemistry)

Formation of Nitrogen Oxides

Spec point — know that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to react, forming oxides of nitrogen

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.

Formation of Sulfur Dioxide

Spec point — explain how the combustion of some impurities in hydrocarbon fuels results in the formation of sulfur dioxide

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

Acid Rain

Spec point — understand how sulfur dioxide and oxides of nitrogen oxides contribute to acid rain

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