Acid Rain: Nitrogen Oxides & Sulfur Dioxide (Edexcel GCSE Chemistry: Combined Science)

Sulfur dioxide

• Sulfur dioxide is a colourless, pungent smelling gas that is a major air pollutant responsible for acid rain
• The sulfur dioxide released mixes with clouds and readily dissolves in rainwater
• SO₂ is a non-metal oxide so it forms an acidic solution in water, hence forming acid rain
• Sources: combustion of fossil fuels - especially coal
• 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)

• Adverse effects: acid rain which causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms. Pollutes crops and water supplies, irritates lungs, throats and eyes

Formation of acid rain from nitrogen and sulfur oxides

• 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
• 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

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
• The temperature in an internal combustion engine can reach over 2000 °C
• Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide:

N₂ (g)+ O₂ (g) ⇌ 2NO (g)

• Nitrogen monoxide reacts further forming nitrogen dioxide:

2NO (g) + O₂ (g) ⇌ 2NO₂ (g)

• Nitrogen dioxide gas 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
• 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

Adverse Effects

• Acid rain with similar effects as SO₂ as well as producing photochemical smog and breathing difficulties, in particular for people suffering from asthma

Catalytic Converters

• They contain a series of transition metal catalysts including platinum, palladium and rhodium
• The metal catalysts are in a honeycomb within the converter to increase the surface area available for reaction
• A series of redox reactions occurs which neutralises the pollutant gases
• Carbon monoxide is oxidised to carbon dioxide:

2CO + O₂ → 2CO₂

• Nitrogen oxides are reduced to N₂ gas:

2NO → N₂ + O₂

2NO₂ → N₂ + 2O₂

• Unburned hydrocarbons are oxidised to carbon dioxide and water:

C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O

Catalytic converters are designed to reduce the polluting gases produced in car exhausts