Acid Rain: Nitrogen Oxides & Sulfur Dioxide (Edexcel GCSE Chemistry)
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Formation of Sulfur Dioxide
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
Acid Rain
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
Formation of Nitrogen Oxides
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
Examiner Tips and Tricks
You don't need to learn all the equations given here. They are shown to illustrate some of the complex processes and variety of products from polluting sources.
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