13.3 ACID RAIN | Environmental Chemistry Air | Chemistry 11

Acid rain is a type of environmental pollution characterized by rainfall or precipitation that has a lower pH (less than 5.6) level than normal. Rain water does not have pH 7 due to slight mixing of aerial $C O_{2}$ which forms a weak acid — carbonic acid.

$C O_{2} + H_{2} O ⟶ H_{2} C O_{3}$

$H_{2} C O_{3} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + H C O_{3 (a q)}^{-}$

These hydronium ions make the rain water slightly acidic (pH ≈ 5.6) but this is not acid rain. Acid rain has a pH typically between 4.0 and 4.5.

13.3.1 Reasons for Acid Rain

Emissions from Fossil Fuels

The primary cause of acid rain is the release of sulphur dioxide ( $S O_{2}$ ) and nitrogen oxides ( $N O_{x}$ ) into the atmosphere, mainly from burning fossil fuels like coal and oil in power plants and vehicles.

Industrial Processes

Industrial activities, such as manufacturing and mining, also release these pollutants, contributing to acid rain.

Reactions Involved

Formation of Sulphuric Acid ( $H_{2} S O_{4}$ ): Sulphur dioxide ( $S O_{2}$ ) reacts with oxygen ( $O_{2}$ ) and water ( $H_{2} O$ ) to form sulphuric acid ( $H_{2} S O_{4}$ ):

$2 S O_{2} + O_{2} + 2 H_{2} O ⟶ 2 H_{2} S O_{4}$

$H_{2} S O_{4} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + S O_{4 (a q)}^{2 -}$

Formation of Nitric Acid ( $H N O_{3}$ ): Nitrogen oxides ( $N O_{x}$ ) react with atmospheric moisture to form nitric acid ( $H N O_{3}$ ):

$N O_{2} + H_{2} O ⟶ H N O_{3}$

$H N O_{3} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + N O_{3 (a q)}^{-}$

13.3.2 Impacts of Acid Rain

Environmental Damage

Acid rain can harm forests, lakes, rivers, and aquatic ecosystems. It damages plant foliage and aquatic life by altering the pH of soil and water, making it difficult for some organisms to survive.

Corrosion of Buildings and Structures

Acid rain accelerates the corrosion of buildings, statues, and infrastructure (a phenomenon sometimes called 'stone leprosy' or 'stone cancer'), causing significant damage to historical landmarks and urban structures.

$C a C O_{3} + H_{2} S O_{4} ⟶ C a S O_{4} (a q) + C O_{2} + H_{2} O$

Human Health

Inhalation of pollutants associated with acid rain, such as fine particulate matter, can have adverse health effects, including respiratory problems and cardiovascular issues.

Economic Costs

Acid rain can lead to increased maintenance and repair costs for infrastructure, as well as reduced agricultural productivity due to soil degradation.

Loss of Biodiversity

Acid rain affects aquatic life by lowering pH levels in water bodies and by causing the formation of aluminium-based gelatinous compounds that clog fish gills, harming fish and amphibians. It can also lead to the decline of sensitive plant and animal species in affected areas.

Global Implications (Transboundary Pollution)

While acid rain primarily affects local and regional areas, the pollutants responsible for its formation can travel long distances in the atmosphere, contributing to transboundary pollution and affecting ecosystems in distant regions.

13.3.3 Control of Acid Rain

Efforts to combat acid rain have included:

Scrubbers (flue gas desulphurisation units) in power plant chimneys to remove $S O_{2}$ before release into the atmosphere.
Use of catalytic converters in vehicles to reduce $N O_{x}$ emissions.
Switching to cleaner/renewable energy sources to reduce fossil fuel combustion.
International agreements and regulations to limit emissions across borders.

$C O_{2} + H_{2} O ⟶ H_{2} C O_{3}$

$H_{2} C O_{3} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + H C O_{3 (a q)}^{-}$

These hydronium ions make the rain water slightly acidic (pH ≈ 5.6) but this is not acid rain. Acid rain has a pH typically between 4.0 and 4.5.

13.3.1 Reasons for Acid Rain

Emissions from Fossil Fuels

Industrial Processes

Industrial activities, such as manufacturing and mining, also release these pollutants, contributing to acid rain.

Reactions Involved

Formation of Sulphuric Acid ( $H_{2} S O_{4}$ ): Sulphur dioxide ( $S O_{2}$ ) reacts with oxygen ( $O_{2}$ ) and water ( $H_{2} O$ ) to form sulphuric acid ( $H_{2} S O_{4}$ ):

$2 S O_{2} + O_{2} + 2 H_{2} O ⟶ 2 H_{2} S O_{4}$

$H_{2} S O_{4} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + S O_{4 (a q)}^{2 -}$

Formation of Nitric Acid ( $H N O_{3}$ ): Nitrogen oxides ( $N O_{x}$ ) react with atmospheric moisture to form nitric acid ( $H N O_{3}$ ):

$N O_{2} + H_{2} O ⟶ H N O_{3}$

$H N O_{3} + H_{2} O ⟶ H_{3} O_{(a q)}^{+} + N O_{3 (a q)}^{-}$

Scrubbers (flue gas desulphurisation units) in power plant chimneys to remove $S O_{2}$ before release into the atmosphere.
Use of catalytic converters in vehicles to reduce $N O_{x}$ emissions.
Switching to cleaner/renewable energy sources to reduce fossil fuel combustion.
International agreements and regulations to limit emissions across borders.