PYTR Class

Learning Objective

1. Outline the formation of acid rain and ocean acidification

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Part 1: Formation of Acid Rain

Summary: Formation of Acid Rain

Acid rain is caused by non-metal oxides, primarily sulfur oxides (SO₂, SO₃) and nitrogen oxides (NO, NO₂), which dissolve in rainwater to form acidic solutions.

1. Natural Acidity of Rainwater

• Rainwater is naturally acidic (pH ~5.6) due to dissolved carbon dioxide (CO₂) forming weak carbonic acid (H₂CO₃).
• Acid rain has a lower pH due to additional acidic compounds from pollution.

2. Sulfur Oxides and Acid Rain

• Sulfur dioxide (SO₂) forms from combustion:

• Sulfur dioxide dissolves in rainwater, forming sulfurous acid (H₂SO₃):

• Further oxidation in the atmosphere produces sulfur trioxide (SO₃), which forms sulfuric acid (H₂SO₄):

3. Nitrogen Oxides and Acid Rain

• Formation of nitrogen monoxide (NO):

• Formation of nitrogen dioxide (NO₂) directly or via oxidation of NO:

• Nitrogen dioxide dissolves in rainwater, forming a mixture of nitrous acid (HNO₂) and nitric acid (HNO₃):

• Further oxidation leads to more nitric acid (HNO₃):

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Summary of Acid Rain by Nitrogen Oxides (NOₓ) and Sulfur Oxides (SOₓ):

Property	Nitrogen Oxides (NOₓ)	Sulfur Oxides (SOₓ)
Main Oxides	Nitrogen monoxide (NO) Nitrogen dioxide (NO₂)	Sulfur dioxide (SO₂) Sulfur trioxide (SO₃)
Sources	– Internal combustion engines (vehicles, factories) – Lightning – Biomass burning	– Burning of fossil fuels (coal, oil) – Industrial smelting – Volcanic eruptions
Acids Formed	Nitrous acid (HNO₂) Nitric acid (HNO₃)	Sulfurous acid (H₂SO₃) Sulfuric acid (H₂SO₄)
Environmental Impact	– Forms acid rain (HNO₃) – Causes respiratory problems – Contributes to smog formation	– Forms acid rain (H₂SO₄) – Damages ecosystems and buildings – Reduces air quality
Control Measures	– Catalytic converters in vehicles – Reducing fuel combustion – Using alternative energy sources	– Flue gas desulfurization (scrubbers) – Low-sulfur fuels – Alternative energy sources

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Part 2: Ocean Acidification

Ocean acidification occurs when carbon dioxide (CO₂) dissolves in seawater, forming carbonic acid (H₂CO₃):

Carbonic acid further dissociates into hydrogencarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, releasing hydrogen ions (H⁺) that increase ocean acidity:

This acidification primarily affects surface waters, where CO₂ absorption occurs. It inhibits shell growth in marine organisms and may contribute to reproductive issues in fish.

Exercises