7.2.4 Overall Impact on Acid Rain

NO₂ acts as a catalyst in the oxidation of SO₂ to sulphuric acid, thereby increasing the concentration of another strong acid in the atmosphere.

Key point: NO₂ acts as a catalyst in the oxidation of SO₂ to sulphuric acid, increasing the concentration of acid in the atmosphere.

Summary Table

Intermediate Level

Direct role of NO and NO₂ in acid rain:

1. Formation of NO₂: 2NO + O₂ → 2NO₂

2. Formation of Nitric Acid: 4NO₂ + 2H₂O + O₂ → 4HNO₃

Catalytic role of NO₂ in the oxidation of SO₂:

1. Oxidation of SO₂: SO₂ + NO₂ → SO₃ + NO

2. Formation of Sulphuric Acid: SO₃ + H₂O → H₂SO₄

3. Regeneration of NO₂: 2NO + O₂ → 2NO₂

Key point: NO₂ acts as a catalyst in the oxidation of SO₂ to sulphuric acid, increasing the concentration of acid in the atmosphere.

Important Points

• - Acid rain is defined as rain having a pH less than 5.
• - The acidity of rainwater increases in polluted areas during thunderstorms.
• - SO₂ from power plants (burning fossil fuels) and NO₂ from automobile exhaust dissolve in rainwater, producing acids.
• - The pH of rainwater can be much lower due to the production of H₂SO₄ and HNO₃.
• - Lightning during thunderstorms can also contribute.
• - Rain may have a pH as low as 2.
• - NO and NO₂ play important roles in the formation of acid rain through both direct and catalytic processes.
• - Direct role: NO₂ reacts with water and oxygen to form nitric acid (HNO₃).
• - Catalytic role: NO₂ catalyses the oxidation of SO₂ to SO₃, which then forms sulphuric acid (H₂SO₄).
• - NO₂ acts as a catalyst in the oxidation of SO₂ to sulphuric acid.

Remember

• - Acid rain = pH less than 5.
• - SO₂ from power plants.
• - NO₂ from automobile exhaust.
• - H₂SO₄ and HNO₃ are the main acids in acid rain.
• - Direct role: NO₂ → HNO₃.
• - Catalytic role: NO₂ catalyses SO₂ → SO₃ → H₂SO₄.
• - NO₂ is regenerated in the catalytic cycle.
• - Lightning contributes to acid formation.
• - Rain pH can be as low as 2.

Common Mistakes

Quick Revision

Acid Rain:

The acidity of rainwater increases in polluted areas during thunderstorms e.g SO₂ Sulphur dioxide from power plants during the combustion of fossil fuels and NO₂ form automobiles. Exhaust fumes dissolve in rainwater producing acids. The pH of rainwater can be much lower due to the production of H₂SO₄ and HNO₃. By lightning during the thunderstorm. The rain may have a pH as low as 2. Acid Rain is defined as rain having a pH less than 5.

7.2.1 Role of nitric oxide and nitrogen dioxide in the formation of acid rain:

Nitric oxide (NO) and nitrogen dioxide (NO₂) play important roles in the formation of acid rain through both direct and catalytic processes.

7.2.2 Direct Role in Acid Rain Formation:

Formation of Nitric Acid (HNO₃): Nitric oxide (NO) is initially produced during combustion processes, such as those occurring in vehicle engines and industrial activities. NO is relatively stable but can be oxidized to form NO₂:

• 2NO_(g) + O_2(g) → 2NO_2(g)

• Formation of Nitric Acid:
• NO₂ can further react with hydroxyl radicals (OH) and water in the atmosphere to form nitric acid (HNO₃), a major component of acid rain:

• 4NO_2(g) + 2H₂O_(g) + O_2(g) → 4HNO_3(g)

7.3.3 Catalytic role in the oxidation of sulphur dioxide (SO₂):

Sulphur dioxide (SO₂) is another significant precursor of acid rain, primarily originating from the burning of fossil fuels containing sulphur. The conversion of SO₂ to sulphuric acid (H₂SO₄) can be catalysed by NO₂.

• SO_2(g) + NO_2(g) → SO_3(g) + NO_(g)

• Formation of Sulphuric Acid (H₂SO₄):
• SO₃ readily reacts with water vapour to form sulphuric acid (H₂SO₄):

• SO_3(g) + H₂O_(g) → H₂SO_4(aq)

• Regeneration of NO₂:
• The NO produced in the oxidation of SO₂ can be re-oxidized to NO₂ by reacting with molecular oxygen (O₂), continuing the catalytic cycle:

• 2NO_(g) + O_2(g) → 2NO_2(g)

This cycle allows a small amount of NO₂ to catalyse the conversion of a large amount of SO₂ to sulphuric acid.

• 7.3.3 Overall Impact on Acid Rain:
• NO₂ acts as a catalyst in the oxidation of SO₂ to sulphuric acid, thereby increasing the concentration of another strong acid in the atmosphere