Air quality and climate

1. Overview

This topic explores the chemical composition of the Earth's atmosphere and the impact of human activity on air quality. Understanding the sources and effects of various pollutants is crucial for developing strategies to mitigate global warming and acid rain, ensuring a sustainable environment.

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Key Definitions

• Pollutant: A substance introduced into the environment that has harmful or poisonous effects.
• Complete Combustion: Burning a fuel in a plentiful supply of oxygen to produce carbon dioxide and water.
• Incomplete Combustion: Burning a fuel in a limited supply of oxygen, producing carbon monoxide and/or particulates (soot).
• Greenhouse Gas: A gas that absorbs infrared radiation (thermal energy) in the atmosphere, contributing to the greenhouse effect.
• Photosynthesis: The process by which plants synthesize glucose from carbon dioxide and water using light energy.
• Catalytic Converter: A device fitted to car exhausts to reduce the emission of toxic gases by converting them into less harmful substances.

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Core Content

The Composition of Clean, Dry Air

The atmosphere is a mixture of different gases. For clean, dry air, the approximate proportions are:

• Nitrogen, N₂: 78%
• Oxygen, O₂: 21%
• Remainder (1%): A mixture of noble gases (mainly Argon) and Carbon Dioxide, CO₂.

Air Pollutants: Sources and Adverse Effects

Pollutant	Source	Adverse Effect
Carbon dioxide (CO₂)	Complete combustion of carbon-containing fuels	Increased global warming, leading to climate change.
Carbon monoxide (CO)	Incomplete combustion of carbon-containing fuels	Toxic gas (binds to haemoglobin, preventing oxygen transport).
Particulates (Soot)	Incomplete combustion of carbon-containing fuels	Increased risk of respiratory problems and cancer.
Methane (CH₄)	Decomposition of vegetation; waste gases from animal digestion	Increased global warming, leading to climate change.
Oxides of Nitrogen (NOₓ)	High temperatures in car engines	Acid rain, photochemical smog, and respiratory problems.
Sulfur dioxide (SO₂)	Combustion of fossil fuels containing sulfur compounds	Acid rain.

Strategies to Reduce Environmental Issues

1. Climate Change (Global Warming):
   • Planting trees: Increases CO₂ absorption via photosynthesis.
   • Reduction in livestock farming: Decreases methane (CH₄) emissions.
   • Decreasing use of fossil fuels: Reducing CO₂ emissions at the source.
   • Increasing renewable energy: Using wind, solar, and hydrogen energy.
2. Acid Rain:
   • Catalytic converters: Installed in vehicles to remove NOₓ.
   • Low-sulfur fuels: Reducing the sulfur content in fuels before combustion.
   • Flue gas desulfurisation: Using calcium oxide, CaO(s), to neutralize SO₂ in power station chimneys.

Photosynthesis

Photosynthesis is the natural process by which plants remove CO₂ from the atmosphere.

• Word Equation: carbon dioxide + water → glucose + oxygen

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Extended Content (Extended Only)

The Greenhouse Effect

Greenhouse gases (CO₂ and CH₄) maintain the Earth's temperature through the following mechanism:

1. The sun emits short-wavelength radiation that passes through the atmosphere to the Earth's surface.
2. The Earth absorbs this and re-emits it as long-wavelength thermal energy (infrared).
3. Greenhouse gases absorb this thermal energy and re-emit it in all directions, including back toward Earth.
4. This reduces thermal energy loss to space, causing the temperature of the atmosphere to rise (global warming).

Oxides of Nitrogen and Catalytic Converters

In car engines, the high temperature and pressure cause nitrogen and oxygen from the air to react:

• $N_2(g) + O_2(g) \rightarrow 2NO(g)$
• $N_2(g) + 2O_2(g) \rightarrow 2NO_2(g)$

Removal: Catalytic converters contain transition metal catalysts (like Platinum) that facilitate the reaction between carbon monoxide and nitrogen monoxide to form harmless gases.

• Symbol Equation: $2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)$

Photosynthesis Symbol Equation

The balanced chemical equation for photosynthesis is:

• $6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)$

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Key Equations

1. Photosynthesis (Symbol): $6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)$ (CO₂ = carbon dioxide, H₂O = water, C₆H₁₂O₆ = glucose, O₂ = oxygen)

2. Catalytic Converter Reaction: $2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)$ (CO = carbon monoxide, NO = nitrogen monoxide, CO₂ = carbon dioxide, N₂ = nitrogen)

3. Formation of Sulfur Dioxide: $S(s) + O_2(g) \rightarrow SO_2(g)$ (S = sulfur impurities in fuel, O₂ = oxygen, SO₂ = sulfur dioxide)

4. Flue Gas Desulfurisation: $CaO(s) + SO_2(g) \rightarrow CaSO_3(s)$ (CaO = calcium oxide, SO₂ = sulfur dioxide, CaSO₃ = calcium sulfite)

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that the "Ozone Layer" and "Global Warming" are the same thing. ✓ Right: Global warming is caused by greenhouse gases (CO₂, CH₄) trapping heat; ozone depletion is a separate issue involving CFCs.
• ❌ Wrong: Stating that Nitrogen makes up 21% of the air. ✓ Right: Nitrogen is 78%; Oxygen is 21%.
• ❌ Wrong: Saying Carbon Monoxide causes global warming. ✓ Right: Carbon Monoxide is toxic to humans; Carbon Dioxide causes global warming.

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Exam Tips

• Percentages: You must memorize 78% (N₂) and 21% (O₂). Examiners often ask for these specific values.
• Command Words: If a question asks you to "State the source," keep it brief (e.g., "Incomplete combustion of fuels"). If it says "Explain how greenhouse gases cause global warming," you must mention the absorption and re-emission of thermal energy.
• Catalytic Converters: Ensure you can write the balanced equation $2CO + 2NO \rightarrow 2CO_2 + N_2$ with state symbols (all gases).
• Context: Be prepared to discuss "Flue gas desulfurisation." Remember that Calcium Oxide (CaO) is a basic oxide, so it reacts with the acidic Sulfur Dioxide (SO₂).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State the approximate percentage of nitrogen and oxygen in clean, dry air. [2]

(b) Name two pollutants produced by the incomplete combustion of carbon-containing fuels. [2]

(c) State one adverse effect of carbon monoxide on human health. [1]

Worked Solution:

(a)

1. Nitrogen: $\boxed{78 %}$ Remember the composition of air.

2. Oxygen: $\boxed{21 %}$ Oxygen makes up just over one-fifth of the air.

How to earn full marks:

• Correct percentage for nitrogen.
• Correct percentage for oxygen.

(b)

1. Carbon monoxide Incomplete combustion produces carbon monoxide.

2. Particulates Incomplete combustion also produces soot, which is particulate matter.

How to earn full marks:

• Correctly name carbon monoxide.
• Correctly name particulates.

(c)

1. Toxic gas Carbon monoxide is poisonous.

How to earn full marks:

• State that carbon monoxide is toxic.

Common Pitfall: Remember to state the percentages for nitrogen and oxygen specifically. Don't confuse the effects of carbon monoxide with other pollutants; focus on its toxicity.

Exam-Style Question 2 — Extended Response [8 marks]

Question:

A power plant burns coal, which contains sulfur impurities.

(a) State how sulfur dioxide is produced from burning this coal. [1]

(b) Describe how sulfur dioxide can lead to acid rain. [3]

(c) Name one other air pollutant that contributes to acid rain. [1]

(d) Describe two strategies to reduce the effects of acid rain. [3]

Worked Solution:

(a)

1. Sulfur compounds in the coal react with oxygen during combustion. Burning the coal causes sulfur to react.

How to earn full marks:

• State that sulfur compounds react with oxygen.

(b)

1. Sulfur dioxide dissolves in atmospheric water (rainwater, clouds, fog). Sulfur dioxide is soluble in water.

2. Sulfur dioxide reacts with water and oxygen to form sulfuric acid. A chemical reaction occurs in the atmosphere.

3. The sulfuric acid then falls as acid rain. The acid gets into the rain.

How to earn full marks:

• Sulfur dioxide dissolves in water.
• Sulfur dioxide reacts to form sulfuric acid.
• Sulfuric acid falls as acid rain.

(c)

1. Oxides of nitrogen Nitrogen oxides also contribute to acid rain.

How to earn full marks:

• Name oxides of nitrogen.

(d)

1. Use low-sulfur fuels. Reduces the amount of sulfur dioxide released into the atmosphere.

2. Flue gas desulfurisation with calcium oxide. Calcium oxide neutralises the sulfur dioxide.

3. Use catalytic converters in vehicles. Reduces the emission of nitrogen oxides.

How to earn full marks:

• State using low-sulfur fuels.
• State flue gas desulfurisation with calcium oxide.
• State using catalytic converters in vehicles.

Common Pitfall: Be specific about the chemical reactions involved in acid rain formation. Don't just say "sulfur dioxide causes acid rain"; explain the process. Also, remember that catalytic converters primarily target nitrogen oxides, not sulfur dioxide.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

The diagram shows how thermal energy interacts with the Earth's atmosphere.

(a) Identify the type of radiation emitted from the Earth's surface. [1]

(b) Name two greenhouse gases that contribute to global warming. [2]

(c) Explain how greenhouse gases cause global warming. [3]

Worked Solution:

(a)

1. Thermal energy The Earth emits heat.

How to earn full marks:

• Identify thermal energy.

(b)

1. Carbon dioxide Carbon dioxide is a major greenhouse gas.

2. Methane Methane is another significant greenhouse gas.

How to earn full marks:

• Name carbon dioxide.
• Name methane.

(c)

1. Greenhouse gases absorb thermal energy emitted from the Earth's surface. The gases trap energy.

2. They re-emit this thermal energy in all directions. The gases radiate heat.

3. Some of this re-emitted thermal energy is directed back towards the Earth's surface, reducing thermal energy loss to space. The Earth stays warmer.

How to earn full marks:

• Greenhouse gases absorb thermal energy.
• They re-emit this thermal energy.
• Some thermal energy is directed back towards the Earth.

Common Pitfall: Don't confuse the absorption of incoming solar radiation with the absorption of outgoing thermal radiation. Greenhouse gases primarily affect the outgoing thermal radiation. Also, remember to explain the re-emission of thermal energy back towards Earth, not just the initial absorption.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A farmer is considering different strategies to reduce methane emissions from their livestock.

(a) State the source of methane emissions from livestock farming. [1]

(b) Explain how methane contributes to global warming. [3]

(c) Describe two strategies the farmer could use to reduce methane emissions. [4]

(d) Photosynthesis plays a vital role in maintaining air quality.

(i) State the word equation for photosynthesis. [1]

(ii) State the symbol equation for photosynthesis. [1]

Worked Solution:

(a)

1. Decomposition of vegetation and waste gases from digestion in animals. Methane is released during digestion.

How to earn full marks:

• State that methane comes from decomposition of vegetation and waste gases from digestion in animals.

(b)

1. Methane is a greenhouse gas that absorbs thermal energy emitted from the Earth's surface. Methane traps heat.

2. It re-emits this thermal energy in all directions. Methane radiates heat.

3. Some of this re-emitted thermal energy is directed back towards the Earth's surface, reducing thermal energy loss to space. The Earth stays warmer.

How to earn full marks:

• Methane absorbs thermal energy.
• It re-emits thermal energy.
• Some thermal energy is directed back towards the Earth.

(c)

1. Reduce the number of livestock. Fewer animals mean less methane.

2. Improve animal feed to reduce methane production during digestion. Better digestion reduces methane.

3. Capture methane from manure and use it as a source of energy (biogas). This prevents the methane from escaping into the atmosphere.

4. Plant trees on the farm to absorb carbon dioxide from the atmosphere. Trees act as carbon sinks.

How to earn full marks:

• Reduce the number of livestock.
• Improve animal feed.
• Capture methane from manure.
• Plant trees on the farm.

(d) (i)

1. Carbon dioxide + water → glucose + oxygen The reactants and products of photosynthesis.

How to earn full marks:

• Correct word equation for photosynthesis.

(ii)

1. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ The balanced chemical equation.

How to earn full marks:

• Correct symbol equation for photosynthesis.

Common Pitfall: When discussing methane reduction strategies, focus on practical steps a farmer could take. Don't just say "reduce emissions"; explain how they can be reduced. Also, remember the exact word and symbol equations for photosynthesis, including the balancing numbers.