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.
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.
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
- 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.
- 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
Extended Content (Extended Only)
The Greenhouse Effect
Greenhouse gases (CO₂ and CH₄) maintain the Earth's temperature through the following mechanism:
- The sun emits short-wavelength radiation that passes through the atmosphere to the Earth's surface.
- The Earth absorbs this and re-emits it as long-wavelength thermal energy (infrared).
- Greenhouse gases absorb this thermal energy and re-emit it in all directions, including back toward Earth.
- 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:
- $6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)$
Key Equations
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)
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)
Formation of Sulfur Dioxide: $S(s) + O_2(g) \rightarrow SO_2(g)$ (S = sulfur impurities in fuel, O₂ = oxygen, SO₂ = sulfur dioxide)
Flue Gas Desulfurisation: $CaO(s) + SO_2(g) \rightarrow CaSO_3(s)$ (CaO = calcium oxide, SO₂ = sulfur dioxide, CaSO₃ = calcium sulfite)
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.
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).