Air quality and climate
Composition of Clean, Dry Air
Clean, dry air is a mixture of gases. The composition is dominated by nitrogen and oxygen, with trace amounts of other gases.
| Gas | Approximate Percentage |
|---|---|
| Nitrogen (N_2) | 78% |
| Oxygen (O_2) | 21% |
| Noble gases and Carbon Dioxide (CO_2) | ~1% (remainder) |
Note: The 'remainder' is primarily Argon (a noble gas) and a small, variable amount of Carbon Dioxide. When answering questions about composition, always specify clean and dry air, as water vapor content varies.
Air Pollutants: Sources
Pollutants are substances present in the air that cause harm. Their sources are specific to the chemical processes involved.
| Pollutant | Source |
|---|---|
| Carbon Dioxide (CO_2) | Complete combustion of carbon-containing fuels (e.g., coal, oil, gas). |
| Carbon Monoxide (CO) and Particulates | Incomplete combustion of carbon-containing fuels (due to insufficient oxygen). |
| Methane (CH_4) | Decomposition of vegetation/waste (anaerobic decay) and digestion in animals (e.g., cows). |
| Oxides of Nitrogen (NO_x) | Car engines (high temperature reaction between N_2 and O_2 in air). |
| Sulfur Dioxide (SO_2) | Combustion of fossil fuels containing sulfur impurities (e.g., coal, oil). |
Adverse Effects of Pollutants
Each pollutant has specific environmental or health consequences.
| Pollutant | Adverse Effect |
|---|---|
| Carbon Dioxide (CO_2) | Increased global warming leading to climate change. |
| Carbon Monoxide (CO) | Toxic gas (binds to hemoglobin, reducing oxygen transport). |
| Particulates | Increased risk of respiratory problems and cancer. |
| Methane (CH_4) | Increased global warming leading to climate change. |
| Oxides of Nitrogen (NO_x) | Acid rain, photochemical smog, and respiratory problems. |
| Sulfur Dioxide (SO_2) | Acid rain. |
The Greenhouse Effect and Global Warming
Global warming is caused by the enhanced greenhouse effect. The key greenhouse gases are carbon dioxide and methane.
Mechanism of Global Warming:
- Solar radiation (visible light) passes through the atmosphere and is absorbed by the Earth's surface.
- The Earth re-emits this energy as thermal energy (infrared radiation).
- Greenhouse gases (CO_2 and CH_4) absorb this thermal energy.
- They then reflect (or scatter/re-emit) the thermal energy back towards the Earth's surface.
- This process reduces the loss of thermal energy to space, causing the Earth's average temperature to rise.
Why this matters: The specific keywords absorption, reflection, and emission are critical. You must state that thermal energy is reflected back to Earth, not just 'trapped' vaguely.
Photosynthesis
Photosynthesis is the process by which plants remove carbon dioxide from the atmosphere and produce glucose. It is the reverse of respiration.
Word Equation:
carbon dioxide + water \xrightarrow{\text{light, chlorophyll}} glucose + oxygen
carbon dioxide + water \xrightarrow{\text{light, chlorophyll}} glucose + oxygen
Symbol Equation:
6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2
6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2
Key Conditions:
- Chlorophyll: The green pigment in leaves that absorbs light energy.
- Light: Provides the energy required for the reaction.
- Reactants: Carbon dioxide (from air) and Water (absorbed by roots).
Formation and Removal of Oxides of Nitrogen
Formation in Car Engines:
Inside a car engine, the temperature becomes very high during combustion. At these high temperatures, nitrogen (N_2) and oxygen (O_2) from the air react to form oxides of nitrogen (primarily nitric oxide, NO, which oxidizes further to NO_2).
Inside a car engine, the temperature becomes very high during combustion. At these high temperatures, nitrogen (N_2) and oxygen (O_2) from the air react to form oxides of nitrogen (primarily nitric oxide, NO, which oxidizes further to NO_2).
Removal by Catalytic Converters:
Catalytic converters are fitted in car exhaust systems. They contain metals like platinum or rhodium that act as catalysts. They convert the toxic pollutants (CO and NO_x) into harmless gases.
Reaction Example:
2CO + 2NO \rightarrow 2CO_2 + N_2
2CO + 2NO \rightarrow 2CO_2 + N_2
Here, carbon monoxide (CO) is oxidized to carbon dioxide (CO_2), and nitrogen oxides (NO) are reduced to harmless nitrogen gas (N_2).
Strategies to Reduce Environmental Issues
We must address both Climate Change and Acid Rain.
| Issue | Strategy to Reduce Effects |
|---|---|
| Climate Change (Global Warming) |
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| Acid Rain |
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⚠︎ Photosynthesis Reactants vs. Products
Mistake: Students often think glucose is a reactant or that oxygen is absorbed.
Correction: In photosynthesis, carbon dioxide and water are the reactants (inputs). Glucose and oxygen are the products (outputs). Oxygen is released as a byproduct.
Correction: In photosynthesis, carbon dioxide and water are the reactants (inputs). Glucose and oxygen are the products (outputs). Oxygen is released as a byproduct.
⚠︎ Greenhouse Gas Mechanism
Mistake: Describing the greenhouse effect as 'blocking' sunlight or 'reflecting' sunlight back to Earth.
Correction: Greenhouse gases do not block incoming sunlight. They absorb thermal energy (infrared radiation) emitted by the Earth and reflect it back to the surface. The key is the handling of thermal energy, not visible light.
Correction: Greenhouse gases do not block incoming sunlight. They absorb thermal energy (infrared radiation) emitted by the Earth and reflect it back to the surface. The key is the handling of thermal energy, not visible light.
Describing the Greenhouse Effect
When to use: When asked to explain how greenhouse gases cause global warming.
Why examiners accept this: The mark scheme specifically looks for the sequence of energy transfer. You must mention absorption of thermal energy by the gas and reflection (or re-emission) back to Earth. Vague terms like 'trapping heat' are often insufficient without these specific verbs.
Why examiners accept this: The mark scheme specifically looks for the sequence of energy transfer. You must mention absorption of thermal energy by the gas and reflection (or re-emission) back to Earth. Vague terms like 'trapping heat' are often insufficient without these specific verbs.
Example Answer:
"Greenhouse gases absorb thermal energy radiated from the Earth's surface and reflect it back towards the Earth, reducing the loss of thermal energy to space."
"Greenhouse gases absorb thermal energy radiated from the Earth's surface and reflect it back towards the Earth, reducing the loss of thermal energy to space."
Identifying Sources of Pollutants
When to use: When asked for the source of a specific pollutant.
Why examiners accept this: Examiners require precise sources. For example, 'cars' is too vague for NO_x; you must specify 'car engines' or 'high temperatures in car engines'. For SO_2, 'burning coal' is correct, but 'burning fuels containing sulfur' is more chemically accurate.
Why examiners accept this: Examiners require precise sources. For example, 'cars' is too vague for NO_x; you must specify 'car engines' or 'high temperatures in car engines'. For SO_2, 'burning coal' is correct, but 'burning fuels containing sulfur' is more chemically accurate.
Example Answer:
Q: Source of sulfur dioxide?
A: Combustion of fossil fuels containing sulfur impurities.
Q: Source of sulfur dioxide?
A: Combustion of fossil fuels containing sulfur impurities.
Composition and Pollutants
Q:
State the approximate percentage of nitrogen in clean, dry air.
A:
78%
Q:
Name one pollutant formed by incomplete combustion of hydrocarbons.
A:
Carbon monoxide (or particulates).
Q:
State one adverse effect of sulfur dioxide.
A:
Acid rain.
Greenhouse Effect and Photosynthesis
Q:
Describe how carbon dioxide causes global warming in terms of thermal energy.
A:
Carbon dioxide absorbs thermal energy emitted by the Earth and reflects it back to the surface, reducing thermal energy loss to space.
Q:
Write the word equation for photosynthesis.
A:
carbon dioxide + water \rightarrow glucose + oxygen (in the presence of light and chlorophyll)
Q:
State the symbol equation for photosynthesis.
A:
6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2
Catalytic Converters and Strategies
Q:
Explain how oxides of nitrogen are formed in car engines.
A:
The high temperature in the engine causes nitrogen and oxygen from the air to react.
Q:
State one strategy to reduce acid rain.
A:
Remove sulfur from fossil fuels before burning them (or use catalytic converters / flue gas desulfurization).
Q:
Name two gases produced in a catalytic converter from CO and NO.
A:
Carbon dioxide (CO_2) and Nitrogen (N_2).