Environmental Chemistry

Welcome to this comprehensive study guide on Environmental Chemistry for the CXC Chemistry syllabus (2024-2025). This resource covers all aspects of Environmental Chemistry as required by the examination board.

Learning Objectives:

By the end of this lesson, you should be able to:

Understand the chemistry of the atmosphere and its layers
Describe air pollution, its causes, effects, and control measures
Explain water pollution, its sources, effects, and treatment methods
Discuss soil pollution and its impact on the environment
Evaluate the importance of green chemistry principles
Analyze the impact of human activities on the environment

1. The Earth's Atmosphere

1.1 Composition and Structure of the Atmosphere

The atmosphere is a mixture of gases that surrounds the Earth, held in place by gravity. It plays a crucial role in supporting life and maintaining the Earth's temperature.

Composition of Clean, Dry Air (at sea level):

Nitrogen (N₂): 78.08%
Oxygen (O₂): 20.95%
Argon (Ar): 0.93%
Carbon dioxide (CO₂): 0.04% (increasing due to human activities)
Trace gases: Neon, Helium, Methane, Krypton, Hydrogen, etc.
Variable components: Water vapor (0-4%), dust particles, pollen

1.2 Layers of the Atmosphere

The atmosphere is divided into several layers based on temperature variation with altitude:

Fig 1: Layers of the Earth's Atmosphere

Troposphere (0-12 km): Contains about 75% of all atmospheric gases and almost all water vapor. Weather phenomena occur here. Temperature decreases with altitude (about 6.5°C per km).
Stratosphere (12-50 km): Contains the ozone layer. Temperature increases with altitude due to absorption of UV radiation by ozone.
Mesosphere (50-80 km): Temperature decreases with altitude again. Coldest part of the atmosphere (-90°C).
Thermosphere (80-700 km): Temperature increases with altitude due to absorption of short-wave radiation.
Exosphere (> 700 km): Outermost layer, gradually fades into space.

1.3 The Ozone Layer

The ozone layer is found primarily in the stratosphere between 15-35 km altitude. It plays a crucial role in protecting life on Earth by absorbing harmful ultraviolet radiation from the sun.

Formation and Destruction of Ozone:

Formation:

O₂ + UV radiation → O + O (Photodissociation)
O + O₂ + M → O₃ + M (where M is a third body, usually N₂ or O₂)

Natural destruction:

O₃ + UV radiation → O₂ + O
O + O₃ → 2O₂

1.4 Ozone Depletion

Ozone depletion is the thinning of the ozone layer, most notably observed over Antarctica (the "ozone hole").

Ozone Depletion by CFCs:

Chlorofluorocarbons (CFCs) are stable compounds that can reach the stratosphere where they break down under UV radiation:

CFCl₃ + UV radiation → CFCl₂ + Cl
Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂

The chlorine atom acts as a catalyst and can destroy thousands of ozone molecules before being removed from the cycle.

Example: Impact of the Montreal Protocol

The Montreal Protocol, signed in 1987, is an international agreement to phase out substances that deplete the ozone layer. Since its implementation, there has been a measurable reduction in atmospheric concentrations of many ozone-depleting substances.

CFC replacements include HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons), which are less damaging to the ozone layer but still contribute to the greenhouse effect.

2. Air Pollution

2.1 Definition and Types of Air Pollutants

Air pollution is the presence of substances in the atmosphere at concentrations high enough to cause harm to humans, other organisms, or materials.

Pollutant Type	Examples	Sources	Effects
Primary Pollutants	SO₂, NO, CO, particulates	Directly emitted from sources	Respiratory problems, acid rain, smog
Secondary Pollutants	O₃, NO₂, HNO₃, H₂SO₄	Formed by reactions in the atmosphere	Photochemical smog, acid rain

2.2 Major Air Pollutants

Carbon Monoxide (CO): Colorless, odorless gas formed by incomplete combustion of carbon-containing fuels. Binds to hemoglobin, reducing oxygen transport in the blood.
Sulfur Dioxide (SO₂): Produced by combustion of sulfur-containing fuels. Contributes to acid rain and respiratory problems.
Nitrogen Oxides (NOₓ): Produced by high-temperature combustion processes. Form acid rain and photochemical smog.
Particulate Matter: Solid or liquid particles suspended in air. Classified by size (PM10, PM2.5, etc.). Can penetrate deep into the lungs.
Volatile Organic Compounds (VOCs): Organic chemicals that evaporate easily at room temperature. Contribute to ground-level ozone formation.
Ground-level Ozone (O₃): Secondary pollutant formed by photochemical reactions involving NOₓ and VOCs. Major component of photochemical smog.

2.3 Acid Rain

Acid rain refers to any precipitation (rain, snow, fog) with pH lower than 5.6 (natural rainwater pH due to dissolved CO₂).

Formation of Acid Rain:

Sulfur dioxide reactions:

SO₂ + OH → HOSO₂
HOSO₂ + O₂ → HO₂ + SO₃
SO₃ + H₂O → H₂SO₄

Nitrogen oxide reactions:

2NO + O₂ → 2NO₂
2NO₂ + H₂O → HNO₂ + HNO₃
3HNO₂ → HNO₃ + 2NO + H₂O

Fig 2: Formation and Effects of Acid Rain

Effects of Acid Rain:

Aquatic ecosystems: Lowers pH of lakes and rivers, affecting aquatic life. pH below 5.0 can kill fish eggs, below 4.5 kills adult fish.
Forests: Damages leaves, depletes soil nutrients, exposes trees to disease.
Buildings and monuments: Corrodes metals, erodes stone (especially limestone and marble).
Human health: Can affect respiratory system when acidic particles are inhaled.

2.4 The Greenhouse Effect and Climate Change

The greenhouse effect is a natural process that warms the Earth's surface. Without it, the Earth would be too cold to support life. However, enhanced greenhouse effect due to human activities is causing global warming.

Major Greenhouse Gases:

Carbon dioxide (CO₂): From burning fossil fuels, deforestation
Methane (CH₄): From agriculture, landfills, natural gas leaks
Nitrous oxide (N₂O): From fertilizers, industrial processes
Water vapor (H₂O): Most abundant but short-lived in atmosphere
Chlorofluorocarbons (CFCs): From refrigerants, aerosol propellants

Fig 3: The Greenhouse Effect

Example: Carbon Footprint Calculation

To calculate the carbon footprint of electricity usage:

Determine electricity usage in kWh (e.g., 5,000 kWh per year)
Multiply by the emission factor (e.g., 0.5 kg CO₂ per kWh)
Result: 5,000 kWh × 0.5 kg CO₂/kWh = 2,500 kg CO₂ per year

This shows how individual actions contribute to greenhouse gas emissions.

2.5 Control of Air Pollution

Methods to Control Air Pollution:

Scrubbers: Remove SO₂ by reaction with lime or limestone.
CaCO₃ + SO₂ → CaSO₃ + CO₂
CaSO₃ + ½O₂ → CaSO₄ (gypsum)
Catalytic converters: Convert harmful exhaust gases to less harmful ones.
2CO + 2NO → 2CO₂ + N₂
2CO + O₂ → 2CO₂
4HC + 5O₂ → 4CO₂ + 2H₂O
Electrostatic precipitators: Remove particulate matter by giving them an electric charge and collecting them on oppositely charged plates.
Alternative energy sources: Solar, wind, hydroelectric power to reduce fossil fuel dependence.

3. Water Chemistry and Pollution

3.1 Properties of Water

Water has unique properties due to hydrogen bonding that make it essential for life:

High specific heat capacity: Requires a lot of energy to change temperature, making bodies of water temperature buffers.
High heat of vaporization: Makes evaporative cooling effective.
Expansion upon freezing: Ice floats on water, insulating bodies of water in winter.
Universal solvent: Dissolves many substances due to its polar nature.
Surface tension: Water molecules attract each other, creating a "skin" at the surface.

Fig 4: Water Molecule Structure and Hydrogen Bonding

3.2 Water Pollution

Water pollution is the contamination of water bodies (lakes, rivers, oceans, groundwater) by substances that make the water unsuitable for its intended use.

Major Water Pollutants:

Organic waste: Sewage, animal waste, plant debris. Leads to oxygen depletion.
Pathogens: Disease-causing microorganisms from untreated sewage.
Nutrients: Nitrates and phosphates from fertilizers, causing eutrophication.
Heavy metals: Mercury, lead, cadmium from industrial processes, mining.
Pesticides and herbicides: From agricultural runoff.
Oil and petroleum products: From spills, leaks, improper disposal.
Heat: Thermal pollution from power plants and industrial cooling.

3.3 Eutrophication

Eutrophication is the enrichment of water bodies with nutrients, primarily phosphates and nitrates, leading to excessive plant and algal growth. This results in oxygen depletion, harming aquatic life and disrupting ecosystems.

Fig: The Eutrophication Process

Major Water Pollutants:

Organic waste: Sewage, animal waste, plant debris. Leads to oxygen depletion.
Pathogens: Disease-causing microorganisms from untreated sewage.
Nutrients: Nitrates and phosphates from fertilizers, causing eutrophication.
Heavy metals: Mercury, lead, cadmium from industrial processes, mining.
Pesticides and herbicides: From agricultural runoff.
Oil and petroleum products: From spills, leaks, improper disposal.
Heat: Thermal pollution from power plants and industrial cooling.

3.3 Eutrophication

Eutrophication Process (Diagram)

Diagram showing the process of eutrophication

Glossary

Eutrophication: The process by which water bodies become enriched with nutrients, causing excessive plant growth and oxygen depletion.
Pathogens: Microorganisms that cause disease.
Organic waste: Waste from living organisms that decomposes and reduces oxygen in water.
Thermal pollution: Rise in water temperature due to industrial discharge, affecting aquatic ecosystems.
Heavy metals: Toxic elements like lead and mercury that can bioaccumulate in organisms.
Bioaccumulation: The build-up of substances, like pesticides or heavy metals, in an organism over time.

Self-Assessment

What are three examples of organic waste?
How do nitrates and phosphates affect water bodies?
What is thermal pollution and what causes it?
Name two heavy metals that pollute water.
What are the dangers of eutrophication to aquatic life?

Answers

Sewage, animal waste, and plant debris.
They cause eutrophication, leading to algal blooms and oxygen depletion.
Thermal pollution is the increase in water temperature due to industrial cooling or power plant discharge.
Mercury and lead.
Eutrophication leads to oxygen depletion, which can kill fish and other aquatic organisms.