Environmental Chemistry Class 11 Chapter Notes

Key Environmental Terms

Environmental Pollution

The addition of undesirable substances to the environment, or the alteration of physical, chemical, or biological characteristics of the environment, making it harmful for living organisms.

Pollutant

A substance present in nature, released by human activity, which has a detrimental effect on the environment.

Contaminant

A substance that does not occur naturally in the environment but is introduced by human activity, impacting environmental quality.

Smog

A combination of smoke and fog. There are two types: Classical (reducing) smog and Photochemical (oxidizing) smog.

BOD (Biochemical Oxygen Demand)

The amount of oxygen required by bacteria to break down organic matter present in a certain volume of a sample of water. A low BOD indicates cleaner water.

Greenhouse Effect

The warming of Earth's surface and troposphere caused by gases in the atmosphere (like CO2, CH4, N2O) which absorb outgoing infrared radiation.

Acid Rain

Rain, snow, fog, or dew that is unusually acidic due to atmospheric pollution, primarily from sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions.

Tropospheric Pollution: Air & Water

The troposphere is the lowest layer of the atmosphere, extending up to approximately 10 km from sea level. It is the most turbulent region, where living organisms exist, and it's most susceptible to pollution. Tropospheric pollution refers to the presence of undesirable solid or gaseous particles in the air and contaminants in water at harmful concentrations.

Gaseous Air Pollutants include oxides of sulfur (SO2, SO3), oxides of nitrogen (NO, NO2), carbon monoxide (CO), carbon dioxide (CO2), and hydrocarbons. Sulphur dioxide (SO2) is primarily produced by burning fossil fuels and can cause respiratory diseases, acid rain, and damage to crops. Nitrogen oxides (NOx), formed from vehicles and power plants, contribute to photochemical smog and acid rain. Carbon monoxide (CO) is highly toxic as it binds to hemoglobin, forming carboxyhemoglobin, reducing oxygen transport. Carbon dioxide (CO2) is a major greenhouse gas, leading to global warming.

Particulate Pollutants are tiny solid particles or liquid droplets suspended in air. Examples include dust, mist, fumes, smoke, and smog. Photochemical smog, a key tropospheric phenomenon, is formed by the action of sunlight on nitrogen oxides and hydrocarbons. Its main components are ozone (O3), nitric oxide (NO), acrolein, formaldehyde, and peroxyacetyl nitrate (PAN). It causes eye irritation, respiratory problems, and damage to plants.

Water Pollution occurs when toxic substances enter water bodies, contaminating them. Common water pollutants include pathogens (bacteria, viruses from domestic sewage), organic wastes (biodegradable matter causing a decrease in dissolved oxygen), chemical pollutants (heavy metals like Cd, Pb, Hg, pesticides, PCBs), and suspended solids. High BOD values indicate severe water pollution due to excessive organic matter requiring more oxygen for decomposition.

Stratospheric Pollution: Ozone Depletion

The stratosphere is the atmospheric layer above the troposphere, extending up to 50 km. It hosts the ozone layer, which acts as a protective shield, absorbing harmful ultraviolet (UV) radiation from the sun. Ozone (O3) is constantly formed and decomposed in the stratosphere: oxygen molecules (O2) are dissociated by UV radiation into oxygen atoms (O), which then react with O2 to form O3.

Ozone depletion refers to the gradual thinning of the ozone layer, primarily over the polar regions (leading to the ozone hole). The main culprits are chlorofluorocarbons (CFCs), synthetic chemicals previously used in refrigerators, air conditioners, and aerosol sprays. CFCs are stable in the troposphere but reach the stratosphere where UV radiation breaks them down, releasing chlorine free radicals (Cl•).

These chlorine radicals act as catalysts, initiating a chain reaction that breaks down ozone molecules:

1. CF2Cl2(g) + UV → Cl•(g) + CF2Cl•(g)
2. Cl•(g) + O3(g) → ClO•(g) + O2(g)
3. ClO•(g) + O(g) → Cl•(g) + O2(g)

The chlorine radical is regenerated, allowing it to destroy thousands of ozone molecules before being removed. This depletion leads to increased UV radiation reaching Earth's surface, causing skin cancer, cataracts, immune system damage, and harming phytoplankton.

International efforts like the Montreal Protocol (1987) have led to the phasing out of CFCs, showing positive signs of ozone layer recovery.

Pollutant Classification: Primary vs. Secondary & Biodegradable vs. Non-Biodegradable

Key Points to Remember

• Acid Rain: Caused primarily by oxides of sulfur (SO2) and nitrogen (NOx) released from industries and vehicles. pH of rain becomes less than 5.6.
• Greenhouse Gases (GHGs): Key GHGs are CO2 (major contributor), CH4, N2O, CFCs, and O3 (tropospheric). They trap heat, leading to global warming.
• Eutrophication: The process by which a body of water becomes overly enriched with minerals and nutrients (e.g., from fertilisers), leading to excessive growth of algae and depletion of dissolved oxygen.
• Minamata Disease: Caused by mercury poisoning (specifically methylmercury) in water, affecting the nervous system.
• Ital-Itai Disease: Caused by cadmium poisoning, leading to severe bone and kidney damage.
• Green Chemistry: A philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances. Focus on atom economy and using renewable feedstocks.
• Strategy to control pollution: 'Reduce, Reuse, Recycle' and proper waste management are crucial steps.
• Sources of Soil Pollution: Pesticides, insecticides, herbicides, industrial waste, and agricultural run-off.

Exam Tip: Mastering Environmental Chemistry

For Environmental Chemistry, focus on cause-and-effect relationships. Be prepared to explain the mechanisms of ozone depletion (chlorine radical chain reaction) and the formation of acid rain with relevant chemical equations. Differentiate clearly between photochemical smog and classical smog, and between the greenhouse effect and global warming. Memorize the main greenhouse gases and their sources. Pay attention to the impact of different pollutants (e.g., CO toxicity, effects of SO2, NOx). Understanding BOD is frequently tested. Green Chemistry principles are also important for short answer questions.

Practice Questions with Solutions

• Q: Name any two components of photochemical smog. A: Ozone (O3) and Peroxyacetyl Nitrate (PAN).
• Q: What is the primary cause of ozone layer depletion? A: Chlorofluorocarbons (CFCs) releasing chlorine free radicals in the stratosphere.
• Q: Define BOD and explain its significance. A: BOD is the amount of oxygen consumed by microorganisms to decompose organic matter in water. A higher BOD indicates more organic pollution and thus lower dissolved oxygen, threatening aquatic life.
• Q: List two adverse effects of acid rain. A: Damage to buildings and monuments (e.g., marble corrosion) and acidification of lakes and soil, harming aquatic life and vegetation.

Frequently Asked Questions