Class 11 Chemistry Notes Chapter 14 (Chapter 14) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 14: Environmental Chemistry from your NCERT Exemplar. This chapter is crucial, not just for your Class 11 exams, but also frequently features in various government competitive exams. Pay close attention as we break down the key concepts.

Chapter 14: Environmental Chemistry - Detailed Notes for Competitive Exams

1. Introduction

• Definition: Environmental Chemistry deals with the study of the origin, transport, reactions, effects, and fates of chemical species in the environment.
• Scope: It encompasses the chemistry of the atmosphere, hydrosphere (water), lithosphere (soil and rocks), and biosphere, and the effects of human activities on them.
• Environmental Segments: Atmosphere, Hydrosphere, Lithosphere, Biosphere.
• Environmental Pollution: Undesirable changes in our surroundings (air, water, soil) that have harmful effects on plants, animals, and human beings.
• Pollutant: Any substance (solid, liquid, or gas) present in concentrations harmful to the environment. Pollutants can be:
  • Biodegradable: Rapidly broken down by natural processes (e.g., domestic sewage).
  • Non-biodegradable: Degrade very slowly or not at all (e.g., DDT, plastics, heavy metals, nuclear wastes).

2. Atmospheric Pollution

• The atmosphere is divided into layers: Troposphere (0-10 km), Stratosphere (10-50 km), Mesosphere, Thermosphere.
• Atmospheric pollution is generally studied as Tropospheric and Stratospheric pollution.

A. Tropospheric Pollution (Region of human activity)

• Causes: Presence of undesirable solid or gaseous particles in the air.
• Major Pollutants:
  • Gaseous Air Pollutants:
    • Oxides of Sulphur (SOx):
      • Source: Burning of fossil fuels (coal, oil) containing sulphur, volcanic eruptions.
      • Reactions:
        • 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
        • SO₂(g) + O₃(g) → SO₃(g) + O₂(g)
        • SO₂(g) + H₂O₂(aq) → H₂SO₄(aq)
        • SO₃(g) + H₂O(l) → H₂SO₄(aq) (Acid Rain)
      • Effects: Respiratory diseases (asthma, bronchitis), irritation to eyes, stiffness of flower buds, acid rain (corrodes buildings, damages ecosystems).
    • Oxides of Nitrogen (NOx - mainly NO and NO₂):
      • Source: Lightning, combustion of fossil fuels (automobiles, power plants) at high temperatures.
        • N₂(g) + O₂(g) --(High Temp)--> 2NO(g)
        • 2NO(g) + O₂(g) → 2NO₂(g)
        • NO(g) + O₃(g) → NO₂(g) + O₂(g) (Contributes to ozone destruction)
      • Effects: Respiratory problems, damage to plant leaves, acid rain (HNO₃ formation), contributes to smog formation, harmful to textiles and metals. NO₂ causes red haze in traffic.
    • Hydrocarbons:
      • Source: Incomplete combustion of fuel (automobiles), natural processes.
      • Types: Methane (CH₄) is a greenhouse gas. Others are carcinogenic.
      • Effects: Form photochemical smog, carcinogenic effects.
    • Oxides of Carbon:
      • Carbon Monoxide (CO):
        • Source: Incomplete combustion of carbon-based fuels (automobiles, firewood, coal), natural processes.
        • Effects: Highly poisonous. Binds to haemoglobin (~300 times more strongly than O₂) forming carboxyhaemoglobin, reducing oxygen-carrying capacity of blood. Causes headache, dizziness, cardiovascular issues, and death at high concentrations.
      • Carbon Dioxide (CO₂):
        • Source: Respiration, combustion of fossil fuels, decomposition of limestone, volcanic eruptions.
        • Effects: Essential for photosynthesis but increased concentration leads to Global Warming (Greenhouse Effect). Not traditionally considered a pollutant but its excess is harmful.
  • Particulate Pollutants: Finely divided solid or liquid particles suspended in air.
    • Types: Smoke, Dust, Mist, Fumes.
    • Viable Particulates: Minute living organisms (bacteria, fungi, moulds, algae).
    • Non-Viable Particulates:
      • Smoke: Solid/mixture particles from combustion (organic matter, fossil fuels).
      • Dust: Fine solid particles (>1 µm) from crushing, grinding, natural sources (sand storms).
      • Mist: Liquid droplets produced by condensation or spraying (e.g., H₂SO₄ mist, herbicides/insecticides).
      • Fumes: Condensed vapours, generally metallic oxides from metallurgical operations.
    • Effects: Respiratory problems, reduced visibility, deposition on surfaces. Particulates < 5 microns can penetrate deep into lungs. Lead particulates (from leaded petrol) affect nervous system development.
• Smog: Combination of smoke and fog.
  • Classical Smog (London Smog):
    • Conditions: Cool, humid climate.
    • Components: Smoke, fog, SO₂.
    • Nature: Reducing mixture.
    • Occurs: Early morning, low temperature.
  • Photochemical Smog (Los Angeles Smog):
    • Conditions: Warm, dry, sunny climate.
    • Components: NOx, hydrocarbons, ozone (O₃), PAN (Peroxyacetyl Nitrate), acrolein, formaldehyde. Formed by photochemical reactions involving sunlight.
    • Nature: Oxidising mixture.
    • Formation: Initiated by sunlight acting on NOx and hydrocarbons. Results in high concentrations of ozone (tropospheric ozone is harmful) and PAN.
    • Effects: Eye irritation, headache, chest pain, throat dryness, cough, respiratory difficulties, cracking of rubber, damage to plants, corrosion of metals/building materials, reduced visibility (brown/yellowish haze due to NO₂).
    • Control: Catalytic converters in automobiles, planting specific trees (Pinus, Pyrus, Vitis etc. metabolize NOx).

B. Acid Rain

• Definition: Rainwater with pH below 5.6 (normal rain pH is ~5.6 due to dissolved CO₂ forming H₂CO₃).
• Causes: Presence of SO₂ and NOx in the atmosphere, which react with water vapour and oxygen to form H₂SO₄ and HNO₃.
  • 2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq)
  • 4NO₂(g) + O₂(g) + 2H₂O(l) → 4HNO₃(aq)
• Effects: Damages buildings and statues (especially marble - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), harms aquatic life (lowers pH of water bodies), damages forests and crops, leaches heavy metals into water sources.

C. Greenhouse Effect and Global Warming

• Greenhouse Effect: Natural process where certain atmospheric gases (Greenhouse Gases) trap heat radiated from the Earth's surface, keeping the planet warm enough for life.
• Greenhouse Gases (GHGs): CO₂, CH₄, O₃, N₂O, Chlorofluorocarbons (CFCs), Water Vapour.
• Global Warming: Enhancement of the natural greenhouse effect due to increased concentrations of GHGs from human activities (burning fossil fuels, deforestation, agriculture), leading to a rise in global average temperature.
• Effects of Global Warming: Climate change (altered weather patterns, increased frequency of extreme events), melting of glaciers and polar ice caps, sea-level rise, impacts on ecosystems and agriculture.

D. Stratospheric Pollution - Ozone Layer Depletion

• Ozone Layer: Found in the stratosphere (10-50 km), protects Earth from harmful UV-B radiation (280-320 nm).
• Formation/Depletion Balance: Ozone is naturally formed and destroyed by UV radiation:
  • O₂(g) --(UV)--> O(g) + O(g)
  • O(g) + O₂(g) ⇌ O₃(g) (Formation/Destruction)
• Ozone Depletion: Thinning of the ozone layer, especially over Antarctica (the "ozone hole").
• Causes: Release of synthetic chemicals, primarily Chlorofluorocarbons (CFCs) or Freons (used as refrigerants, propellants, solvents). Also, NOx released by supersonic jets or industrial processes.
• Mechanism (CFCs):
  1. CFCs are stable in the troposphere and diffuse into the stratosphere.
  2. UV radiation breaks down CFCs, releasing chlorine free radicals (Cl•).
     • CF₂Cl₂(g) --(UV)--> Cl•(g) + •CF₂Cl(g)
  3. Chlorine radicals catalytically destroy ozone:
     • Cl•(g) + O₃(g) → ClO•(g) + O₂(g)
     • ClO•(g) + O(g) → Cl•(g) + O₂(g)
  4. The Cl• radical is regenerated, allowing one Cl• atom to destroy thousands of O₃ molecules.
• Role of Polar Stratospheric Clouds (PSCs): During winter over poles, PSCs form. They provide surfaces for reactions that convert chlorine reservoirs (like ClONO₂ and HCl) into active forms (like Cl₂), which are photolyzed by sunlight in spring to release Cl•, leading to massive ozone depletion (Ozone Hole).
• Effects of Ozone Depletion: Increased UV-B radiation reaching Earth, causing skin cancer (melanoma), cataracts, sunburns, damage to immune system, harm to phytoplankton, effects on plant growth and crop yields.
• Control: Montreal Protocol (international treaty to phase out ozone-depleting substances like CFCs). Use of substitutes like hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) - though HFCs are potent greenhouse gases.

3. Water Pollution

• Definition: Contamination of water bodies (lakes, rivers, oceans, groundwater) usually as a result of human activities, making it unfit for its intended uses.
• Sources: Point sources (specific discharge points like pipes from factories) and Non-point sources (diffused sources like agricultural runoff, acid rain).
• Major Water Pollutants and Their Effects:
  • Pathogens: Disease-causing microorganisms (bacteria, viruses, protozoa) from domestic sewage and animal excreta. Cause waterborne diseases like cholera, typhoid, dysentery.
  • Organic Wastes: Biodegradable matter (sewage, leaves, grass clippings, food waste). Consumed by bacteria, which depletes dissolved oxygen (DO) in water.
    • Biochemical Oxygen Demand (BOD): Amount of dissolved oxygen needed by aerobic microorganisms to break down organic material present in a given water sample at certain temperature over a specific time period (usually 5 days at 20°C).
    • High BOD indicates high level of organic pollution. Clean water generally has BOD < 5 ppm. Polluted water can have BOD ≥ 17 ppm.
    • Low DO (due to high BOD) kills fish and other aquatic life.
  • Chemical Pollutants:
    • Heavy Metals: Cadmium (Cd), Mercury (Hg), Lead (Pb), Nickel (Ni). From industries, mining, paints. Toxic to humans and aquatic life (e.g., Minamata disease from Hg poisoning, Itai-Itai disease from Cd poisoning).
    • Pesticides/Insecticides: DDT, Aldrin, BHC. Runoff from agriculture. Toxic, persistent, biomagnify in food chains.
    • Polychlorinated Biphenyls (PCBs): Used in electrical equipment, plastics. Persistent, carcinogenic.
    • Fertilizers (Phosphates and Nitrates): Runoff from agriculture. Cause Eutrophication.
    • Industrial Chemicals: Acids, alkalis, detergents, solvents.
• Eutrophication: Nutrient enrichment of water bodies (mainly by nitrates and phosphates), leading to excessive growth of algae (algal bloom). When algae die, their decomposition consumes large amounts of DO, leading to oxygen depletion and death of aquatic animals. Reduces water quality.
• International Standards for Drinking Water: Specify maximum permissible limits for various ions and contaminants.
  • Fluoride (F⁻): Up to 1 ppm beneficial (prevents tooth decay). > 1.5 ppm causes mottled teeth (fluorosis). > 10 ppm harmful (bone/teeth damage).
  • Lead (Pb): Max limit 50 ppb. Can damage kidney, liver, reproductive system.
  • Sulphate (SO₄²⁻): Max limit 500 ppm. Excess causes laxative effect.
  • Nitrate (NO₃⁻): Max limit 50 ppm. Excess causes methemoglobinemia (Blue Baby Syndrome) in infants.
  • Other metals (Fe, Mn, Al, Cu, Zn, Cd): Have prescribed limits.

4. Soil Pollution

• Definition: Build-up of persistent toxic compounds, chemicals, salts, radioactive materials, or disease-causing agents in soil, which have adverse effects on plant growth and human/animal health.
• Sources:
  • Pesticides/Herbicides/Insecticides/Fungicides: Persistent organic pollutants accumulate in soil, enter food chain.
  • Industrial Wastes: Disposal of untreated industrial effluents, fly ash, heavy metals.
  • Urban Wastes: Sewage sludge, plastics, glass, discarded materials.
  • Agricultural Practices: Excessive use of fertilizers.
• Control: Proper waste disposal (segregation, recycling, composting), controlled use of pesticides and fertilizers, bioremediation.

5. Strategies to Control Environmental Pollution

• Waste Management:
  • Segregation of waste at source.
  • Reduction, Reuse, Recycling (3 R's).
  • Proper treatment of sewage and industrial effluents before discharge.
  • Incineration (controlled burning) for certain types of waste.
  • Secure landfills for non-biodegradable waste.
• Green Chemistry:
  • Definition: Design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.
  • Principles: Prevention of waste, atom economy, less hazardous chemical syntheses, designing safer chemicals, safer solvents and auxiliaries, design for energy efficiency, use of renewable feedstocks, reduce derivatives, catalysis, design for degradation, real-time analysis for pollution prevention, inherently safer chemistry for accident prevention.
  • Examples: Using H₂O₂ for bleaching paper instead of Cl₂, synthesizing ibuprofen more efficiently (higher atom economy), using supercritical CO₂ as a solvent.

Key Terms to Remember:

• BOD, COD (Chemical Oxygen Demand)
• Eutrophication
• Biomagnification
• Greenhouse Gases
• Acid Rain
• Smog (Classical vs Photochemical)
• PAN (Peroxyacetyl Nitrate)
• Ozone Hole
• CFCs
• Green Chemistry
• Pollutant (Biodegradable vs Non-biodegradable)

Multiple Choice Questions (MCQs)

1. Which of the following is responsible for the depletion of the ozone layer in the upper strata of the atmosphere?
   (a) Polyhalogens (CFCs)
   (b) Ferrocene
   (c) Fullerenes
   (d) Freons

2. Classical smog occurs in places with:
   (a) High temperature, dry climate
   (b) Low temperature, high humidity, presence of SO₂
   (c) High concentration of NOx and hydrocarbons
   (d) Intense sunlight and presence of O₃

3. Biochemical Oxygen Demand (BOD) is a measure of:
   (a) Industrial pollution
   (b) Dissolved oxygen needed by microbes to decompose organic waste
   (c) Amount of carbon dioxide released during decomposition
   (d) Dissolved salts in water

4. Which of the following is NOT a greenhouse gas?
   (a) Carbon dioxide (CO₂)
   (b) Methane (CH₄)
   (c) Oxygen (O₂)
   (d) Nitrous oxide (N₂O)

5. Acid rain is primarily caused by the dissolution of which oxides in rainwater?
   (a) CO₂ and CO
   (b) SO₂ and NOx
   (c) N₂O and NH₃
   (d) O₃ and CH₄

6. Excessive concentration of nitrate (NO₃⁻) in drinking water can cause:
   (a) Minamata disease
   (b) Fluorosis
   (c) Blue Baby Syndrome (Methemoglobinemia)
   (d) Itai-Itai disease

7. Photochemical smog is characterized by high concentrations of:
   (a) SO₂, Fog, Smoke
   (b) CO, CO₂, Particulates
   (c) O₃, PAN, NOx
   (d) H₂SO₄, HNO₃, Dust

8. Which component of photochemical smog is responsible for the brownish haze and has oxidizing properties?
   (a) Carbon monoxide (CO)
   (b) Sulphur dioxide (SO₂)
   (c) Nitrogen dioxide (NO₂)
   (d) Methane (CH₄)

9. Green Chemistry aims to:
   (a) Increase the use of non-renewable resources
   (b) Design chemical processes that generate hazardous waste
   (c) Reduce or eliminate the use and generation of hazardous substances
   (d) Promote the use of persistent organic pollutants

10. Eutrophication in lakes is caused by:
    (a) Depletion of nutrients like nitrates and phosphates
    (b) Enrichment of water with nutrients leading to algal blooms
    (c) Increase in heavy metal concentration
    (d) Presence of pathogens

Answer Key for MCQs:

1. (d) Freons (Freons are a type of CFC, which are polyhalogen compounds, but Freons is the more specific common name often used in this context. If both (a) and (d) were options and referred to the same thing, the question might be ambiguous, but Freons are the classic example taught). Let's refine: CFCs are the primary cause. Freon is a trade name for certain CFCs. Polyhalogens is a broader class. Given the options, Freons is the most direct answer related to ozone depletion taught in this context. Self-correction: Option (a) Polyhalogens is technically broader and includes CFCs. However, 'Freons' is specifically linked to ozone depletion in textbooks. Let's stick with (d) as the most intended answer, but acknowledge CFCs are the chemical class.
2. (b) Low temperature, high humidity, presence of SO₂
3. (b) Dissolved oxygen needed by microbes to decompose organic waste
4. (c) Oxygen (O₂)
5. (b) SO₂ and NOx
6. (c) Blue Baby Syndrome (Methemoglobinemia)
7. (c) O₃, PAN, NOx
8. (c) Nitrogen dioxide (NO₂)
9. (c) Reduce or eliminate the use and generation of hazardous substances
10. (b) Enrichment of water with nutrients leading to algal blooms

Make sure you understand the underlying concepts behind each point and MCQ. Environmental chemistry is very relevant today, and understanding these basics is essential. Good luck with your preparation!