Class 11 Notes

Class 11 Chemistry Notes Chapter 7 (Environmental Chemistry) – Chemistry Part-II Book

Philoid

Philoid

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Chemistry Part-II
Alright class, let's focus today on a very crucial chapter for your exams and for understanding the world around us – Chapter 7, Environmental Chemistry from your NCERT Class 11 Part II book. This chapter deals with the chemical phenomena occurring in the environment, focusing heavily on pollution and its management. Pay close attention, as questions from this area frequently appear in various government exams.

Environmental Chemistry: Detailed Notes for Exam Preparation

1. Introduction

  • Definition: Environmental Chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It deals with the origin, transport, reactions, effects, and fates of chemical species in the air, water, and soil environments, and the effect of human activity on these.
  • Environmental Segments: Lithosphere (Solid Earth), Hydrosphere (Water bodies), Atmosphere (Gaseous envelope), Biosphere (Living organisms).
  • Environmental Pollution: The effect of undesirable changes in our surroundings that have harmful effects on plants, animals, and human beings. A substance causing pollution is known as a pollutant. Pollutants can be solid, liquid, or gaseous, produced due to human activities or natural happenings.

2. Atmospheric Pollution

  • The atmosphere is divided into layers: Troposphere (0-10 km above sea level, contains ~80% air mass, weather phenomena occur here) and Stratosphere (10-50 km above sea level, contains the ozone layer).

  • Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.

    A. Tropospheric Pollution: Occurs in the lowest region of the atmosphere.
    * Gaseous Air Pollutants:
    * Oxides of Sulphur (SOx): Primarily Sulphur Dioxide (SO₂).
    * Sources: Burning of fossil fuels containing sulphur (coal, oil), volcanic eruptions.
    * Effects: Respiratory diseases (asthma, bronchitis, emphysema), irritation to the eyes, stiffness of flower buds. Major contributor to Acid Rain.
    * Reaction Example (Acid Rain): 2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq)
    * Oxides of Nitrogen (NOx): Primarily Nitric Oxide (NO) and Nitrogen Dioxide (NO₂).
    * Sources: High-temperature combustion (automobile engines, power plants), lightning.
    * Effects: Respiratory problems, damage to plant leaves, damage to textiles and metals. Contributes to Acid Rain and Photochemical Smog.
    * Reaction Example (Acid Rain): 2NO(g) + O₂(g) → 2NO₂(g); 3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)
    * Hydrocarbons: (Methane, Benzene etc.)
    * Sources: Incomplete combustion of fuel in automobiles, industrial processes.
    * Effects: Carcinogenic (cancer-causing), harm plants, contribute to Photochemical Smog.
    * Carbon Monoxide (CO):
    * Sources: Incomplete combustion of carbon-based fuels (automobile exhaust, firewood, coal), natural processes.
    * Effects: Highly poisonous. Binds to haemoglobin (~200-300 times more strongly than O₂) forming carboxyhaemoglobin, reducing the oxygen-carrying capacity of blood, leading to headache, dizziness, asphyxiation, and death.
    * Carbon Dioxide (CO₂):
    * Sources: Respiration, burning of fossil fuels, deforestation, cement manufacturing.
    * Effects: Major contributor to the Greenhouse Effect and Global Warming. Not traditionally toxic at normal atmospheric levels but its increasing concentration is a major concern.

    *   **Greenhouse Effect and Global Warming:**
    *   *Greenhouse Gases:* CO₂, Methane (CH₄), Water Vapour (H₂O), Nitrous Oxide (N₂O), Ozone (O₃), Chlorofluorocarbons (CFCs).
    *   *Mechanism:* These gases trap outgoing infrared (heat) radiation from the Earth's surface, warming the atmosphere. This natural effect keeps the Earth warm enough for life.
    *   *Global Warming:* Enhancement of the greenhouse effect due to increased concentrations of greenhouse gases from human activities, leading to a rise in global average temperature.
    *   *Consequences:* Melting of glaciers and polar ice caps, rising sea levels, changes in weather patterns, impacts on ecosystems.

*   **Acid Rain:**
    *   Rainwater with a pH below 5.6 (normal rain pH is ~5.6 due to dissolved CO₂ forming weak carbonic acid).
    *   *Causes:* Oxides of sulphur (SO₂) and nitrogen (NOx) reacting with water vapour in the atmosphere to form sulphuric acid (H₂SO₄) and nitric acid (HNO₃).
    *   *Effects:* Damages buildings and statues (especially marble - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), harms aquatic life, damages forests, leaches heavy metals into water sources, affects soil fertility.

*   **Particulate Pollutants:** Minute solid particles or liquid droplets suspended in the air.
    *   *Types:*
        *   *Viable:* Bacteria, fungi, moulds, algae (can cause diseases).
        *   *Non-viable:* Classified by size and nature:
            *   *Smoke:* Solid/mixed particles from combustion (e.g., cigarette smoke, burning fossil fuels).
            *   *Dust:* Fine solid particles (>1µm) from crushing, grinding (e.g., sand, sawdust, fly ash).
            *   *Mist:* Liquid droplets from spray liquids or condensation (e.g., sulphuric acid mist, herbicides/insecticides).
            *   *Fumes:* Condensed vapours, generally metallic oxides from processes like smelting.
    *   *Effects:* Reduce visibility. Particles < 5 microns can penetrate deep into lungs causing respiratory issues. PM2.5 (particulate matter ≤ 2.5 µm) is particularly harmful. Can carry adsorbed toxic substances.

*   **Smog:** Combination of smoke and fog.
    *   **Classical Smog (London Smog):** Occurs in cool, humid climates. Mixture of smoke, fog, and SO₂. It is *reducing* in nature.
    *   **Photochemical Smog (Los Angeles Smog):** Occurs in warm, dry, sunny climates. Results from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories.
        *   *Components:* Ozone (O₃), Formaldehyde, Acrolein, Peroxyacetyl nitrate (PAN), Nitrogen oxides (NOx).
        *   *Formation:* Requires sunlight. NOx + Hydrocarbons --(Sunlight)--> PAN, O₃, Aldehydes etc.
        *   *Effects:* Eye irritation, headache, chest pain, coughing, damage to plants (ozone, PAN), corrosion of metals, cracking of rubber. It is *oxidizing* in nature.
        *   *Control:* Catalytic converters in vehicles (reduce NOx and hydrocarbon emissions), planting specific trees (e.g., Pinus, Juniparus) that metabolize NOx.

    B. Stratospheric Pollution:
    * Ozone Layer Depletion:
    * Location: Stratosphere (contains ~90% of atmospheric ozone).
    * Function: Absorbs harmful ultraviolet (UV-B) radiation from the sun, protecting life on Earth.
    * Depletion Cause: Primarily Chlorofluorocarbons (CFCs or freons - used as refrigerants, propellants). Other substances include NOx (from supersonic jets), CCl₄.
    * Mechanism: UV radiation breaks down CFCs in the stratosphere, releasing chlorine free radicals (Cl•). These radicals catalytically destroy ozone.
    * CF₂Cl₂(g) --(UV)--> Cl•(g) + •CF₂Cl(g)
    * Cl•(g) + O₃(g) → ClO•(g) + O₂(g)
    * ClO•(g) + O(g) → Cl•(g) + O₂(g) (Regenerates Cl•, continuing the cycle)
    * Ozone Hole: Region of exceptionally depleted ozone over Antarctica, particularly evident during spring (Sept-Oct). Unique atmospheric conditions (Polar Stratospheric Clouds) facilitate the reactions.
    * Effects of Ozone Depletion: Increased UV-B radiation reaching Earth leads to skin cancer (melanoma), cataracts, sunburn, damage to plants, reduced phytoplankton productivity.

3. Water Pollution

  • Contamination of water bodies (lakes, rivers, oceans, groundwater) usually as a result of human activities.

  • Sources:

    • Point Sources: Discharge pollutants at specific locations through drain pipes or ditches (e.g., factories, sewage treatment plants). Easily identifiable.
    • Non-point Sources: Diffuse sources, discharge over a wide area (e.g., agricultural runoff, acid rain, storm-water drainage). Difficult to regulate.

  • Major Water Pollutants and their Effects:

    • Pathogens: Disease-causing microorganisms (bacteria, viruses, protozoa).

      • Source: Domestic sewage, animal excreta.
      • Effects: Cause waterborne diseases like cholera, typhoid, dysentery, jaundice.

    • Organic Wastes: Biodegradable matter (leaves, grass clippings, sewage, food waste).

      • Source: Domestic sewage, animal waste, food processing factories, agricultural runoff.
      • Effects: Decomposed by bacteria, consuming dissolved oxygen (DO) in water. High levels lead to oxygen depletion, harming aquatic life.

    • Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.

      • Significance: High BOD indicates a high level of biodegradable organic pollution. Clean water generally has BOD < 5 ppm; highly polluted water can have BOD ≥ 17 ppm.

    • Chemical Pollutants:

      • Heavy Metals: Cadmium (Cd), Lead (Pb), Mercury (Hg).
        • Sources: Industrial discharge, mining, paints, batteries.
        • Effects: Highly toxic, accumulate in organisms. Cd causes Itai-Itai disease (bone/kidney damage). Hg causes Minamata disease (neurological damage). Pb damages kidneys, liver, reproductive system.
      • Pesticides & Herbicides: (e.g., DDT, Aldrin, BHC).
        • Sources: Agricultural runoff.
        • Effects: Toxic to non-target organisms (fish, birds), persistent in the environment, undergo Biomagnification (increase in concentration at successive trophic levels).
      • Polychlorinated Biphenyls (PCBs):
        • Sources: Used as cleaning solvents, transformer fluids.
        • Effects: Persistent, carcinogenic.
      • Fertilizers (Phosphates and Nitrates):
        • Sources: Agricultural runoff.
        • Effects: Cause Eutrophication. High nitrate concentration in drinking water can cause Methemoglobinemia or Blue Baby Syndrome (reduces oxygen transport by blood, especially in infants).

    • Eutrophication: Nutrient enrichment of water bodies (mainly by nitrates and phosphates) leading to excessive growth of algae (algal bloom).

      • Consequences: Algal blooms cover the surface, blocking sunlight. When algae die, their decomposition consumes large amounts of dissolved oxygen, leading to oxygen depletion and death of fish and other aquatic organisms. Reduces water quality.

  • International Standards for Drinking Water: (Key values to remember)

    • Fluoride (F⁻): Deficiency causes tooth decay. Recommended concentration ~1 ppm. Excess (> 1.5-2 ppm) causes fluorosis (mottling of teeth, bone defects).
    • Lead (Pb): Max limit 50 ppb (parts per billion). Toxic.
    • Sulphate (SO₄²⁻): Max limit 500 ppm. Excess causes laxative effect.
    • Nitrate (NO₃⁻): Max limit 50 ppm. Excess causes Methemoglobinemia.
    • Other parameters include pH (6.5-8.5), Total Dissolved Solids (TDS < 500 ppm), Hardness, presence of metals like Fe, Mn, Al, Cu, Zn within limits.

4. Soil Pollution

  • Buildup of persistent toxic compounds, chemicals, salts, radioactive materials, or disease-causing agents in soil, which have adverse effects on plant growth and animal health.
  • Sources:
    • Pesticides: Herbicides (e.g., sodium chlorate NaClO₃, sodium arsenite Na₃AsO₃), Insecticides (e.g., DDT, BHC), Fungicides (organo-mercury compounds). They are often non-biodegradable and persist in soil.
    • Industrial Wastes: Effluents containing toxic chemicals, heavy metals.
    • Fertilizers: Excessive use can alter soil chemistry and pollute groundwater.
    • Improper Waste Disposal: Dumping of plastics, electronic waste, biomedical waste.

5. Strategies to Control Environmental Pollution

  • Waste Management: Proper handling, treatment, and disposal of waste.
    • Segregation: Separating waste into biodegradable and non-biodegradable, recyclable and non-recyclable.
    • Reduction: Minimizing waste generation at the source.
    • Reuse: Using items multiple times.
    • Recycling: Processing waste materials to create new products (e.g., paper, plastic, metal).
    • Dumping: Landfills for non-biodegradable waste (should be managed properly to prevent leaching).
    • Composting: Biological decomposition of biodegradable organic waste.
    • Incineration: Burning waste at high temperatures (can cause air pollution if not controlled).
  • Air Pollution Control: Using scrubbers (remove gases like SO₂), electrostatic precipitators (remove particulate matter), catalytic converters (in vehicles to convert NOx, CO, hydrocarbons into N₂, CO₂, H₂O).
  • Water Pollution Control: Sewage treatment plants, effluent treatment plants (ETPs) for industries.

6. Green Chemistry

  • Definition: The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It aims to achieve pollution prevention at the molecular level.
  • Goal: To minimize environmental impact through sustainable chemistry practices.
  • Examples in Daily Life/Industry:
    • Using H₂O₂ (hydrogen peroxide) for bleaching paper instead of chlorine gas (Cl₂). H₂O₂ produces water as a byproduct, while Cl₂ can form toxic chlorinated organic compounds.
    • Synthesizing chemicals using safer solvents (like water or supercritical CO₂) instead of hazardous organic solvents (like benzene, CCl₄).
    • Using catalysts to improve reaction efficiency and reduce waste.
    • Developing biodegradable materials.

This covers the core concepts of Environmental Chemistry relevant for your exams. Remember the specific pollutants, their sources, effects, key phenomena like acid rain, smog, ozone depletion, BOD, eutrophication, and the basic principles of green chemistry.

Multiple Choice Questions (MCQs)

  1. Which of the following is a major component of Classical Smog (London Smog)?
    (a) Ozone (O₃)
    (b) Sulphur dioxide (SO₂)
    (c) Peroxyacetyl nitrate (PAN)
    (d) Nitrogen dioxide (NO₂)

  2. The excessive discharge of phosphates and nitrates into a water body leads to:
    (a) Increased dissolved oxygen
    (b) Eutrophication
    (c) Decreased Biochemical Oxygen Demand (BOD)
    (d) Reduced algal growth

  3. Minamata disease is caused by the consumption of fish contaminated with:
    (a) Lead
    (b) Cadmium
    (c) Mercury
    (d) Arsenic

  4. Which gas binds most strongly to haemoglobin, reducing the oxygen-carrying capacity of blood?
    (a) Carbon dioxide (CO₂)
    (b) Sulphur dioxide (SO₂)
    (c) Nitrogen dioxide (NO₂)
    (d) Carbon monoxide (CO)

  5. The phenomenon responsible for the depletion of the ozone layer in the stratosphere is primarily caused by:
    (a) Carbon dioxide (CO₂)
    (b) Methane (CH₄)
    (c) Chlorofluorocarbons (CFCs)
    (d) Sulphur dioxide (SO₂)

  6. A high value of Biochemical Oxygen Demand (BOD) in a water sample indicates:
    (a) The water is highly pure.
    (b) The water contains a high level of dissolved oxygen.
    (c) The water is highly polluted with biodegradable organic matter.
    (d) The water is rich in aquatic life.

  7. Acid rain is caused by the atmospheric pollution of:
    (a) CO₂ and CO
    (b) SO₂ and NOx
    (c) CH₄ and O₃
    (d) CFCs and Hydrocarbons

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

  9. The permissible limit of fluoride ion (F⁻) in drinking water, above which it can cause mottling of teeth (fluorosis), is approximately:
    (a) 0.1 ppm
    (b) 0.5 ppm
    (c) 1.0 ppm
    (d) 2.0 ppm (or slightly above 1.5 ppm)

  10. Green Chemistry aims to:
    (a) Increase the use of hazardous solvents for better reactions.
    (b) Maximize waste production to study its effects.
    (c) Design chemical processes that reduce or eliminate hazardous substances.
    (d) Focus only on cleaning up existing pollution.

Answers to MCQs:

  1. (b)
  2. (b)
  3. (c)
  4. (d)
  5. (c)
  6. (c)
  7. (b)
  8. (c)
  9. (d) (NCERT mentions >2 ppm causes brown mottling, 1 ppm is recommended, values slightly above 1.5 ppm start causing issues)
  10. (c)

Study these notes thoroughly. Focus on the key terms, pollutants, and phenomena. Good luck with your preparation!

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