Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental Chemistry studies the origin, transport, reactions, effects, and fates of chemical species in air, water, and soil.

Smog — two types:

• Reducing smog (winter): SO₂ + soot + fog from coal combustion — London smog, 1952.
• Photochemical/oxidant smog (summer): NO₂ + ozone + PAN from sunlight reacting with vehicle exhaust.

Key formulas to memorise:

• BOD = (D₁ − D₂) × dilution factor (D = Dissolved Oxygen)
• COD = (VS − VB) × N × 8 × 1000 / V (in mg/L, N = normality of FAS)

High-yield exam pointers:

• Ozone depletion: CFCs (CF₂Cl₂, CFCl₃) → UV → Cl• radical → destroys ~10⁶ O₃ molecules per Cl•.
• Greenhouse gases: CO₂ (~60% contribution), CH₄, N₂O, CFCs — trap infrared radiation.
• Clean water DO: ~8 mg/L; polluted water DO < 4 mg/L.
• Acid rain threshold: pH < 5.6.
• Eutrophication: phosphate detergents trigger algal blooms; DDT causes bioaccumulation.

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🟡 Standard — Regular Study (2d–2mo)

Smog Types and Composition

Reducing smog forms during winter in cold climates when coal (high sulfur content) undergoes incomplete combustion. It contains sulfur dioxide (SO₂), unburnt carbon particles (soot), and water vapour fog. The London smog of 1952 killed ~4,000 people in four days. SO₂ reacts with water to form sulfurous acid: SO₂ + H₂O → H₂SO₃, causing respiratory irritation.

Photochemical smog is a summer phenomenon driven by sunlight. Nitrogen oxides (NO, NO₂) and volatile organic compounds (VOCs) from vehicle exhaust undergo photochemical reactions:

NO₂ → NO + O (photolysis by UV < 420 nm)
O + O₂ → O₃ (Chapman cycle)
NO₂ + O₃ → NO + O₂

Ozone and peroxyacetyl nitrate (PAN, CH₃COOOONO₂) are the principal oxidants. PAN causes eye irritation and plant leaf chlorosis.

Ozone Layer Depletion Mechanism

CFCs used as refrigerants (CF₂Cl₂, CFCl₃) diffuse into the stratosphere where UV radiation cleaves the C–Cl bond: CFCl₃ + UV → •Cl + •CFCl₂. The chlorine radical enters a catalytic cycle:

•Cl + O₃ → ClO• + O₂
ClO• + O₃ → •Cl + 2O₂

One Cl• destroys approximately 10⁶ ozone molecules before being sequestered. Montreal Protocol (1987) phased out CFCs globally.

Water Pollution Parameters

BOD (Biochemical Oxygen Demand): oxygen consumed by aerobic bacteria over 5 days at 20°C to degrade organic matter. High BOD (> 3 mg/L) indicates severe organic pollution.

COD (Chemical Oxygen Demand): measures total oxygen required to chemically oxidise both organic and inorganic matter using acidified potassium dichromate. COD > BOD because it oxidises materials bacteria cannot digest.

DO (Dissolved Oxygen): clean water holds ~8 mg/L. Fish die when DO drops below 4 mg/L. Eutrophication depletes DO as decomposers consume algae.

Greenhouse Effect

Infrared radiation emitted by Earth’s surface is absorbed and re-emitted by greenhouse gases (CO₂, CH₄, N₂O, CFCs), warming the troposphere. CO₂ contributes approximately 60% of the warming effect. Methane’s GWP is 28× that of CO₂ over 100 years.

Green Chemistry Principles

Atom economy measures how efficiently atoms are incorporated into the desired product: (Molar mass of desired product / Sum of molar masses of all reactants) × 100. Catalytic converters exemplify green chemistry: Pt–Pd–Rh catalyst converts CO → CO₂, NOx → N₂, and unburnt hydrocarbons → CO₂ + H₂O.

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🔴 Extended — Deep Study (3mo+)

Chapman Cycle and Ozone Balance

The stratospheric ozone layer (15–35 km altitude) absorbs 98% of harmful UV-B (280–315 nm). The Chapman mechanism in simplified form:

O₂ + UV (λ < 242 nm) → 2O
O + O₂ + M → O₃ + M (M = N₂ or O₂, carries away excess energy)
O₃ + UV → O₂ + O
O + O → O₂ (slowest step)

Steady-state ozone concentration results from the balance between these reactions.

Classical vs. Photochemical Smog: Comparative Analysis

Classical smog dominates industrial cities with coal dependency and thermal inversions. Visibility drops due to particulates (fly ash: Si, Al, Fe, Ca oxides). Combustion of leaded fuel produced lead tetramethyl: 2Pb(CH₃)₄ + O₂ → 2PbO + 4•CH₃. Modern unleaded petrol eliminates this pathway.

Photochemical smog intensity depends on NOx concentration, solar radiation intensity (peak at 12–1 pm), and VOC availability. Peak ozone concentration occurs 2–4 hours after NO₂ peaks. Urban canyons trap pollutants, increasing photochemical smog severity.

DDT and Biomagnification

Dichloro-diphenyl-trichloroethane (DDT) is persistent and lipophilic. At each trophic level, concentration increases approximately 10× (biomagnification). Rachel Carson’s Silent Spring (1962) documented eggshell thinning in birds due to DDE (DDT metabolite). Biological concentration factor = concentration in organism / concentration in water.

Phosphate Eutrophication Mechanism

Synthetic detergents contain sodium tripolyphosphate (STPP, Na₅P₃O₁₀) as a builder (30–40%). Phosphates enter waterways → algae bloom → surface coverage blocks light → submerged plants die → decomposers proliferate → DO collapses → fish kills. Phosphate limit for lakes: < 0.02 mg/L to prevent eutrophication.

Common NEET Exam Traps

1. Confusing BOD and COD purpose: BOD measures biodegradable organics; COD measures total oxidisable matter. A factory discharging phenol (biologically recalcitrant) can have low BOD but high COD — students frequently choose wrong option.
2. Ozone vs. Smog ozone: Stratospheric ozone (> 20 km) is beneficial (UV shield); tropospheric ozone (formed at ground level) is a pollutant causing respiratory damage.
3. Misidentifying smog type: Reducing smog is winter, associated with coal; oxidant smog is summer, driven by vehicles.

Practice Prompts

1. A river water sample has BOD = 10 mg/L and COD = 200 mg/L. What does this indicate about the pollutant nature? What biological consequences follow?
2. Calculate atom economy for the reaction: 2CH₃Cl + Na₂S → (CH₃)₂S + 2NaCl used in pesticide synthesis. Would this meet green chemistry criteria?

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