Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental chemistry traces the origin, reactions, and sinks of chemical species across the troposphere, stratosphere, hydrosphere, and lithosphere, with focus on pollutants from human activity. The single most-tested calculation is BOD — Biochemical Oxygen Demand measured in mg/L over 5 days at 20 °C, where higher values indicate more biodegradable organic pollution. A second recurring computation is water hardness expressed as the CaCO₃ equivalent in mg/L: hardness = (2.5 × Ca²⁺ mg/L) + (4.1 × Mg²⁺ mg/L). Two contrasting phenomena dominate the atmosphere: greenhouse gases (CO₂, CH₄, N₂O, CFCs, H₂O) absorb outgoing IR and warm the lower atmosphere, while stratospheric ozone formed by UV-driven O₂ dissociation (λ < 240 nm) shields the surface from UV-B. For JEE Main, expect 1 question worth 3% weightage — usually a concept-MCQ on smog type, BOD interpretation, or the CFC–ozone chain. Memorise that one Cl radical catalytically destroys ≈ 10⁵ O₃ molecules (Molina–Rowland hypothesis).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Atmospheric Structure and Chemistry

The atmosphere is layered by temperature gradient: troposphere (0–12 km, site of weather, life, and most pollution), stratosphere (12–50 km, hosts the protective ozone layer), mesosphere (50–85 km), and thermosphere (>85 km). In the stratosphere, UV-C photons (< 240 nm) split O₂ into O atoms, which then combine with O₂ to form O₃; the steady-state ozone layer absorbs harmful UV-B (280–315 nm).

Greenhouse Effect and Global Warming

Greenhouse gases — CO₂, CH₄, N₂O, CFCs, tropospheric H₂O — are transparent to incoming visible light but absorb outgoing infrared. Each gas has a Global Warming Potential (GWP) per unit mass; CH₄ has ~28× the GWP of CO₂ over 100 years, while CFCs reach thousands. The CO₂-equivalent emission = mass × GWP.

Smog: Two Chemically Opposite Phenomena

Classical (London) smog is reducing — a winter mixture of SO₂, smoke particulates, and fog, formed when sulphur in coal burns incompletely. Photochemical (Los Angeles) smog is oxidising — summer sunlight drives NO₂ → NO + O, and the atomic oxygen produces O₃, PAN (peroxyacetyl nitrate, CH₃C(O)OONO₂), formaldehyde, and acrolein from vehicular hydrocarbons and NOₓ.

Acid Rain

Oxidation of SO₂ and NOₓ yields strong acids that lower rainwater pH below the natural 5.6:

• 2 SO₂ + O₂ → 2 SO₃; SO₃ + H₂O → H₂SO₄
• 4 NO₂ + O₂ + 2 H₂O → 4 HNO₃

Acid rain leaches Al³⁺ from soils, damages marble (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), and acidifies lakes.

Water Quality: BOD and COD

BOD quantifies only the biodegradable organic load: BOD₅ = (DO_initial − DO_final) mg/L after 5 days at 20 °C. COD measures total oxidisable matter (both biodegradable and non-biodegradable) using hot acidic KMnO₄ or K₂Cr₂O₇ — always COD > BOD for the same water sample.

Eutrophication

Excess nitrates and phosphates (from fertilisers, detergents, sewage) trigger algal blooms; decomposition consumes dissolved O₂, killing aquatic life.

Pollutant Classification

Primary pollutants are emitted directly (CO, SO₂, NO, particulate matter, hydrocarbons). Secondary pollutants form by atmospheric reactions (O₃, PAN, H₂SO₄, HNO₃, smog).

Quick Reference Table

Parameter	Formula / Value	Indicates
BOD	(DO_i − DO_f) mg/L, 5 d, 20 °C	Biodegradable pollution
COD	KMnO₄ oxidation	Total oxidisable load
Hardness	2.5[Ca²⁺] + 4.1[Mg²⁺] mg/L as CaCO₃	Soap-scum, scale
Acid rain pH	< 5.6	SO₂ / NOₓ oxidation
AQI	0–500 composite	PM2.5, PM10, SO₂, NO₂, O₃, CO

JEE Main Pattern

Expect one MCQ matching smog type to chemistry, a BOD/COD interpretation, or the CFC–ozone catalytic cycle. Numerical questions on hardness or pH appear occasionally.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Stratospheric Ozone Destruction in Detail

Chlorofluorocarbons (e.g., CFCl₃, CF₂Cl₂) are stable in the troposphere and diffuse upward. UV-C (λ < 220 nm) in the stratosphere cleaves them:

CFCl₃ → CFCl₂• + Cl• ; Cl• + O₃ → ClO• + O₂ ; ClO• + O → Cl• + O₂

The regenerated Cl• propagates a chain — one chlorine atom destroys ~10⁵ ozone molecules before being sequestered as HCl or ClONO₂. This is the Molina–Rowland hypothesis, experimentally confirmed and the basis of the 1987 Montreal Protocol banning CFCs; replacements include HCFCs and HFCs (which still have GWP but no Cl).

Edge Cases and Cross-Topic Links

• Tropospheric vs stratospheric ozone: the same molecule is pollutant below (oxidant in smog, harms lungs) and protector above. JEE traps often flip this.
• PAN stability: CH₃C(O)OONO₂ is a secondary pollutant and an eye-irritant; it is thermally unstable and decomposes above ~80 °C.
• Biomagnification of DDT and mercury up the food chain links environmental chemistry with biomolecules and p-block discussions of heavy-metal toxicity.
• Green chemistry: 12 IUPAC principles — prevention, atom economy, less-hazardous synthesis, designing safer chemicals, safer solvents, energy efficiency, renewable feedstocks, reduce derivatives, catalysis, design for degradation, real-time pollution monitoring, inherently safer accident prevention. Direct JEE question: which principle is being applied?

Common Mistakes

1. Calling classical smog “oxidising” — it is reducing; only photochemical smog is oxidising.
2. Treating SO₂ as the sole acid-rain precursor — NOₓ contributes comparably over continental India.
3. Equating high COD with safe water — COD only measures oxidation, not biodegradability or toxicity.
4. Forgetting that greenhouse warming is cumulative; atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to >420 ppm today.

Worked Micro-Example

A water sample has Ca²⁺ = 40 mg/L and Mg²⁺ = 12 mg/L. Total hardness as CaCO₃ equivalent = (2.5 × 40) + (4.1 × 12) = 100 + 49.2 = 149.2 mg/L — moderately hard.

If DO_i = 8.0 mg/L and DO_f = 4.2 mg/L after 5 days at 20 °C, BOD₅ = 3.8 mg/L — within permissible limits for river discharge (< 3 mg/L for bathing; this sample needs aeration).

Practice Prompts

1. The atmosphere’s stratospheric ozone is formed by photolysis of O₂. Write the two-step mechanism and explain why UV-B (not UV-C) reaches the surface in reduced amounts today.
2. Compare classical and photochemical smog: identify the season, the dominant chemical, the reducing/oxidising nature, and one pollutant unique to each type.

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