Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental chemistry for JEE Advanced tests pollution chemistry across three compartments: atmosphere, hydrosphere, and lithosphere. The single highest-yield sub-topic is stratospheric ozone depletion by CFCs — remember the photolysis step CF2Cl2 + hv → CF2Cl· + Cl· (λ < 220 nm) and the catalytic chain Cl· + O3 → ClO· + O2; ClO· + O → Cl· + O2, which destroys ~10⁵ ozone molecules per Cl radical. The second must-know is the greenhouse effect: IR-active gases (CO₂, CH₄, N₂O, CFCs, H₂O) absorb Earth’s outgoing IR and re-radiate it; CO₂’s natural buffering of rainwater gives pH ≈ 5.6, so acid rain is defined as pH < 5.6. Third, water-quality BOD₅ = (DO_initial − DO_final) over 5 days at 20 °C — clean water BOD < 1 mg/L, polluted river water > 5 mg/L. Distinguish classical (London, SO₂-based, winter) from photochemical (Los Angeles, NOₓ + VOC + sunlight, summer) smog.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Atmospheric Chemistry: Smog and Ozone

Classical smog forms when SO₂ from coal combustion reacts with particulates and water droplets under cool, foggy conditions. It is reducing in character (containing H₂S, SO₂). Photochemical smog is oxidising — NOₓ (NO + NO₂) and volatile organic compounds (VOCs) react under UV light to produce ozone (O₃), PAN (peroxyacetyl nitrate, CH₃COOONO₂), aldehydes, and peroxyacids. NO₂ photolysis is the trigger: NO₂ + hv (λ < 400 nm) → NO + O·; the O· then yields O₃, while NO reacts with peroxy radicals to regenerate NO₂. PAN is a secondary pollutant and a powerful eye irritant.

Stratospheric Ozone Hole

Ozone forms naturally by the Chapman cycle: O₂ + hv (λ < 240 nm) → 2O·, then O· + O₂ + M → O₃ + M (M = third body, N₂ or O₂). CFCs (e.g. CF₂Cl₂, CFCl₃) are stable in the troposphere but photolyse in the stratosphere, releasing Cl· radicals. Over Antarctica, polar stratospheric clouds (PSCs) host heterogeneous reactions such as HCl + ClONO₂ → Cl₂ + HNO₃, which release photolysable Cl₂ each spring. The resulting catalytic chain gives the annual ozone hole (lowest O₃ column over the Antarctic each September–October).

Acid Rain

SO₂ and NOₓ oxidise to H₂SO₄ and HNO₃; rainwater pH drops below the natural 5.6. Damage mechanisms: marble (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂; CaSO₄ is flaky), iron corrosion, soil leaching of Ca²⁺/Mg²⁺, and fish die-off in lakes below pH 4.5.

Water Quality Parameters

Parameter	Meaning	Clean water	Polluted
DO	Dissolved O₂ (mg/L)	> 6	< 4 (hypoxia)
BOD₅	Biodegradable load at 20 °C, 5 d	< 1	> 5
COD	Total oxidisable matter	< 10	> 50
Hardness	Ca²⁺ + Mg²⁺ as CaCO₃ (mg/L)	< 75 soft	> 200 hard

Hardness = 2.5[Ca²⁺] + 4.12[Mg²⁺] (mg/L as CaCO₃). Temporary hardness (bicarbonates) is removed by boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. Permanent hardness (chlorides, sulphates) needs ion exchange or washing soda.

Eutrophication and Thermal Pollution

Excess nitrate/phosphate from fertilisers and detergents triggers algal blooms; on decay the bloom consumes DO, killing fish. Thermal pollution from power-plant effluents lowers DO because gas solubility falls with temperature (O₂ solubility at 25 °C ≈ 8.3 mg/L vs 14.6 mg/L at 0 °C).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Global Warming and Greenhouse Gases

The greenhouse factor quantifies a molecule’s radiative forcing relative to CO₂. The CO₂–carbonic acid equilibrium (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻) buffers natural rainwater to pH ≈ 5.6 — the threshold below which rain is “acid”. Global warming potential (GWP) over 100 yr: CO₂ = 1, CH₄ ≈ 21, N₂O ≈ 310, CFC-12 ≈ 10,900. Methane’s short lifetime (~12 yr) but high per-molecule absorption makes it potent. Black carbon (PM 2.5) also warms by ~0.3–0.6 W m⁻² when deposited on snow.

Edge Cases and JEE-Specific Traps

• BOD vs COD: COD always exceeds BOD because it oxidises both biodegradable and non-biodegradable (e.g., cellulose, plastics) matter using strong oxidants like KMnO₄ or K₂Cr₂O₇.
• SO₂ is both acidic and reducing; a common trap option labels it “oxidising”. Use SO₂ + 2H₂O → H₂SO₃ to remember the Bronsted behaviour.
• CFCs are NOT greenhouse gases in JEE MCQs that ask about ozone-only depletion — they are, in fact, the strongest per-molecule greenhouse gases, so read the wording carefully.
• AQI integrates eight pollutants (PM2.5, PM10, SO₂, NO₂, O₃, CO, Pb, NH₃); in 2019 CPCB revised breakpoints.
• The Montreal Protocol (1987) phased out CFCs; replacements HFCs and HCFCs still have GWP issues (Kigali Amendment, 2016).

Worked Example

A 2.0 L water sample has initial DO = 8.5 mg/L. After 5 days at 20 °C, DO = 2.5 mg/L. BOD₅ = 8.5 − 2.5 = 6.0 mg/L — indicates significant organic pollution (threshold for river-water discharge in India is BOD₃ < 3 mg/L for surface waters).

Common Mistakes

1. Calling classical smog “photochemical” — they are mutually exclusive by season and chemistry.
2. Forgetting the third body M in O· + O₂ + M → O₃ + M; without M, energy is not dissipated and O₃ would not stabilise.
3. Confusing temporary hardness (boiling-removable) with permanent (not removed by boiling).

Practice Prompts

1. Why does the ozone hole appear over Antarctica in spring rather than continuously? (Hint: PSC chemistry + sunlight return.)
2. A lake has [Ca²⁺] = 60 mg/L and [Mg²⁺] = 24 mg/L. Calculate total hardness as CaCO₃ equivalents and classify the water.

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