Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental chemistry studies the chemical species in air, water and soil and how human activity alters their natural balance. For MDCAT, master five pillars: air pollutants and smog types, stratospheric ozone depletion by CFCs, the greenhouse effect, acid rain chemistry, and water-quality indicators (BOD and COD). Remember the key equations:

• pH = −log[H⁺]; Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C
• SO₂ + H₂O → H₂SO₃ (the sulfurous acid that drives acid rain)
• 2 NO₂ + H₂O → HNO₃ + HNO₂ (nitric/nitrous acid in acid rain)
• BOD₅ = (DOᵢ − DO_f) mg/L

High-yield traps: classical smog is SO₂-based and reducing, photochemical smog is oxidising (ozone + NOx); stratospheric O₃ is a UV shield, but tropospheric O₃ is a pollutant. CFCs destroy ozone via Cl• radical chain catalysis, not directly.

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Atmospheric structure and major pollutants

The troposphere (0–12 km) holds weather and most human emissions; the stratosphere (12–50 km) contains the ozone layer. Primary pollutants enter directly — CO, CO₂, SO₂, NO, hydrocarbons, particulate matter (PM2.5/PM10) and CFCs. Secondary pollutants form by atmospheric reactions, e.g., O₃, PAN, HNO₃ and H₂SO₄ aerosols.

Smog: classical vs photochemical

Classical (London) smog forms in cool, humid conditions from SO₂ + smoke particulates; it is reducing and irritates the lungs. Photochemical (Los Angeles) smog forms in sunlight when NOₓ and unburnt hydrocarbons react: NO₂ + hν → NO + O•, then O• + O₂ → O₃. Result is a brown oxidising haze rich in ozone and peroxyacetyl nitrate (PAN).

Stratospheric ozone depletion

CFC-12 (CF₂Cl₂) drifts up, and UV-C breaks a C–Cl bond: CFCl₃ → CFCl₂• + Cl•. The catalytic cycle is:

• Cl• + O₃ → ClO• + O₂
• ClO• + O → Cl• + O₂
• Net: O₃ + O → 2 O₂

A single Cl atom destroys ~10⁵ O₃ molecules before being deactivated, which is why the Montreal Protocol (1987) phased out CFCs.

Greenhouse effect and global warming

CO₂, CH₄, N₂O, CFCs and H₂O vapour absorb outgoing IR and re-radiate it. Rising atmospheric CO₂ (>420 ppm today) from fossil-fuel combustion is the principal driver of warming; CH₄ traps ~28× more IR per molecule than CO₂ but has a shorter residence time.

Acid rain

Sulfur in coal/oil oxidises to SO₂, then SO₃, which forms H₂SO₄ in clouds; vehicle and power-station NOₓ forms HNO₃. Rain pH below 5.6 damages marble (CaCO₃), leaches Al³⁺ into lakes (killing fish), and acidifies soil.

Water quality: BOD and COD

BOD₅ measures the dissolved oxygen consumed by microbes in 5 days — an index of biodegradable organic load (sewage). COD measures total oxidisable matter using hot acidified KMnO₄ or K₂Cr₂O₇; COD > BOD because it includes non-biodegradable compounds. Eutrophication of lakes occurs when phosphate/nitrate runoff causes algal blooms; decay of the algae strips O₂ and kills aquatic life.

Green chemistry

Twelve guiding principles (Anastas & Warner, 1998) promote atom economy, safer solvents, catalysis, design for degradation and energy efficiency, reducing hazardous substances at the source rather than treating waste afterwards.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge cases and exam traps

• Ozone is context-dependent. In the stratosphere, O₃ absorbs harmful UV-B (λ 280–315 nm), protecting DNA. In the troposphere it is a toxic oxidant and a key component of photochemical smog.
• CFCs do not directly attack O₃. Destruction is catalytic and proceeds via Cl•/ClO• radicals; polar stratospheric clouds (PSCs) provide the surface that releases Cl₂ from HCl + ClONO₂, activating the cycle each spring.
• Acid rain is not CO₂ rain. Natural rain is mildly acidic (pH ≈ 5.6) because CO₂ + H₂O ⇌ H₂CO₃. True acid rain (pH < 5) is dominated by H₂SO₄ and HNO₃ from SO₂ and NOₓ, not by carbonic acid.
• BOD vs COD: BOD₅ uses 20 °C incubation with aerobic bacteria and reflects biodegradable organics only; COD is a chemical test (e.g., reflux with K₂Cr₂O₇/H₂SO₄) and is always ≥ BOD. A high COD/BOD ratio suggests non-biodegradable industrial effluent.

Connections

Ozone depletion, greenhouse warming and acid rain are linked through the same emissions — burning fossil S- and N-containing fuels releases both SO₂/NOₓ and CO₂. Halogenated refrigerants connect ozone chemistry (Cl• catalysis) with greenhouse forcing (CFCs are also potent IR absorbers). Eutrophication links to detergents (polyphosphates) and to fertiliser overuse, tying water chemistry to agricultural chemistry.

Common mistakes

Writing that “CO₂ causes ozone depletion” or that “CFCs cause acid rain”; these are separate phenomena. Confusing PAN (a smog component) with PAN-peroxide lab reagent. Treating PM2.5 as a gas — it is a fine particulate that penetrates alveoli.

Worked numeric

A water sample has DOᵢ = 8.4 mg/L and after 5 days DO_f = 4.2 mg/L. Then BOD₅ = 8.4 − 4.2 = 4.2 mg/L, indicating moderate organic pollution (clean river ≈ 1 mg/L; raw sewage ≈ 200 mg/L). If the same sample gives COD = 14 mg/L, the COD/BOD ratio ≈ 3.3, hinting at a non-biodegradable industrial component alongside biodegradable waste.

Practice prompts

1. Write the catalytic cycle by which a Cl• radical destroys two stratospheric ozone molecules and identify the species that regenerates Cl•.
2. A lake has pH 4.8 after acid deposition. Calculate [H⁺] and [OH⁻] at 25 °C, and state which acid (H₂SO₃, H₂SO₄, HNO₃) is the most likely contributor from coal-fired power stations.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.