Environmental Chemistry and Green Chemistry

Environmental Chemistry and Green Chemistry

🟢 Lite — Quick Review (1h–1d)

Environmental chemistry studies chemical changes in air, water, and soil, and the impact of human activity; green chemistry designs products and processes that minimise hazardous waste. Key formulas: BOD = (D₁ − D₂)/V, pH = −log[H⁺], [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, and atom economy = (mass of desired product / total mass of reactants) × 100%. NECO SSCE high-yield points: (1) BOD measures biodegradable organic pollution — high BOD = polluted water. (2) Greenhouse gases (CO₂, CH₄, N₂O, CFCs) trap infrared radiation, while ozone depletion is caused specifically by CFCs releasing Cl• radicals in the stratosphere — these are two separate phenomena. (3) The 12 Principles of Green Chemistry include prevention, atom economy, safer solvents, catalysis, and design for degradation.

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🟡 Standard — Regular Study (2d–2mo)

Air Pollution

Major air pollutants include CO (incomplete combustion of carbon, binds haemoglobin as carboxyhaemoglobin), SO₂ and NO₂ (acid rain precursors — H₂SO₄ and HNO₃), CFCs (stratospheric ozone destruction), and lead compounds from tetraethyl lead in petrol (now banned in Nigeria since 2017). Photochemical smog forms when NO₂ and hydrocarbons react under sunlight to produce ozone (O₃) at ground level and peroxyacetyl nitrate (PAN).

Water Pollution and BOD

Biochemical Oxygen Demand (BOD) measures the dissolved oxygen consumed by microbes decomposing organic waste over 5 days at 20 °C. Clean river water has BOD < 5 mg/L; polluted water exceeds 10 mg/L. Formula: BOD (mg/L) = (D₁ − D₂)/V, where D₁ is initial DO, D₂ is final DO (after 5 days), and V is sample volume in litres. Eutrophication occurs when nitrate/phosphate fertiliser runoff causes algal blooms; the algae die, decomposers consume oxygen, and aquatic life suffocates.

Sewage Treatment

Primary treatment is physical: screening, sedimentation, and skimming remove solids. Secondary treatment is biological: bacteria in aeration tanks break down dissolved organics; activated sludge is returned to the system. Tertiary treatment uses chlorination, filtration, and sometimes reverse osmosis to remove residual pathogens and nutrients.

Soil Pollution

Bioaccumulation is the build-up of toxins (e.g., DDT, mercury) in an organism over time. Biomagnification is the increasing concentration of these toxins up the food chain — apex predators (eagles, humans) carry the highest body burden. Persistent organic pollutants (POPs) are non-biodegradable and lipid-soluble.

Green Chemistry

Anastas and Warner’s 12 Principles (1998) include: prevention of waste, atom economy, less hazardous synthesis, designing safer chemicals, safer solvents, energy efficiency, renewable feedstocks, reduced derivatives, catalysis, design for degradation, real-time pollution monitoring, and inherently safer chemistry.

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🔴 Extended — Deep Study (3mo+)

Edge Cases and Common Misconceptions

A frequent NECO trap equates the greenhouse effect with ozone depletion. The greenhouse effect is warming from CO₂, CH₄, and N₂O absorbing outgoing infrared radiation; the ozone hole is Cl•-catalysed destruction of stratospheric O₃ by CFCs. Methane has a Global Warming Potential (GWP) of 25 over 100 years and N₂O of 298, both relative to CO₂ = 1, so a kilogram of CH₄ traps 25× the heat of a kilogram of CO₂. Atmospheric lifetime also differs: CH₄ lasts ~12 years, while CFCs persist 50–100+ years.

Atom Economy Worked Example

Reaction: CH₃CH₂OH → CH₂=CH₂ + H₂O (dehydration, 96% H₂SO₄ catalyst, 170 °C). Molar masses: ethanol = 46, ethene = 28, water = 18. Atom economy = (28 / 46) × 100% = 60.87%. This explains why industrial chemists prefer reactions such as the addition of HBr to ethene (atom economy = 100%), where all reactant atoms end up in the product.

Connections to Other Topics

• Acids and bases: acid rain is just dilute H₂SO₄ + HNO₃ with pH below 5.6; calculate [H⁺] from pH.
• Kinetics: CFC–ozone reactions proceed via Cl• chain catalysis: Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂. One Cl atom destroys ~100,000 O₃ molecules.
• Electrolysis/electrochemistry: tertiary water treatment sometimes uses electrochemical oxidation of pathogens.

Practice Prompts

1. State three sources of water pollution and explain why a high BOD value indicates low water quality.
2. List five of the 12 Principles of Green Chemistry and explain how catalysis improves atom economy in the Haber process.

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