Environmental Chemistry
Syllabus Unit: Inorganic Chemistry
Why it matters
Chapter overview
Environmental chemistry tracks pollutants through air, water and soil — their sources, chemistry, and effects on ecosystems and health — and pairs that with green chemistry's approach of designing processes that avoid creating pollution in the first place. NEET draws heavily on named mechanisms here: the catalytic ozone-destruction cycle by CFCs, the composition difference between classical and photochemical smog, and BOD as the standard measure of organic water pollution.
1Air Pollution & Its Control
Air pollutants split into gaseous and particulate types, and the two great smogs of history — London's reducing smog and Los Angeles's oxidising smog — form by completely different chemistry.
Learning Objectives
- Classify air pollutants into gaseous (SOx, NOx, CO, hydrocarbons, O3) and particulate (dust, smoke, fumes, mist) types with examples.
- Differentiate classical (reducing) smog from photochemical (oxidising) smog by composition and the weather conditions each requires.
- Describe the formation of PAN (peroxyacetyl nitrate) and its role as an eye and lung irritant in photochemical smog.
- Explain the working principle of a catalytic converter and an electrostatic precipitator in pollution control.
Key Terms
Air pollutants fall into two broad classes: gaseous pollutants (SO2, NO2, CO, CO2, hydrocarbons, ozone) and particulate pollutants (smoke, dust, mist, fumes — fine solid or liquid particles suspended in air). Both classes contribute to the two classic smog types NEET tests.
Classical (reducing) smog
Classical smog, notorious from 1950s London, forms when SO2 released from burning coal combines with fog and smoke particulates in cold, humid winter weather. It is chemically reducing (SO2 is a reducing agent) and causes severe respiratory distress; the 1952 Great Smog of London killed thousands.
Photochemical (oxidising) smog
- Forms in warm, sunny, dry climates (e.g. Los Angeles) with heavy vehicular traffic.
- NO2 from exhaust photolyses in sunlight: NO2 → NO + O; the released O atom combines with O2 to form ozone: O + O2 → O3.
- Ozone and unburnt hydrocarbons react further to form PAN, formaldehyde and acrolein — potent eye, throat and lung irritants.
Control measures include catalytic converters (Pt/Pd/Rh catalysts convert CO and hydrocarbons to CO2/H2O and NOx to N2/O2 in vehicle exhaust — unleaded petrol is essential since lead poisons the catalyst) and electrostatic precipitators (charge particulates so they are attracted to and collected on oppositely charged plates before flue gases are released).