Application of Chemistry to Society and the Environment
Core ideas
Chemistry is applied within environmental, economic and social constraints. Understand green chemistry principles such as atom economy, waste prevention, safer solvents and energy efficiency; distinguish renewable and non-renewable resources; and use life-cycle thinking rather than judging a process by one stage. Link combustion stoichiometry to CO2 emissions, acid rain to SOx and NOx, and water treatment to coagulation, filtration, chlorination and pH adjustment. Know the chemistry of batteries, corrosion and industrial processes such as Haber and Contact, and evaluate risk as hazard combined with exposure rather than hazard alone. Sustainability decisions also involve ethics, regulation, cost and social acceptability.
Traps
Candidates often say a substance is “safe because it is natural”, confuse the greenhouse effect with ozone depletion, or compare fuels by volume rather than by energy or CO2 per mole. Do not ignore stoichiometric coefficients when calculating emissions, and do not treat recycling as automatically better without considering transport and processing energy. Hazard is not risk: a corrosive substance poses little risk when sealed and unused. Percent yield and atom economy are different measures - one concerns practical efficiency, the other theoretical incorporation of atoms.
Worked example
Compare CO2 from complete combustion of methane and octane per mole of fuel. CH4 + 2O2 → CO2 + 2H2O gives 1 mol CO2 per mol CH4 (16 g), or 2.75 g CO2 per g CH4. 2C8H18 + 25O2 → 16CO2 + 18H2O gives 8 mol CO2 per mol C8H18 (114 g), or 3.09 g CO2 per g octane. Rule of thumb: write the balanced equation first, convert to moles, then compare on a common basis such as per mole, per gram or per kJ.