Topic 3 of 5
Use reactions to reduce harmful emissions
Treat the pollutant before release where possible.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Pure onlyUse reactions to reduce harmful emissions
Treat the pollutant before release where possible.
A catalytic converter speeds reactions that turn some exhaust pollutants into less harmful products. Carbon monoxide is oxidised to carbon dioxide while nitrogen monoxide can be reduced to nitrogen: 2CO(g) + 2NO(g) -> 2CO2(g) + N2(g). Unburned hydrocarbons can also be oxidised to carbon dioxide and water. The catalyst increases rate; it is not used up overall.
This reduces CO, nitrogen oxides and unburned hydrocarbons, but does not make the exhaust free of environmental impact: carbon dioxide still enters the atmosphere. Nitrogen atoms end up mainly in N2, not in an unspecified harmless gas.
- Neutralise acid already present
Adding suitable limestone to an acidified lake can consume H+ and reduce acidity. It treats an effect rather than preventing emissions.
- Remove sulfur dioxide from flue gas
Contact exhaust with a limestone-containing treatment system. Acidic sulfur dioxide reacts with calcium carbonate.
- Oxidise the captured sulfite
Further oxidation can form calcium sulfate; the sulfur is retained in a solid product instead of released as SO2.
A simplified sequence is CaCO3(s) + SO2(g) -> CaSO3(s) + CO2(g), followed by 2CaSO3(s) + O2(g) -> 2CaSO4(s). Actual flue-gas systems often use a wet slurry; the equations show the net chemical changes.