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K324 / 2027
Maintaining Air Quality

Lesson 3 of 5

Use reactions to reduce harmful emissions

Treat the pollutant before release where possible.

O-Level 6092 (2026) / SEC G3 K324 (2027)

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  • I can explain pollution-control reactions.Syllabus

    Syllabus K324, 12(d). Explain pollution-control reactions

Use 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.

Calcium carbonate has two applications
  1. 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.

  2. Remove sulfur dioxide from flue gas

    Contact exhaust with a limestone-containing treatment system. Acidic sulfur dioxide reacts with calcium carbonate.

  3. 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.

Check your understanding

In a catalytic converter, 2CO + 2NO -> 2CO2 + N2. Which reactant is oxidised and which is reduced?

  1. CO is oxidised to CO2, and NO is reduced to N2
  2. NO is oxidised to N2, and CO is reduced to CO2
  3. Both are oxidised, because both turn into safer gases
Show the answer

CO is oxidised to CO2, and NO is reduced to N2

CO gains oxygen to become CO2, so it is oxidised. NO loses its oxygen to become N2, so it is reduced. Two pollutants are removed in one redox reaction.