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9476 / 2027
Organic Chemistry

Lesson 17 of 34

Combustion products have different environmental effects

A catalytic converter reduces several pollutants but does not remove carbon dioxide emissions.

A-Level 9476 (2026-2027)

  • I can explain environmental consequences of hydrocarbon use.Syllabus

    Syllabus 9476, 11.4(h). Explain environmental consequences of hydrocarbon use.

Combustion products have different environmental effects

A catalytic converter reduces several pollutants but does not remove carbon dioxide emissions.

Internal-combustion emissions
EmissionOrigin and consequenceCatalytic treatment
COIncomplete combustion; interferes with oxygen transport in the body.2CO + O2 → 2CO2.
NOxNitrogen and oxygen react under high-temperature engine conditions; contributes to air pollution, acidic deposition and photochemical smog.A representative reduction is 2NO + 2CO → N2 + 2CO2.
Unburnt hydrocarbonsFuel escapes complete combustion; contributes to photochemical pollution with NOx in sunlight.Oxidise to CO2 and H2O on the catalytic surface.
CO2Product of complete combustion; increased atmospheric amounts contribute to the enhanced greenhouse effect.Ordinary exhaust catalysts do not remove it.

Gases such as CO2, CH4 and N2O absorb infrared radiation given off by the Earth. More of these gases in the air strengthens the greenhouse effect and changes the climate. This is a different cause from ozone depletion and from local poisoning. One pollutant can add to more than one problem, but the chemical causes are different.

A catalytic converter needs to be hot enough, and the gases must reach its active sites. Removing CO, NOx and unburnt hydrocarbons makes exhaust cleaner. But the amount of fuel burned and its carbon content still decide how much CO2 is made. To judge a fuel, look at its energy output, its incomplete-combustion products and its other emissions, not one label alone.

Check your understanding

Which statement about a car's catalytic converter is correct?

  1. It removes NO by oxidising it to nitrogen dioxide on the metal
  2. It removes CO2 by reducing it to carbon monoxide on the catalyst
  3. It works best in the first minutes after a cold start of the car
  4. It removes NO by reacting it with CO: 2NO + 2CO -> N2 + 2CO2
Show the answer

It removes NO by reacting it with CO: 2NO + 2CO -> N2 + 2CO2

NO forms when N2 and O2 combine at the high temperature in the engine. On the hot catalyst, CO reduces NO to N2 and is oxidised to CO2, removing two pollutants at once.