Engines run hot enough to make nitrogen and oxygen react. The nitrogen oxides that escape catalyse acid rain, feed photochemical smog and, together with fertiliser run-off, upset the nitrogen balance of rivers.
At the very high temperatures inside an engine (or in lightning), nitrogen and oxygen react: N2 + O2 → 2NO. In air NO is oxidised to NO2. Catalytic converters remove NO and CO together: 2CO + 2NO → 2CO2 + N2. In the atmosphere NO2 acts as a catalyst for the oxidation of SO2 to SO3, and NO2 also forms nitric acid, so both add to acid rain. NO and unburnt hydrocarbons in sunlight form photochemical smog containing peroxyacetyl nitrate (PAN). Nitrate fertilisers washed into water cause eutrophication.
Run the engine model in the simulator at different temperatures, then fit a catalytic converter and watch the exhaust composition change.
| Process | Equation | Note |
|---|---|---|
| in the engine | N2 + O2 → 2NO | needs very high temperature (endothermic, strong N≡N) |
| in the air | 2NO + O2 → 2NO2 | brown, toxic gas |
| catalytic converter | 2CO + 2NO → 2CO2 + N2 | Pt/Rh on a ceramic honeycomb |
| catalysing SO2 oxidation | NO2 + SO2 → NO + SO3; NO + ½O2 → NO2 | NO2 regenerated |
| acid rain | 4NO2 + O2 + 2H2O → 4HNO3 | nitric acid |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why are nitrogen oxides formed in car engines?
Which reaction occurs in a catalytic converter?
What is the role of NO2 in the atmospheric oxidation of SO2?
Which pollutant is a component of photochemical smog?
Nitrogen compounds cause two further problems: one in city air, one in rivers and lakes.
In the oxidation of SO2, NO2 is a catalyst: it is used in the first step and regenerated in the second, so it is not used up overall. A catalytic converter removes CO, NO and hydrocarbons; it produces CO2, it does not remove it.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which pair of equations shows NO2 acting as a catalyst in the oxidation of SO2?
What is the oxidation number of nitrogen in NO and in NO2?
In eutrophication, why do fish die?
In the catalytic converter reaction 2CO + 2NO → 2CO2 + N2, which species is reduced?
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