Nitrogen Oxides

Nitrogen oxides (written NOx, mainly NO and NO2) are the pollutants that form from the air itself. Inside a hot engine the unreactive nitrogen and oxygen of the atmosphere are forced to combine, and the products go on to cause acid rain and smog unless a catalytic converter intercepts them.

Key facts

Property	Detail
Formulae	NO and NO2 (collectively NOx)
Colours	NO is colourless; NO2 is brown
Bonding / structure	Simple covalent molecules
Main source	High-temperature combustion in engines and power stations
Classification of NO2	Acidic oxide
Main exam roles	Air pollution, acid rain, catalytic converters

Where this compound appears in 0620

• Air quality and climate change (Topic 11): NOx formation, its effects, and removal by catalytic converters.
• Redox and oxidation numbers (Topic 7): the catalytic-converter reaction is a neat redox example.
• Oxides (Topic 8): NO2 is an acidic oxide contributing to acid rain.

Formation

N2(g) + O2(g) -> 2NO(g)

This only happens at very high temperatures (above about 1000 C), such as inside an engine — nitrogen and oxygen do not react in ordinary air because the N2 triple bond is very strong. The nitrogen monoxide then meets more oxygen:

2NO(g) + O2(g) -> 2NO2(g) (brown gas)

Reactions you must know

Acid rain (nitric acid):

4NO2(g) + O2(g) + 2H2O(l) -> 4HNO3(aq)

This contributes to acid rain alongside sulfur dioxide, damaging buildings, lakes and vegetation.

Removal in a catalytic converter:

2CO(g) + 2NO(g) -> 2CO2(g) + N2(g)

One reaction removes two pollutants at once: toxic carbon monoxide and the acid-rain precursor nitrogen monoxide become harmless carbon dioxide and nitrogen. The catalyst (platinum/rhodium on a honeycomb) gives a large surface area and is not used up.

Other effects and sources

Beyond acid rain, nitrogen oxides contribute to photochemical smog in cities, which irritates the lungs and worsens breathing problems. Small amounts also form naturally when lightning heats the air enough for nitrogen and oxygen to react — but the exam focus is on the far larger quantities produced by traffic and power stations.

Worked exam question

A car’s catalytic converter removes carbon monoxide and nitrogen monoxide from the exhaust gases in a single reaction.

(a) Explain why nitrogen and oxygen from the air react inside the engine but not in the atmosphere outside. (2 marks)

(b) Write the equation for the reaction that removes carbon monoxide and nitrogen monoxide. (1 mark)

(c) State which substance is oxidised in this reaction and explain your answer. (2 marks)

Mark scheme

• (a) The engine reaches a very high temperature [1]; this provides enough energy for the normally unreactive nitrogen and oxygen to react (the strong N2 bond is only overcome at high temperature) [1]
• (b) 2CO + 2NO -> 2CO2 + N2 [1]
• (c) Carbon monoxide is oxidised [1]; it gains oxygen to become carbon dioxide (carbon changes from +2 to +4) [1]

Examiner note: in (c) the nitrogen monoxide is reduced (it loses oxygen / N goes from +2 to 0), so oxidation must refer to CO. “Oxygen is added to carbon monoxide” is the quickest way to secure the mark.

Common exam mistakes

• Thinking NOx comes from the fuel. The nitrogen comes from the air (N2), not from petrol or diesel; it reacts with oxygen only because the engine is so hot.
• Calling NO the brown gas. NO is colourless — it is NO2 that is brown, formed when NO meets more oxygen.
• Saying the converter “burns” or “filters” the gases. It catalyses a redox reaction turning CO and NO into CO2 and N2; it does not filter particulates and the catalyst is not consumed.
• Blaming acid rain on sulfur dioxide only. NOx also produces acid rain, forming nitric acid (4NO2 + O2 + 2H2O -> 4HNO3).