2.4.2 Nitrogen Oxides

Natural occurrence of nitrogen oxides

• Due to its lack of reactivity, only under extreme conditions will nitrogen react with oxygen to form gaseous nitrogen oxides
• An example of these extreme conditions is lightning which can trigger the formation of nitrogen(II) and nitrogen(IV) oxides (NO and NO₂ respectively)
• The chemical equations for these reactions are:

N₂(g) + O₂(g) → 2NO(g)

N₂(g) + 2O₂(g) → 2NO₂(g)

Man-made occurrence of nitrogen oxides

• In the engine of a car, a mixture of air and fuel is compressed and ignited by a spark
• Air consists of 78% of nitrogen and 21% of oxygen
• Under the high pressure and temperature inside a car engine, nitrogen can react with oxygen to form nitrogen oxides
• These nitrogen oxides are released into the atmosphere through the car’s exhaust fumes

Catalytic removal of nitrogen oxides

• The nitrogen oxides released through cars’ exhaust fumes pollute the atmosphere
• Many car exhaust systems are therefore fitted with catalytic converters to reduce the pollutants from motor vehicles
• The nitrogen oxides are reduced on the surface of the hot catalyst (e.g. platinum) to form the unreactive and harmless nitrogen gas which is then released from the vehicle’s exhaust pipe into the atmosphere
• The chemical reaction for the reduction of nitrogen oxide to nitrogen gas by the catalyst is as follows:

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

A catalytic converter helps reduce the pollutants from motor vehicles

• Nitrogen oxides are examples of primary pollutants because they are given off directly into the air from the source of pollution
• Examples of pollution sources are car exhausts and power plants
• Nitrogen oxides are extra dangerous as they can react with substances in the air to make secondary pollutants
• These are pollutants that are not given off directly into the air from human activity
• Exhaust fumes contain another primary pollutant called volatile organic compound (VOCs)
• These are unburnt hydrocarbons from fuel and their oxidised products
• VOCs react with nitrogen oxides in air to form peroxyacetyl nitrate (PAN, CH₃CO₃NO₂)
• Sunlight provides the energy needed to start off the reactions of VOCs and nitrogen oxides in air, so they are also called photochemical reactions
• PAN is one of the harmful pollutants found in photochemical smog
• ‘Smog’ is derived from ‘smoke’ and ‘fog’
• PAN affects the lungs and eyes and in high concentrations plant-life

Primary & secondary pollutant types & their pollution source table

The diagram shows the formation of PAN from the photochemical reaction between VOCs and nitrogen oxide

Formation of acid rain by nitrogen oxides

• As mentioned earlier, lightning strikes trigger the formation of nitrogen(II) and nitrogen (IV) oxides in air:

2NO(g) + O₂(g) ⇌ 2NO₂(g)

• The air also contains oxygen and tiny droplets of water that make up clouds
• The nitrogen(IV) oxide (NO₂) dissolves and reacts in water with oxygen as follows:

2NO₂(aq) + H₂O(l) + ½O₂(g) → 2HNO₃(aq)

• When the clouds rise, the temperature decreases, and the droplets get larger
• When the droplet containing dilute nitric acid are heavy enough, they will fall down as acid rain

The diagram shows the formation of acid rain by the oxidation of nitrogen(IV) oxide

Nitrogen oxide as a catalyst

• Acid rain also contains dilute sulfuric acid (H₂SO₄)
• Sulfur(IV) oxide (SO₂) is another pollutant found in the atmosphere
• When SO₂ is oxidised, it forms SO₃ which reacts with rainwater to form dilute sulfuric acid as follows:

SO₃(g) + H₂O(l) → H₂SO₄(aq)

• Nitrogen oxides can directly cause acid rain but can also act as catalysts in the formation of acid rain
• NO₂ catalyses the oxidation of SO₂ to SO₃:

NO₂(g) + SO₂(g) → SO₃(g) + NO(g)

• • The formed NO gets oxidised to regenerate NO₂:

NO(g) + ½ O₂(g) → NO₂(g)

• The regenerated NO₂ molecule can get again oxidise another SO₂ molecule to SO₃ which will react with rainwater to form H₂SO₄

The formation of dilute sulfuric acid is catalysed by the nitrogen oxides