STAGE 2 CHEMISTRY
TOPIC 1: MONITORING THE ENVIRONMENT
1.2 – Photochemical Smog
Aaron Tieu
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LEARNING OBJECTIVES
• You should be able to:
qExplain the formation of NO, NO2 and O3, with the aid of
equations
qState the cause of photochemical smog
qState the harmful effects of photochemical smog
qExplain, with aid of equation on how to reduce the risk of
photochemical smog
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SCIENCE UNDERSTANDING
Nitrogen oxides are formed in high-temperature engines and furnaces.
q Write equations for the formation of nitrogen oxides NO and NO2.
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EARTH’S ATMOSPHERE
• Made up of a mixture of gases with varying abundance:
Ø Nitrogen 78.1%
Ø Oxygen 20.9%
Ø Argon 0.9%
Ø Carbon dioxide 0.04%
Ø Others: neon, helium, methane, nitrous oxide, ozone, nitrogen
dioxide
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NITROGEN
• Exists as a diatomic molecule, N2
• Held together by a strong triple covalent bond à unreactive
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NITROGEN
• High energy processes (natural and anthropogenic) that can allow
for reaction of nitrogen gas:
Natural Anthropogenic
Lightning Internal combustion engines
Volcanic activity Energy production
Bushfires Industrial furnaces
• High temperature breaks existing strong triple covalent bonds
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NITROGEN
• In high temperature conditions, nitrogen gases reacts with oxygen
to form nitric oxide:
N2(g) + O2(g) ⟶ 2NO(g)
• Nitric oxide reacts with oxygen gas to form nitrogen dioxide:
2NO(g) + O2(g) ⟶ 2NO2(g)
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NITROGEN
• Alternatively, nitrogen oxide can be formed from the reduction of
nitrogen compounds by microorganisms in soils.
NO2- + 2H+ + e- ⟶ NO + H2O
• Will revisit in subtopic 4.3 Soils
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SCIENCE UNDERSTANDING
Nitrogen oxides and ozone are pollutants in the troposphere that are
associated with photochemical smog.
q Describe and write equations showing the role of nitrogen oxides
in the formation of ozone in the troposphere.
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PHOTOCHEMICAL SMOG
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PHOTOCHEMICAL SMOG
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PHOTOCHEMICAL SMOG
• Smog = smoky fog
• Poisonous mixture of smoke, fog, air and other pollutants.
• Caused by the build-up of primary and secondary pollutants.
• Primary pollutants: Directly emitted into the atmosphere.
• Nitrogen oxides, NOx i.e. NO, NO2 • SO2
• Unburnt hydrocarbons • CO
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PHOTOCHEMICAL SMOG
• Secondary pollutants: Formed from the reaction of primary
pollutants.
Ø These reactions are often aided by sunlight (photochemical)
• NO2
• O3
• HNO3, H2SO4
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PHOTOCHEMICAL SMOG
• More common in densely populated cities with windless, sunny
conditions, where temperature inversions occur.
• Urban areas with large number of motor vehicles
• https://aqicn.org/map/world/
Discussion Question:
• What factors contribute to bad air pollution?
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PHOTOCHEMICAL SMOG
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PHOTOCHEMICAL SMOG
• Primarily caused by the emission of pollutants by motor vehicles.
• Nitrogen dioxide (NO2) and ozone (O3) are two major components of
photochemical smog.
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PHOTOCHEMICAL SMOG
• Ozone Formation
Photodissociation of nitrogen dioxide:
NO2(g) + uv ⟶ NO(g) + O(g)
O(g) = oxygen free radical (have unpaired electrons, very reactive)
O(g) + O2(g) ⟶ O3(g)
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SCIENCE UNDERSTANDING
Nitrogen oxides and ozone are pollutants in the troposphere that are
associated with photochemical smog.
qDescribe the harmful effects of nitrogen oxides and ozone in the
troposphere.
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EFFECTS OF PHOTOCHEMICAL SMOG
• Nitrogen oxides
• Visual pollutant (NO2)
• Damage health of animals and plants, respiratory irritant
• Can contribute to acid rain
• Eye irritation
Ø Reaction with moisture to produce acids (i.e. acid rains)
e.g. 2NO2(g) + H2O(l) ⟶ HNO2(aq) + HNO3(aq)
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EFFECTS OF PHOTOCHEMICAL SMOG
• Ozone
• Respiratory irritant
Ø Damage tissue lining lungs
Ø Elderly, young and those with asthma and bronchitis at higher
risk
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EFFECTS OF PHOTOCHEMICAL SMOG
• Ozone
• Impacts on plant growth
Ø Damages leaves, limiting gas exchange in leaves (important for
photosynthesis)
Ø Ozone also enters the plant cells, it is a strong oxidizing
agent and effects photosynthesis and other cellular reactions
Ø Plants more susceptible to disease and weather extremes
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EFFECTS OF PHOTOCHEMICAL SMOG
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EFFECTS OF PHOTOCHEMICAL SMOG
• Ozone
• Attacks polymers e.g. rubber
• Ozone acts as an oxidising agent
Ø Weakens polymer – no longer can carry out its role more
efficiently
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EFFECTS OF PHOTOCHEMICAL SMOG
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SCIENCE UNDERSTANDING
Nitrogen oxides and ozone are pollutants in the troposphere that are
associated with photochemical smog.
qDescribe and write equations showing how catalytic converters
reduce the quantities of nitrogen oxides generated by motor
vehicles.
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CATALYTIC CONVERTERS
• Convert some precursors of photochemical smog
to less harmful pollutants.
• Fitted to exhaust pipes of all modern vehicles.
• Made of an open, porous (honeycomb) matrix
coated with a platinum, palladium and/or
rhodium catalyst.
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CATALYTIC CONVERTERS
• Solution to reduce the risk of photochemical smog
• Nitric oxide and carbon monoxide converted to less harmful gases
2NO(g) + 2CO(g) ⟶ N2(g) + 2CO2(g)
More Harmful Less Harmful
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CATALYTIC CONVERTERS
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CATALYTIC CONVERTERS
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Your Turn!
• Complete practice questions worksheet
