H7
Chemistry and the Atmosphere
Human activity has polluted the atmosphere. This chapter investigates the nature and
the source of this pollution and the techniques used to monitor the concentration of
gases in the air.
Major constituents of
the atmosphere
 Nitrogen
78.08%
 Oxygen
20.95%
 Argon
0.93%
The percentage of water varies from 0.5% to 5% and is
measured as relative humidity.
Minor constituents of
the atmosphere
(Insert Conquering chemistry HSC page 227 Table 7.1)
Troposphere
Region of the atmosphere closest to the Earth in which the
temperature generally decreases. (From sea level to a height of
15 km)
Stratosphere
Region of the atmosphere in which the temperature increases
as the altitude increases. (From 15km to 50km)
Major air pollutants
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Carbon monoxide
Oxides of nitrogen (NO and NO2)
Hydrocarbons
Other volatile organic compounds (VOCs)
Particulates
Lead
Sulfur dioxide
Radioactivity
Carcinogen compounds
Fluoride
Chlorofluorocarbons (CFCs)
Ozone
Ozone
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Allotrope of the element carbon
Ozone in the stratosphere forms the ozone layer that
protects all life forms by filtering out short-wavelength
ultraviolet light.
Ozone is poisonous to humans and other life forms
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Photochemical smog
Ozone is produced by the reactions
NO2 + sunlight
NO + O
O + O2
O3
The NO2 is produce in car exhausts. NO which is also
produced reacts with ozone and reduces its concentration in
the atmosphere.
NO + O3
NO2 + O2
The concentration of ozone in the air is a result of these two
processes and therefore a result of the relative amounts of NO
and NO2 produced by car exhausts.
In conditions where the NO2 production is high,
photochemical smog becomes a problem.
Catalytic exhausts in
motor vehicles
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Rhodium catalyses the removal of nitric oxide before it is
converted to nitrogen dioxide.
2CO(g) + 2NO(g)
2CO2(g) + N2(g)
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Platinum catalyses the conversion of carbon monoxide
and hydrocarbons to carbon dioxide and water.
2CO(G) + O2(g)
2CO2(g)
C5H12(g) + 8O2(g)
5CO2(g) + 6H2O(g)
Coordinate bonds
A covalent bond in which both of the shared electrons come
from the one atom.
Coordinate covalent bonds are found in
 Ozone
 Carbon monoxide
 The ammonium ion
 Complex ions such as Ag(NH3)2+ produced when a Lewis
base reacts with a Lewis acid.
Reactivity of the
allotropes of oxygen
When O2 reacts the double bond is broken to produce two
oxygen atoms. This requires a considerable amount of energy
and consequently this allotrope is not as reactive.
The O3 molecule reacts by splitting off one atom of oxygen
and leaving the O2 molecule. This requires less energy and the
O3 molecule is more reactive than the O2.
Ultraviolet radiation
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u.v.-A (400 to 320nm) generally beneficial as it facilitates
photosynthesis and forms Vitamin D
u.v.-B (320 to 280nm): causes skin cancers, eye cataracts,
decreased immune response and damages plants.
u.v.-C (<280nm): more harmful than u.v.-B but there is
no u.v.-C in the solar radiation reaching Earth.
Ozone in the stratosphere absorbs virtually all the u.v.-B and
any u.v.-C not already absorbed by O2.
Haloalkanes
Compounds in which one or more H atoms of an alkane have
been replaced by a halogen atom.
Haloalkenes
Compounds in which one or more H atoms of an alkene have
been replaced by a halogen atom.
Names haloalkanes
and haloalkenes
Straight chained haloalkanes and haloalkenes are named using
the following rules
 bromo-, chloro-, fluoro- and iodo- are used as prefixes to
the hydrocarbon name
 the position of the halogen atom is denoted by a number
and di-, tri-, tetra- are used when there is more than one
atom of any halogen. A location number is given for each
halogen atom.
 If more than one type of halogen is present they are listed
alphabetically with any di-, tri-, tetra- being ignored.
Chlorofluorocarbons
CFCs are compounds containing chlorine, fluorine and carbon
only. (They contain no hydrogen atoms)
CFCs and ozone
u.v. light breaks up CFC molecules
CCl3F + u.v. light
Cl + CCl2F
The chlorine atom produced can react with an ozone molecule.
Cl + O3
ClO + O2
A chain reaction results and one chlorine molecule can destroy
thousands of ozone molecules.
The extremely cold conditions of an Antarctic winter
produced solid particles that can catalyse the reaction.
HCl + ClONO2
Cl2 + HNO3
The chlorine molecules produced by the Antarctic winter
decompose in the spring and the chlorine atoms produced
begin to destroy ozone molecules
Cl2 + u.v. light
2Cl
Other ozone
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Halons
destroying
compounds
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Carbon tetrachloride
1,1,1-trichloroethane
Alternatives to CFCs
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Hydrochlorofluorocarbons (HCFCs)
Hydrofluorocarbons (HFCs)