1 New Way Chemistry for Hong Kong A- Level Book 4 1 Chemistry and the Environment 46.1 46.1 Introduction Introduction 46.2 46.2 Air Pollution Air Pollution 46.3 46.3 Water Pollution Water Pollution 46.4 46.4 Solid Waste Solid Waste 46.5 46.5 Pollution Control in Hong Kong Pollution Control in Hong Kong Chapter 46 Chapter 46
Chapter 46. Chemistry and the Environment. 46.1 Introduction 46.2 Air Pollution 46.3 Water Pollution 46.4 Solid Waste 46.5 Pollution Control in Hong Kong. 46.1 Introduction (SB p.220). Pollution found in Hong Kong. 46.1 Introduction (SB p.220). - PowerPoint PPT Presentation
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Chapter 46
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New Way Chemistry for Hong Kong A-Level Book 4
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46.1 Introduction (SB p.220)
Pollution is the introduction of waste matter or energy into the
environment by humans. It causes damages or deterioration to living
systems and/or the environment
A pollutant refers to any substance or energy produced through
human activities which, at an unacceptable level, causes damages to
the environment or is harmful to living systems
New Way Chemistry for Hong Kong A-Level Book 4
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46.2 Air Pollution (SB p.221)
Major air pollutants in the modern city are CO(g), SO2(g), NOx(g)
and small particulates (e.g. soot and lead)
Produced by burning fossil fuels in car engines, industrial plants
and power plants
Major Air Pollutants and their Sources
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produced by incomplete combustion of petrol, coal, and wood
e.g. C8H18(g) + 9O2(g)
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2. Sulphur dioxide
produced by combustion of sulphur-containing fuels
large proportion of SO2 are generated at power stations, metal
smelters in which sulphur-containing fuels are used
Power station
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3. Nitrogen oxide
produced when fuels are burnt at high temperatures
the level of NOx is noticeably high because of high-proportions of
diesel-driven vehicles running
diesel engines produce less CO but a lot more NOx
4. Hydrocarbons
formed by evaporation of organic materials, e.g. petrol, diesel,
etc
found in car exhaust as unburnt hydrocarbons
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5. Ozone
produced by the reaction of oxygen gas with free oxygen atoms which
are formed from the reaction between NOx and hydrocarbons in
sunlight
O2(g) + O • O3(g)
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6. Particulates
include black smoke (fine carbon particles), dust and soot (coarse
carbon particles), so small that remain suspend in air for a long
period of time
produced during operations in incinerators, factories, diesel
vehicles, construction sites and coal or charcoal burners
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Source of CO
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Source of SO2
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Source of NOx
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Source of particulates
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46.2 Air Pollution (SB p.224)
High levels of pollutants can cause headaches, nausea, irritation
of the eyes, nose and throat and ill-feeling generally
Some pollutants may lead to respiratory diseases
Effects of Polluted Air on the Environment
Some plants like lichens will not grow in cities because of high
levels of pollutants as they are more sensitive to O3 and SO2 than
humans.
Lichens cannot grow in seriously polluted air
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1. Carbon monoxide
poisonous, colourless, tasteless and odourless gas
when one inhales it, it will enter the blood stream and bind
preferentially to haemoglobin in the red blood cells replacing
O2
reduction of O2 supply to organs and tissues
causes death even in relatively low concentrations
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2. Sulphur dioxide
colourless, toxic gas
form acid rain
damage buildings and forests
The plant on the left (from Germany) is damaged by acid rain
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3. Nitrogen dioxide
irritate lungs, lower the resistance to respiratory
infections
gives rise to photochemical smog
impair lung development in young children
form acid rain
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4. Hydrocarbons
5. Ozone
irritate respiratory system causing cough, chest pain and throat
and eyes irritation and increase susceptibility to respiratory
infections
damage plants
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6. Particulates
environmental impact depends on size and nature
very fine particles (diameter less than 1 m) are the most
hazardous
cannot be filtered by hairs and mucus in the nose and the
respiratory track.
drawn deep into the lungs
impair lung functions, hinder the gas exchange, damage the lung
tissue and cause respiratory illness
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The sources and effects of some air pollutants
Air pollutant
Incomplete combustion of carbon-containing fuels
Reduce the amount of oxygen delivered to body organs and tissues,
and can cause death
Sulphur dioxide
Nitrogen oxides
From motor car exhaust, electric power stations, incinerators and
industrial plants
Irritate the lungs and lower the resistance to respiratory
infections, and cause the formation of photochemical smog and acid
rain
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The sources and effects of some air pollutants (cont’d)
Air pollutant
Cause photochemical smog; some are carcinogenic
Ozone
Particulates
From refuse incineration, factories, diesel vehicles, construction
sites and coal or charcoal burners
Impair the lung function by hindering gas exchange and damaging
lung tissues, and cause respiratory illness
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The harmful effects of pollutants depend on:
1. Concentration &
2. The duration of exposure
Take CO as an example:
If the concentration of CO is 600 – 700 ppm, inhaling it for an
hour brings about barely detectable effects
The concentration of CO of 4000 ppm or higher can cause death in
less than one hour
If the concentration of CO is 1000 ppm, the people will die from CO
poisoning after inhaling for 4 hours
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46.2 Air Pollution (SB p.226)
Parts per million (ppm) is one way to describe the concentration of
pollutants
The number of molecules of pollutants per million (106) molecules
in air. (It is a ratio with no unit)
It is equal to the number of volumes of pollutants per million
volumes of air based on the Avogadro’s law.
Representation of Concentration of Pollutants
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46.2 Air Pollution (SB p.226)
e.g. 50 ppm of CO means 50 cm3 of pollutant for every
1 000 000 cm3 of air
If the concentration is represented by percentage by volume, the
unit is readily converted to ppm by multiplying with a factor of 10
000
e.g. 0.1 % by volume = 1 000 ppm
The concentrations of pollutants are commonly expressed in g m–3
nowadays
Using ideal gas equation, PV = nRT, g m–3 and ppm can be
interconverted
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Example 46-1
Express the concentration of 0.05 ppm carbon monoxide measured at
298 K (25°C) and 101 kPa (1 atm) in g m–3.
(R.a.m. : C = 12.0, O = 16.0;
ideal gas constant = 8.314 J K–1 mol–1)
Answer
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Solution:
In 1 m3 of air, there is m3 carbon monoxide.
Using the ideal gas equation,
PV = nRT
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Solution: (cont’d)
Mass of CO
∴ The concentration of carbon monoxide is 57.1 g m–3
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(a) (i) State the major air pollutants.
(ii) For each kind of pollutants, state one of the harmful
effects.
Answer
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Air pollutant
Harmful effect
Carbon monoxide
Reduce the amount of oxygen delivered to body organs and tissues,
and can also cause death
Sulphur dioxide
Nitrogen oxides
Irritate the lungs and lower the resistance to respiratory
infections, and cause the formation of photochemical smog and acid
rain
Hydrocarbons
Cause the formation of photochemical smog and some are
carcinogenic
Ozone
Particulates
Impair the lung function by constricting respiratory passage and
damaging lung tissues, and cause respiratory illnesses
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Check Point 46-1 (cont’d)
(b) Given that the temperature is 0°C and the pressure is
1 atm (i.e. 101 kPa), express the following concentrations of the
gases in the units given.
(i) Express 10 ppm NO2 in % by volume;
(Ideal gas constant = 8.314 J K–1 mol–1; R.a.m.: N = 14.0,
S = 32.0, O = 16.0, C = 12.0)
Answer
= 0.001%
of NO2 by volume.
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Check Point 46-1 (cont’d)
(b) Given that the temperature is 0°C and the pressure is
1 atm (i.e. 101 kPa), express the following concentrations of the
gases in the units given.
(ii) Express 0.3 ppm SO2 in g m–3;
(Ideal gas constant = 8.314 J K–1 mol–1; R.a.m.: N = 14.0,
S = 32.0, O = 16.0, C = 12.0)
Answer
(b) (ii) In 1 m3 of air mass of SO2
= 854.4 g
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Check Point 46-1 (cont’d)
(b) Given that the temperature is 0°C and the pressure is
1 atm (i.e. 101 kPa), express the following concentrations of the
gases in the units given.
(iii) Express 10 000 g m–3 CO in ppm;
(Ideal gas constant = 8.314 J K–1 mol–1; R.a.m.: N = 14.0,
S = 32.0, O = 16.0, C = 12.0)
Answer
46.2 Air Pollution (SB p.227)
(b) (iii) Let the volume of CO in 1 m3 of air be V.
V = 8.03 10–6 m3
=
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Check Point 46-1
(b) Given that the temperature is 0°C and the pressure is
1 atm (i.e. 101 kPa), express the following concentrations of the
gases in the units given.
(iv) Express 600 g m–3 SO2 in % by volume.
(Ideal gas constant = 8.314 J K–1 mol–1; R.a.m.: N = 14.0,
S = 32.0, O = 16.0, C = 12.0)
Answer
46.2 Air Pollution (SB p.227)
(b) (iv) Let the volume of SO2 in 1 m3 of air be V.
V = 2.11 10–7 m3
% by volume =
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46.2 Air Pollution (SB p.230)
Unpolluted rainwater is slightly acidic, with pH value of about
5.7
CO2(g) + H2O(l) H2CO3(aq) pH 5.7
Rainwater reacts with CO2 to form carbonic acid
Acid Rain
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46.2 Air Pollution (SB p.230)
Rainwater with pH values lower than 5.7 is called acid rain
Acidic gaseous pollutants : SO2 and NO2
In the atmosphere, SO2 is oxidized to SO3 slowly and then dissolves
in rainwater readily
2SO2(g) + O2(g) 2SO3(g)
SO3(g) + H2O(l) H2SO4(aq)
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SO2(g) also dissolves in rainwater to form sulphuric(IV) acid
SO2(g) + H2O(l) H2SO3(aq)
2H2SO3(aq) + O2(g) 2H2SO4(aq)
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NO(g) is formed when burning fossil fuels in automobiles and power
stations.
The NO(g) is then oxidized to NO2(g)
2NO(g) + O2(g) 2NO2(g)
In a series of complex reactions, NO2(g) is oxidized to
HNO3(aq)
4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)
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46.2 Air Pollution (SB p.231)
Acid rain causes a lot of harmful effects on the environment
In water of pH lower than 4.5, calcium metabolism in freshwater
fish will be affected, leading to poor health and stunted
growth
diversity and population of some freshwater species will be
reduced
In soil of pH lower than 4.5, absorption
of essential nutrients (e.g. K+, Ca2+
and Mg2+) will be affected
results in the death of plants
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46.2 Air Pollution (SB p.231)
Acid rain also corrodes metals and accelerates the rate of
deterioration of buildings, rocks and statues
The statue was corroded by acid rain
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A summary of the formation and effects of acid rain
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Photochemical smog is a mixture of pollutants including
particulates, NOx, aldehydes, peroxyacetyl nitrate (PAN) and
unreacted hydrocarbons, etc
Brownish haze that irritates our eyes is an indicator of
photochemical smog
(Note: NO2 is responsible for the brown colour)
Photochemical Smog
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Appearance of a city:
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46.2 Air Pollution (SB p.232)
The formation of photochemical smog is initiated by sunlight and
involves hydrocarbons and nitrogen oxides
The free O reacts with unburnt hydrocarbons, O2 and water vapour in
a series of complex reaction to produce a variety of lachrymatory
and toxic chemicals such as peroxyacetyl nitrate (PAN)
NO2 from automobile exhaust absorbs sunlight and breaks down into
NO and free O
NO2(g) NO(g) + O(g)
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Principal reactions involved in the formation of photochemical smog
(much simplified)
O(g) + O2(g) O3(g)
O(g) + hydrocarbons aldehydes
O3(g) + hydrocarbons aldehydes
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Formation of photochemical smog
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46.2 Air Pollution (SB p.233)
The reactions of the formation of photochemical smog vary with
altitude
wavelength of ultraviolet radiation from the sun determines which
bonds are broken in the reaction
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Photochemical smog causes headaches, eye, nose and throat
irritation, worsening the lung function, coughing and
wheezing
Ozone attacks the C = C linkage in rubber in a process known as
ozonolysis. It cause rubbers and fabrics to deteriorate and car
tyres to crack
46.2 Air Pollution (SB p.233)
New Way Chemistry for Hong Kong A-Level Book 4
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(a) Describe the chemical reactions involved in the formation
of
(i) acid rain;
(a) (i) Acidic gaseous pollutants like sulphur dioxide and
nitrogen dioxide are responsible for the formation of acid
rain.
In the atmosphere, sulphur dioxide is slowly oxidized to sulphur
trioxide which dissolves readily in water droplets to form
sulphuric(VI) acid.
2SO2(g) + O2(g) 2SO3(g)
SO3(g) + H2O(l) H2SO4(aq)
When nitrogen monoxide is released to the atmosphere, it combines
with atmospheric oxygen to form nitrogen dioxide.
2NO(g) + O2(g) 2NO2(g)
In a series of complex reactions, nitrogen dioxide combines
with
oxygen and water vapour to form nitric(V) acid
4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)
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(a) Describe the chemical reactions involved in the formation
of
(ii) photochemical smog.
(a) (ii) Photochemical smog is a mixture of pollutants including
particulates, nitrogen oxides, ozone, aldehydes, peroxyacetyl
nitrate (PAN) and unreacted hydrocarbons.
Nitrogen dioxide from automobile exhaust first absorbs sunlight and
breaks down into nitrogen monoxide and free oxygen atom which is
very reactive.
NO2(g) NO(g) + O(g)
The oxygen atom then reacts with other components of automobile
exhaust (e.g. unburnt hydrocarbons) and those of the atmosphere
(e.g. oxygen and water vapour) in a series of complex reactions to
produce a variety of
lachrymatory substances such as peroxyacetyl nitrate (PAN).
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Very much simplified, some of the principal reactions in the
formation of photochemical smog are as follows:
O(g) + O2(g) O3(g)
O(g) + hydrocarbons aldehydes
O3(g) + hydrocarbons aldehydes
Hydrocarbons + O2(g) + NO2(g)
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Check Point 46-2 (cont’d)
(b) Iron bars and marble stones (CaCO3) are two common building
materials. Describe how sulphuric (VI) acid in acid rain affects
these materials. Explain with the help of equations.
Answer
46.2 Air Pollution (SB p.233)
(b) Iron bars are corroded by sulphuric(VI) acid as shown in the
following equations.
Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g)
CaCO3(s) + H2SO4(aq) CaSO4(aq) + CO2(g) + H2O(l)
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(chlorine-like) smell
harmless in low concentrations
cause respiratory problems and headaches when concentration rises
above 100 ppm
The Ozone layer and Chlorofluorocarbons
Ozone
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In troposphere, O3 can be formed by:
1. reaction between NOx and hydrocarbons in sunlight
2. electric sparks in car engines and electrical appliances
3. lightning
4. passing a stream of O2 through an electric discharge
O3 is very reactive, and does not stay in the atmosphere for a long
time
Sources and Properties
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46.2 Air Pollution (SB p.234)
O3 can be generated (a) by passing a stream of O2 through an
electric discharge or (b) by lightning
(a)
(b)
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In stratosphere,
O3 is formed from atmospheric O2 by absorption of UV radiation of
the right energy (wavelength 250 nm)
dissociates O2 molecules into free O
free O reacts with other O2 to form O3
UV light
O2(g) 2O(g)
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Formation of ozone
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46.2 Air Pollution (SB p.236)
When O3 absorbs UV radiation of wavelength 215 – 295 nm, it
undergoes photodissociation and splits up again
O3(g) O(g) + O2(g)
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This reaction is responsible for vital screening effect of
ozone
It screens out harmful UV radiation
O3 is constantly created and destroyed in the stratosphere and its
concentration remains constant
46.2 Air Pollution (SB p.236)
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46.2 Air Pollution (SB p.236)
The presence of O3 in the stratosphere is very important for living
organisms on the Earth
Although the concentration is very low, it filters out 99% harmful
UV radiation
Thinning of the O3 layer may result in increase in amount of UV
radiation reaching the Earth’s surface
leads to an increase in the incidence of skin cancer, genetic
mutation, cataract and a decrease in crop yield
Desirability of Ozone in the Stratosphere
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Answer
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Check Point 46-3 (cont’d)
(b) Explain how the concentration of ozone is kept constant in the
stratosphere.
Answer
46.2 Air Pollution (SB p.236)
(b) In the stratosphere, ozone is formed from atmospheric oxygen by
absorption of ultraviolet radiation of the right energy (wavelength
250 nm), which dissociates oxygen molecules into free oxygen
atoms.
O2(g) 2O(g)
The oxygen atoms produced react with other oxygen molecules to form
ozone.
O(g) + O2(g) O3(g)
On the other hand, when ozone absorbs ultraviolet radiation of
wavelength 215 – 295 nm, it undergoes photodissociation and splits
up again.
O3(g) O2(g) + O(g)
destroyed, its concentration remains constant.
UV light
UV light
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Chlorofluorocarbons (CFCs) are a family of compounds containing
chlorine, fluorine and carbon
They are unreactive, low flammability and low toxicity
Due to its properties, CFCs have a wide range of uses
Chlorofluorocarbons
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1. As aerosol propellants
CFCs such as CCl3F and CCl2F2 used as propellants
Upon depressurization when the valve is opened, the propellant
vaporizes and expels the materials in the form of aerosol
spray
Uses of CFCs
An aerosol canister
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• odourless
• non-flammable
• stable
• non-toxic
Commonly found aerosol canisters in daily life
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2. As solvents in cleaning electronic components and metals
CFCs such as CCl2FCClF2 can dissolve grease and are widely used as
solvents in cleaning electronic components and metals
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3. As refrigerants
Freon is a series of CFCs (CCl2F2 may be the most important
one)
Freon absorbs heat of vaporization on evaporation, leading to
cooling of the surroundings
Widely use as refrigerants in refrigerators and air-conditioning
units
Advantages of using CFCs:
1. same thermal properties as traditional refrigerants such as SO2
and NH3
2. non-toxic
3. non-corrosive
CFC refrigerant
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4. Blowing agents in foam plastic manufacturing
Volatile CFCs such as CCl3F are incorporated into the plastics when
making foam plastics
The heat evolved during the polymerization reaction vaporizes the
CFC which then forms thing bubbles in the plastics
Use of foam plastics:
Foam plastic products
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Some of the CFCs escape into the atmosphere during use
More CFCs enter the environment when refrigerators and air
conditioning units are scrapped or when foam plastics are disposed
of
CFCs are unreactive and non-biodegradable that they break down in
atmosphere very slowly
CFCs are relatively insoluble in water, therefore they cannot be
removed by either rainfall or dissolution into the ocean.
Causes for the Accumulation of CFCs in the Stratosphere
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They have long lifetime (e.g. CCl3F about 75 yrs)
give plenty of time to travel up to stratosphere
Once in the stratosphere, the UV radiation from the sun is intense
enough to dissociate CFCs molecules
giving Cl free radicals which destroy ozone dramatically through a
series of free radical chain reaction
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Check Point 46-4
(a) Explain why an aerosol propellant should be a good solvent,
chemically unreactive and has a low boiling point.
Answer
46.2 Air Pollution (SB p.238)
(a) It should be a good solvent as it has to dissolve the solute.
It should be chemically unreactive so that it does not react with
the solute or the substances it sprays on. It has a low boiling
temperature so that the aerosol can vaporize very easily when
pressure is released.
New Way Chemistry for Hong Kong A-Level Book 4
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Check Point 46-4
(b) Besides the properties mentioned in (a), what other properties
should an aerosol liquid possess?
Answer
(b) It has to be odourless, non-flammable and non-toxic
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46.2 Air Pollution (SB p.238)
O3 in the stratosphere undergoes photodissociation to give O2 and O
atoms by absorbing UV radiation
O3(g) O2(g) + O(g) ………………… (1)
The O atoms formed further react with other O3 molecules to give O2
molecules
O(g) + O3(g) 2O2(g) ………………… (2)
reduce the concentration of O3
O3 is formed at the same time. Their rates are somewhat
equivalent
the concentration of O3 remains fairly constant
Depletion of the Ozone Layer
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46.2 Air Pollution (SB p.238)
During the past few decades, the concentration of O3 has fallen
over the Antarctica and the Arctic
Evidence shows that O3 depletion is related to release of CFCs to
the atmosphere
The destruction of O3 by CFCs is a complex chemical process
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The free Cl • from the above initiation steps readily react with O3
via the following reactions
Cl • (g) + O3(g) ClO • (g) + O2(g)………………… (5)
ClO • (g) + O3(g) Cl • (g) + 2O2(g) ……………… (6)
Take CCl3F and CCl2F2 as examples:
CCl3F(g) CCl2F • (g) + Cl • (g) …………………… (3)
CCl2F2(g) CClF2 • (g) + Cl • (g)………………….(4)
UV light
UV light
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46.2 Air Pollution (SB p.239)
The depletion of ozone by (5) is faster then that by (2)
The worst issue is that the Cl • consumed in the (1) is regenerated
in (2)
1 Cl • can destroy many O3 molecules effectively in series of chain
reactions
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46.2 Air Pollution (SB p.239)
Since 1975, scientists have detected a continuous and severe drop
in the concentration of ozone in the layer over the
Antarctica
In 1987, it was found that half of the Antarctica’s ozone had
disappeared over a region twice of the size of the United States,
creating a huge hole in the ozone layer
At the altitude of 18 km, the concentration of ozone drop as much
as 50% of the normal value
The concentration of ozone decreased up to 3 % was observed in the
Northern hemisphere
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46.2 Air Pollution (SB p.239)
The maps of the O3 layer over the Antarctica indicated by the Total
Ozone Mapping Spectrometer (TOMS) from 1979 to 1992
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46.2 Air Pollution (SB p.240)
Ozone depletion increases the absorption of UV light by the
stratosphere which will lead to
adverse consequences on aquatic and terrestrial ecosystems
reduced crop yield
Solution:
minimize the use of CFCs and develop suitable substitutes which do
not affect the environment adversely
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46.2 Air Pollution (SB p.240)
In 1987, Montreal Protocol on Substances that Deplete the Ozone
Layer was signed in Montreal, Canada.
The Protocol requires the phasing out of the Ozone Depleting
Substances (ODS)
Control of the Ozone Depletion Problem
Ozone depleting substances
Chlorofluorocarbons (CFCs)
Tetrachloromethane
1,1,1-Trichloroethane
Hydrochlorofluorocarbons (HCFCs)
Freezing of consumption at base level starting 1st January 1996
Import for local consumption banned by 1st January 2003
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Consumption of ozone depleting substances in Hong Kong
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46.2 Air Pollution (SB p.241)
To fulfill the Hong Kong SAR’s international obligations, the Ozone
Layer Protection Ordinance was enacted in July 1989 to control the
use of ozone depleting susbstances
The Ordinance prohibits the manufacture of such substances and
imposes controls on import and export of these substances through
legislation and licensing provisions
Possible solutions:
Installation of recovery and recycling units to recycle CFCs from
used refrigeration systems and motor vehicles air
conditioners
Using substitutes for CFCs
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46.2 Air Pollution (SB p.241)
Compounds with low ozone depletion potential have been developed to
replace CFCs, but they suffer from some disadvantages:
1. HCFCs and HFCs
The presence of H atom makes the compounds less stable and do not
reach the stratosphere in high concentration
low % of Cl and low ozone depletion potential
damage the ozone layer if overused
HFCs do not contain Cl and thus are “ozone safe”, but the questions
on their toxicity and safety are still unsolved
Possible Alternatives for Chlorofluorocarbons
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2. Butane and propane
not expensive and readily available
but they are flammable and poisonous
3. Water and steam
effective for cleaning applications in some occasions but the
applications are rather limited
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Possible alternatives for CFCs and some of their properties
Compound
Toxicity
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Check Point 46-5
Give some methods for reducing the depletion of the ozone
layer.
Answer
46.2 Air Pollution (SB p.242)
Firstly, the consumption of CFCs and other ozone depleting
substances should be controlled. Secondly, recovery and recycling
units to recycle CFCs used refrigeration system and motor vehicle
air conditioners could be installed in order to reduce the amount
of CFCs released to the atmosphere. Thirdly, substitutes for CFCs
such as butane and propane could be used as they do not
contain chlorine, they are “ozone safe”
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46.3 Water Pollution (SB p.242)
Water pollution is the degradation of the quality of water by the
introduction of chemical, physical or biological wastes into
rivers, streams, lakes and oceans.
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1. Livestock waste
• When there is an excessive amount of organic matter is discharged
into water, the existing micro-organisms will grow and multiply
rapidly
oxygen is rapidly depleted
Due to lack of oxygen, anaerobic micro-organisms begin to partially
break down the organic matter, releasing noxious gases like
methane, ammonia and hydrogen sulphide
aquatic organisms are endangered
The Causes of Water Pollution and its Effects on the
Environment
46.3 Water Pollution (SB p.243)
New Way Chemistry for Hong Kong A-Level Book 4
*
46.3 Water Pollution (SB p.243)
The river is seriously polluted by livestock waste, causing the
death of aquatic organisms
New Way Chemistry for Hong Kong A-Level Book 4
*
2. Oil spillages
Oil clogs the feathers of sea birds and prevent them from flying or
swimming
It also interferes with the insulation provided by the feathers, so
affected birds may die of cold or pneumonia
46.3 Water Pollution (SB p.243)
When birds try to clean their feathers with their beaks, they may
be poisoned through the ingestion of oil
New Way Chemistry for Hong Kong A-Level Book 4
*
Oil deposited on beaches
may drive away holiday-
Oil spillages are difficult to clean up
New Way Chemistry for Hong Kong A-Level Book 4
*
3. Residues of pesticides
Pesticides are synthetic organic compounds used to eliminate flying
and crawling pests
When pesticides enter water, it cause immediate toxic effects on
aquatic life
46.3 Water Pollution (SB p.244)
Pesticides are non-biodegradable and may accumulate along the food
chain until the toxic level is reached
New Way Chemistry for Hong Kong A-Level Book 4
*
46.3 Water Pollution (SB p.244)
New Way Chemistry for Hong Kong A-Level Book 4
*
(a) it resists degradation and remains in the
environment
(b) it is very soluble in fat, including the fat in
milk and the body fat of animals, with the potential for damaging
internal organs
(c) it affects the reproduction of birds, fish, etc. and weakens
the shells of eggs
(d) many species of insects can develop resistance to DDT
The use of DDT has been banned in many parts of the world including
Hong Kong
46.3 Water Pollution (SB p.244)
New Way Chemistry for Hong Kong A-Level Book 4
*
Synthetic detergents are the major pollutant present in
sewage
46.3 Water Pollution (SB p.244)
New Way Chemistry for Hong Kong A-Level Book 4
*
Discharge of detergents causes eutrophication (massive growth of
algae in water bodies resulting in deaths of other plants and
animals due to depletion of oxygen, space and food) brought about
by phosphates found in some detergents
46.3 Water Pollution (SB p.244)
Eutrophication produces a mat of algae on the surface of
water
New Way Chemistry for Hong Kong A-Level Book 4
*
Ingestion of detergents by aquatic organisms can cause death
46.3 Water Pollution (SB p.244)
Eutrophication causes fish to suffocate and die
New Way Chemistry for Hong Kong A-Level Book 4
*
5. Industrial effluents
Toxic materials present in industrial effluents vary with the types
of industries involved
They may contain heavy metals (e.g. Cd, Cr, Cu, Hg, Ni, Pb, Zn),
cyanides, polychlorinated hydrocarbons, etc.
Bioaccumulation of heavy metals is a process by which the heavy
metals become more concentrated as they move up food chains
46.3 Water Pollution (SB p.244)
New Way Chemistry for Hong Kong A-Level Book 4
*
One of the worst known case of toxicity is the bioaccumulation of
mercury found in Minamata Bay, Japan, in 1950s.
It was caused by consumption of fish that has been contaminated by
mercury let off from a chemical manufacturing plant
121 people is poisoned after eating contaminated fish, and 46 of
them died
46.3 Water Pollution (SB p.244)
New Way Chemistry for Hong Kong A-Level Book 4
*
The causes and effects of water pollution
Cause
Harmful effects
Livestock waste
Release noxious gases, and leads to the death of aquatic
organisms
Oil spillages
Clog the feathers of sea birds and prevent them from flying or
swimming Interfere with the insulation of the feathers of birds,
and cause the death of birds from cold or pneumonia Poison birds by
ingestion Prevent dissolution of oxygen, and cause the death of
aquatic organisms Spoil recreational resorts
New Way Chemistry for Hong Kong A-Level Book 4
*
The causes and effects of water pollution (cont’d)
Cause
Residues of pesticides
Accumulate along food chain Have the potential for damaging
internal organs Exert harmful effects on reproduction of
animals
Detergents in sewage
Cause eutrophication, and lead to the death of aquatic
organisms
Industrial effluents
New Way Chemistry for Hong Kong A-Level Book 4
*
Dissolved oxygen (DO) can be used as an indicator of oxygen content
in water
Water Quality
Dissolved Oxygen
It is measured in mg of oxygen per dm3 of water or percentage
saturation of dissolved oxygen
The maximum concentration of dissolved oxygen in water at 16°C is
10 mg dm–3
New Way Chemistry for Hong Kong A-Level Book 4
*
46.3 Water Pollution (SB p.246)
Normally, fish require 4 – 6 mg dm–3 of oxygen for survival
A minimum DO is also needed for reducing the odour caused by
anaerobic degradation of organic matter
If a water sample contains 6 mg dm–3 of dissolved O2 at 16°C, the
percentage saturation of DO is:
The sample is said to be 60% saturated with oxygen
New Way Chemistry for Hong Kong A-Level Book 4
*
DO in lakes and rivers is rapidly depleted if organic waste is
released into the water
Sources of organic waste: human and livestock sewage and industrial
waste from paper mills, tanneries and food processing plant
Organic waste in aquatic ecosystems is decomposed by aerobic
decomposers, primarily bacteria and fungi
If the water is overloaded with organic waste, aerobic decomposers
proliferate and DO will be consumed more rapidly than it can be
replaced from the atmosphere
46.3 Water Pollution (SB p.246)
New Way Chemistry for Hong Kong A-Level Book 4
*
If the level of DO falls below 5 mg dm–3, fish start to die
If the concentration of DO continues to fall, other marine animals
and aerobic bacteria will die too
When the DO is completely consumed, decomposition of organic matter
will be taken over by anaerobic bacteria
The water begins to smell unpleasant because different pathways of
decomposition are proceeding
Many of the end produces have displeasing odours
e.g. H2S, NH3, amine and phosphorous compounds
46.3 Water Pollution (SB p.246)
New Way Chemistry for Hong Kong A-Level Book 4
*
Quality standards of water for different uses
Water use
Public water supply Water contact recreation Fish propagation and
wildlife Industrial water supply Agricultural water supply
Shellfish harvesting
4.0 4.0 – 5.0 4.0 – 6.0 (depending on fish species) 3.0 – 5.0
(depending on use) 3.0 – 5.0 (based on application) 4.0 – 6.0
(depending on local conditions)
New Way Chemistry for Hong Kong A-Level Book 4
*
The quantity of Esherichia coli (E.coli) is also used for
indicating the general water quality
E. coli is a kind of bacteria found in human faeces and often used
as an indicator of sewage pollution
High E. coli count indicates great faecal contamination and high
health risk
Low DO level indicates the severe depletion of oxygen in the
water
46.3 Water Pollution (SB p.247)
New Way Chemistry for Hong Kong A-Level Book 4
*
46.3 Water Pollution (SB p.247)
The E. coli counts of four typhoon shelters in Hong Kong
New Way Chemistry for Hong Kong A-Level Book 4
*
46.3 Water Pollution (SB p.247)
The dissolved oxygen levels of four typhoon shelters in Hong
Kong
New Way Chemistry for Hong Kong A-Level Book 4
*
Winkler method is used to determine the amount of dissolved oxygen
in water
Procedure of Winkler method
1. A water sample of known volume is collected with a
250.0 cm3 volumetric flask
2. A little MnSO4(aq) solution and a little alkaline solution of
KI(aq) are added to the volumetric flask
46.3 Water Pollution (SB p.248)
Determination of Dissolved Oxygen in a Water Sample
New Way Chemistry for Hong Kong A-Level Book 4
*
In alkaline solution, DO in the sample oxidize Mn2+ to Mn3+
8OH–(aq) + 4Mn2+(aq) + O2(aq) + 2H2O(l) 4Mn(OH)3(s)
brown ppt
46.3 Water Pollution (SB p.248)
3. Once the precipitation is finished, a little conc. H2SO4 is
introduced into the solution well below its surface
4. The flask is stoppered and mixed until the brown precipitate of
Mn(OH)3(s) disappears the Mn(OH)3(s) converts I– into I2 while
itself reduced to Mn2+
2Mn(OH)3(s) + 2I–(aq) + 6H+(aq)
2Mn2+(aq) + I2(aq) + 6HO(l)
*
5. The iodine formed is determined by titration with a standard
sodium thiosulphate solution. A few drops of starch solution is
added as indicator when the end point approaches. The complete
disappearance of blue colour indicates the end point of the
titration
46.3 Water Pollution (SB p.248)
New Way Chemistry for Hong Kong A-Level Book 4
*
From the above stoichiometric equations,
1 mol of O2 4 mol of Mn(OH)3 2 mol of I2
After determining the no. of moles of I2 produced, the number of
moles of O2 present in the original sample can be calculated
46.3 Water Pollution (SB p.248)
New Way Chemistry for Hong Kong A-Level Book 4
*
BOD is used as an indicator of the extent of organic pollution in
water
If the water has a high BOD, this implies that a lot of organic
waste is present and much oxygen is required to break down the
waste
A low BOD value indicates the presence of only a small amount of
organic mater and there is little pollution
46.3 Water Pollution (SB p.248)
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is the amount of oxygen required to
break down the organic matter present in a water sample
New Way Chemistry for Hong Kong A-Level Book 4
*
Natural clean water has a BOD of about 1 to 4 mg dm–3
If the BOD is greater than the amount of O2 replenished, then some
fish and aquatic life may suffocate.
anaerobic micro-organisms begin to multiply
producing unpleasant odour
BOD is determined in the laboratory by incubating in a water sample
for 5 days at 20°C and measuring the amount of O2 consumed
It is known as five-day BOD or BOD5
46.3 Water Pollution (SB p.249)
New Way Chemistry for Hong Kong A-Level Book 4
*
If a sample has an initial DO of 8 mg dm–3, and in five days it has
dropped to 2 mg dm–3
Then BOD5 = (8 – 2) mg dm–3 = 6 mg dm–3
BOD5 value is commonly used to indicate the relative oxygen
requirements in treating effluents and polluted waters
46.3 Water Pollution (SB p.249)
The degree of pollution indicated by biochemical oxygen demand
(BOD5)
BOD5 (mg dm–3)
*
Check Point 46-6
(a) The solubility of oxygen in water is 9 mg dm–3 at 20°C and 1
atm. In a stream, the water is found to be 40% saturated with
oxygen. List some possible uses of the stream water.
Answer
(a) Dissolved oxygen of the stream water
= 9 mg dm–3
= 3.6 mg dm–3
Therefore, the stream water can be used as industrial water supply
and agricultural water supply.
New Way Chemistry for Hong Kong A-Level Book 4
*
(b) (i) What is the significance of BOD?
(ii) If the initial DO of a water sample is 30 mg dm–3 and the
value has dropped to 10 mg dm–3 in five days, calculate the BOD5 of
the water sample
Answer
46.2 Air Pollution (SB p.249)
(b) (i) BOD stands for biochemical oxygen demand. It is the amount
of oxygen required to break down the organic matter present in a
water sample. It is commonly used as an indicator of the extent of
organic pollution in water. A high BOD of water implies that a lot
of organic waste is present and much oxygen is required to break
down the waste. A low BOD indicates the presence of only a small
amount of organic matter and there is little organic
pollution
(ii) BOD5 = 30 mg dm–3 – 10 mg dm–3
= 20 mg dm–3
*
46.4 Solid Pollution (SB p.249)
In Hong Kong, over 16 820 tonnes of solid waste was disposed of
daily in 2001
Solid waste is produced as a result of:
(1) activities of domestic households
(2) commercial operations
(3) manufacturing industries
(4) construction sites
*
Composition of solid waste disposal in Hong Kong in 2001
New Way Chemistry for Hong Kong A-Level Book 4
*
Composition of domestic waste in Hong Kong in 1999
New Way Chemistry for Hong Kong A-Level Book 4
*
major disposal method for municipal solid
water in Hong Kong
Potential environmental problems:
leachate (highly polluting water percolating
through decaying waste at landfills)
Disposal of Solid Waste
New Way Chemistry for Hong Kong A-Level Book 4
*
Solution to the problems:
Wells are installed at the landfills to extract the gas and burns
it off at very high temperatures
The movement of landfill gas is monitored and controlled throughout
the operation of the landfills to reduce emissions to
atmosphere
Landfill gas is used as an energy source in some countries
Leachate may be treated in sewage treatment plants
New Way Chemistry for Hong Kong A-Level Book 4
*
Advantages:
high temperature kills disease-causing
heat produced can be used as energy source
some incinerators equipped with facilities
for recovering valuable metals (e.g. Fe, Al)
from waste
New Way Chemistry for Hong Kong A-Level Book 4
*
Disadvantages:
e.g. metals (Be, Cd, Cr, Pb and Hg) are released
large amount of CO2 produced adds to the problem of greenhouse
effect
New Way Chemistry for Hong Kong A-Level Book 4
*
plastics do not undergo decomposition at landfill sites
incineration of plastic waste produces air pollutants (e.g. HCl)
which can cause acid rain
small fish have been found dead with their digestive tracts clogged
by fragments of plastic foam ingested
sea animals have been suffocated to death by plastic bag
use of plasticizers and CFCs in the manufacture of plastics leads
to further ecological and environmental problems
Pollution Problems Associated with the Disposal of Plastics
New Way Chemistry for Hong Kong A-Level Book 4
*
46.4 Solid Pollution (SB p.252)
The enzymes in micro-organisms tend to attack the ends of the
polymer chains only
plastics undergo extremely slow degradation
Attempts have been made to develop plastics which are more
degradable
e.g. biopolymers, photodegradable plastics and synthetic
biodegradable plastics
Possible Solutions to Pollution Problems of Plastics
Development of Degradable Plastics
*
Biopolymers
Paracoccus
Bacillus
Spirullum
*
Poly(hydroxybutyrate) (PHB)
a natural polyester made by certain bacteria which can be broken
down by micro-organisms found in soil and natural water sources
under the anaerobic condition of a landfill within 9 months
The main disadvantage is more expensive (15 times) than
polyethene
46.4 Solid Pollution (SB p.252)
New Way Chemistry for Hong Kong A-Level Book 4
*
Photodegradable Plastics
These groups enhance the ability of the plastics to absorb UV
radiation
When the plastics are exposed to UV radiation from sunlight, the
long polymer chains start to break down into small fragments
Photodegradable plastics have light-sensitive functional groups
such as the carbonyl group ( ) incorporated into their polymer
chains
New Way Chemistry for Hong Kong A-Level Book 4
*
Synthetic Biodegradable Plastics
When micro-organisms consume starch or cellulose, the plastics are
broken down into tiny pieces
The biodegradation is speeded up greatly as surface area have been
greatly increased
Synthetic biodegradable plastics are made by incorporating starch
or cellulose into the polymers during productions
New Way Chemistry for Hong Kong A-Level Book 4
*
1. The products of biodegradation cause water pollution
2. The rate of biodegradation is still slow for the large quantity
of plastic waste generated
46.4 Solid Pollution (SB p.253)
Products made of degradable plastics
New Way Chemistry for Hong Kong A-Level Book 4
*
Apply on thermoplastics only
The plastics in the waste are separated, cleaned, pulverized, and
remoulded into new plastic items
Recycling of Plastics
*
A flowchart of the general recycling process
New Way Chemistry for Hong Kong A-Level Book 4
*
46.4 Solid Pollution (SB p.254)
The collection of clean and uncontaminated plastic waste is the key
step to the success of direct recycling, however this is the most
difficult step
Other constraints on the recycling of plastic waste in Hong Kong
include:
(1) high land and labour costs
(2) low environmental awareness
(4) space limitation
New Way Chemistry for Hong Kong A-Level Book 4
*
46.4 Solid Pollution (SB p.255)
The use of plastic coding system is one of the pre-requisites for
successful plastic recycling
facilitate the sorting process
A commonly used and internationally recognized coding system, SPI
(Society of Plastics Industry, USA) is adopted
Seven code number: 1 – 6 refer to specific polymers; 7 refers to
all other types
New Way Chemistry for Hong Kong A-Level Book 4
*
The plastic coding system
*
The plastic coding system
New Way Chemistry for Hong Kong A-Level Book 4
*
46.4 Solid Pollution (SB p.256)
Incineration of plastics to produce energy is recycling of the
energy content stored in materials
Then energy obtained from burning plastic waste in incinerators can
be used for heating or for generating electricity
The drawback is that certain plastics release toxic gases when
burnt, such as PVC gives out HCl when burnt
Recycling of Energy
*
Calorific values of plastics and various materials
Material
Polystyrene Poly(ethene) Heating oil Fats Leather Polyvinyl
chloride (PVC) Paper Wood
46 000 46 000 44 000 37 800 18 900 18 900 16 800 16 000
New Way Chemistry for Hong Kong A-Level Book 4
*
46.4 Solid Pollution (SB p.256)
Large plastic molecules are broken down on the surface of a hot
silica catalyst to smaller, more useful molecules
Pyrolysis is carried out between 400°C and 800°C
The products are separated by fractional distillation and used to
make plastics and other chemicals
Recycling of Chemicals
Pyrolysis is the thermal decomposition of a material at a high
temperature and in the absence of air
New Way Chemistry for Hong Kong A-Level Book 4
*
Recycling of chemicals from plastics by pyrolysis
New Way Chemistry for Hong Kong A-Level Book 4
*
Pyrolysis product
Substance manufactured
Poly(ethene) Poly(propene) Town gas Rubber Various plastics (e.g.
polystyrene)
New Way Chemistry for Hong Kong A-Level Book 4
*
Answer
(a) The benefits are as follows:
(1) To save the limited resources like petroleum.
(2) Energy saved from burning of plastic waste in incinerators can
be used for heating or generating electricity.
(3) To produce useful chemicals for production of
plastics or other purpose
*
(b) What are the difficulties in recycling plastics?
Answer
(b) The difficulties are:
(1) Plastic waste collected for recycling should be clean and
uncontaminated, and it is difficult to achieve.
(2) There are different kinds of plastics. We have to sort the
waste, plastics and eliminate the unrecyclable one,
and it is time-consuming and difficult.
New Way Chemistry for Hong Kong A-Level Book 4
*
The Environmental Protection Department (EPD) was establish in 1986
to coordinate and carry out pollution prevention and control
activities
Its tasks include:
2. Enforcing environmental legislation
3. Monitoring environmental quality
4. Providing facilities for collection, transfer, treatment and
disposal of many types of waste
New Way Chemistry for Hong Kong A-Level Book 4
*
5. Advising on the environmental implications of town planning and
new policies
6. Handling pollution complaints and incidents
7. Raising awareness and support in the community for environmental
initiatives
46.5 Pollution Control in Hong Kong (SB p.258)
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.258)
Premature combustion of petrol in engines results in knocking power
loss
Tetraethyllead(V) (TEL) are added to the fuel to prevent
knocking
Toxic volatile lead halides formed during combustion pass out with
exhaust gas
Lead is toxic. It causes anaemia and damages the nervous system and
kidney
Lead can inactivate the catalyst in catalytic converters
must use unleaded petrol in cars fitted with catalytic
converters
Measures to Improve Air Quality
Use of Unleaded Petrol
*
46.5 Pollution Control in Hong Kong (SB p.259)
NOx, CO and unburnt hydrocarbons are major air pollutants coming
from car exhaust
Installation of catalytic converters can help reduce the emission
of these pollutants
The converter contains precious metals, including Rh and Pt (or
Pd), as catalyst
When car exhaust containing CO, unburnt hydrocarbons and NOx is
passed through the converter, Rh catalyzes the reduction of
NO
Installation of Catalytic Converters in Car Exhaust Systems
Rh
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.259)
The heterogeneous catalysts are supported on a honeycomb structure
that allows the best contact between the car exhaust and the
catalyst surface
The catalysts can be poisoned by lead compounds easily
cars fitted with catalytic converters use unleaded petrol
only
Pt catalyzes the oxidation of CO and unburnt hydrocarbons such as
C7H16
2CO(g) + O2(g) 2CO2(g) (Oxidation)
Pt
Pt
*
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.260)
In the past, Hong Kong suffered from serious air pollution caused
by SO2 emission from burning sulphur-containing fuels
∴ Starting from 1990, the sulphur contents in liquid fuels (e.g.
diesel, kerosene) and solid fuels (e.g. coal, coke, charcoal) are
limited to 0.5% and 1% by weight respectively
Starting from 2000, the limitation is tightened
The sulphur content in diesel is further limited to not more than
0.05% by weight
Limitation of Sulphur Content in Fuels
New Way Chemistry for Hong Kong A-Level Book 4
*
SO2 is released during combustion of coal which contains
sulphur
Using coal of low sulphur content, the SO2 released is
reduced
Further reduction on SO2 emission by using flue gas
desulphurization (FGD) system
The FGD utilizes the “wet limestone-gymsum scrubbing process”
able to remove up to 90% of SO2 emitted
Flue gas from the boiler is directed to the absorber which contains
a recycled slurry of limestone for removal of SO2
Desulphurization of Flue Gas
*
SO2(g) + H2O(l) H2SO3(aq) Absorption
gypsum
*
46.5 Pollution Control in Hong Kong (SB p.261)
The treated flue gas leaves the absorber and directed into chimney
for dispersal into the atmosphere
The gypsum has a purity of over 90% which can be exported for
industrial application (e.g. raw material for making cement)
Flue gas desulphurization plant on Lamma Island
New Way Chemistry for Hong Kong A-Level Book 4
*
A schematic diagram of a flue gas desulphurization system
New Way Chemistry for Hong Kong A-Level Book 4
*
Coal contains certain proportion of ash that cannot be burnt
Coal-burning power plants generate tonnes of ash each day
electrostatic precipitators are equipped in power plants to remove
ash
Over 99% of fly ash is removed from flue gas
The ash collected is used for making construction materials (e.g.
cement) or for landfill purpose
Installation of Electrostatic Precipitators in Power Plants
New Way Chemistry for Hong Kong A-Level Book 4
*
Working principle of electrostatic precipitators:
1. The flue gas from boiler is passed through a chamber in the
electrostatic precipitator where particulates become charged
2. The charged particles get attracted on the collecting wall
3. The charges on the particles are neutralized on the wall where
they deposit and removed
4. The cleaned flue gas is discharged through chimney
46.5 Pollution Control in Hong Kong (SB p.262)
New Way Chemistry for Hong Kong A-Level Book 4
*
An electrostatic precipitator
*
46.5 Pollution Control in Hong Kong (SB p.262)
NOx include NO and NO2 which are formed during the combustion of
fuels in 2 ways:
1. N2 in the air reacts with O2 at high temperature near the
combustion chamber to form NO
N2(g) + O2(g) 2NO(g)
NO2 is formed when NO releases into the atmosphere and reacts with
O2
2NO(g) + O2(g) 2NO2(g)
2. Fossil fuels may contain nitrogen compounds. When these fuels
are burnt, the nitrogen compounds are oxidized to NOx
Installation of Low Nitrogen Oxide Burners in Power Plants
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.263)
Low nitrogen oxides burners utilizes a combustion control method in
which the operating conditions for combustion is modified to reduce
NOx formation
Reduction in flame temperature in the combustion zone and reduction
in availability of oxygen in the NOx formation zone can reduce the
production of NOx up to 50%
By controlling the emission of SO2 and NOx, the pollution caused by
burning of fossil fuels in power plants has been greatly
reduced
New Way Chemistry for Hong Kong A-Level Book 4
*
Answer
46.2 Air Pollution (SB p.265)
As less toxic lead compounds released when the running cars run on
unleaded petrol, air quality can be improved.
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.266)
In Hong Kong, all wastewater is discharged by one route or another
into the sea
At present, only part of the wastewater receives some sort of water
treatment before discharge
Sewage treatment is a series of processes to remove contaminants
from used water before it enters the waterways
Measures to Improve Water Quality
Treatment of Sewage
*
The sewage treatment plant in Shatin
New Way Chemistry for Hong Kong A-Level Book 4
*
Procedure:
1. The sewage is first passed through a series of screens that
remove large objects
2. Then it is passed through grit chambers to settle out grit
particles
3. Then it is sent to the sedimentation tanks where most of the
suspended solids settle out and are moved as “primary sludge”
Screening and Sedimentation (Primary Treatment)
New Way Chemistry for Hong Kong A-Level Book 4
*
After the primary treatment, about 60% of the solids are removed
and the BOD is reduced by 35%
The solids that accumulate can be purified and used at reclamation
sites
46.5 Pollution Control in Hong Kong (SB p.266)
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.266)
Sewage after primary treatment is passed into aeration tanks where
it is mixed with air and a mixture of micro-organisms (activated
sludge)
Micro-organisms digest the organic matter in this oxygen-rich
environment
Aeration is need to provide O2 to support the activities of
micro-organisms
The treated effluent is 90% free of suspended solids and the BOD is
reduced by 95%
Digestion of Pollutants by Micro-organisms (Secondary
Treatment)
New Way Chemistry for Hong Kong A-Level Book 4
*
The treated sewage is chlorinated to kill the micro-organisms
It is then discharged into the waterways or further treated to
remove some inorganic species (e.g. nitrate(V) and
phosphate(V))
The sludge removed by primary and secondary treatment is treated to
kill pathogens and to biodegrade more organic matter
The treated sludge is dried and disposed of into the sea or at
landfill site
New Way Chemistry for Hong Kong A-Level Book 4
*
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.267)
Rapid economic growth has resulted in the establishment of a
quickly expanding industrial sector. This led to the production of
vast amount of chemical waste
Major producers of chemical wastes in Hong Kong:
manufacturers of electroplating, electronic, electrical and
scientific products
Kinds of waste: Acids, alkalis, organic solvents, cyanides, heavy
metals and oily waste
These wastes are corrosive, flammable, poisonous and may give off
toxic or explosive vapours
Treatment of Chemical Waste from Industry and Labobatories
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.268)
Composition of chemical waste treated at the chemical waste
treatment centre in 2001
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.268)
In Hong Kong, the Chemical Waste Treatment Centre (CWTC) on Tsing
Yi Island provides proper waste treatment services for local waste
producers
There are 3 major waste treatment systems in the Centre:
1. Oil/water separation system
2. Physical/chemical treatment system
It treats water-based chemical waste such as acids, alkalis, and
wastewater containing heavy metal salts
3. Incineration
It used for destructing hazardous organic waste such as pesticides
and cyanide waste
New Way Chemistry for Hong Kong A-Level Book 4
*
46.5 Pollution Control in Hong Kong (SB p.269)
Physical methods include separation of waste oil from oily water
mixture in the oil/water separation system
It also separates water from all residue formed in neutralization
and precipitation processes
Physical Methods
*
1. Neutralization of acids and bases
Slaked lime (Ca(OH)2) is used to neutralize acidic waste, while
sulphuric(VI) acid is used to neutralize alkaline waste
H+(aq) + OH–(aq) H2O(l)
Chemical Methods
*
Precipitation reaction
alkalis are used to remove metals from a number of metal-containing
waste (e.g. waste solution of Fe, Zn, Cu and Ni)
e.g. Fe from spent FeCl3 etchant is removed by precipitation
Fe3+(aq) + 3OH–(aq) Fe(OH)3(s)
Spent NH4Cl & CuCl2 are treated with NaOH to recover the
marketable product, CuO
NH4+(aq) + OH–(aq) NH3(aq) + H2O(l)
Cu2+(aq) + 2OH–(aq) Cu(OH)2(s)
Cu(OH)2(s) CuO(s) + H2O(l)
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NH3 produced is treated with H2SO4 to form (NH4)2SO4 which is also
a marketable product
2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)
New Way Chemistry for Hong Kong A-Level Book 4
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Reduction reaction
Soluble Cr(VI) compounds are usually found in the effluent of
metal-finishing plants
Toxic Cr(VI) compounds are first reduced to non-toxic Cr(III)
compounds and then precipitated as hydroxide
Common reducing agents used: Na2SO3, SO2, FeSO4
2H2CrO4(aq) + 3Na2SO3(aq) + 3H2SO4(aq)
Cr2(SO4)3(aq) + 3Na2SO4(aq) + 5H2O(l)
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46.5 Pollution Control in Hong Kong (SB p.270)
Incineration destroys most of the toxic organic compounds,
hazardous medical waste and cyanide waste by decomposing them into
carbon dioxide and various gases
Contaminants in the flue gas are removed by scrubbers
Advantages:
Eliminating the problem of liquid waste disposal
Incineration
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A simplified flow diagram of the chemical waste treatment
process
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.273)
HKSAR government has been actively promoting the 3Rs concept
(Reduce, Reuse and Recycle) to arise the public environmental
awareness and promote waste avoidance, minimization, recovery for
reuse and recycling
Major kinds of recyclable waste recovered:
paper, plastics, ferrous metal and non-ferrous metal
Waste paper is recycled locally to produce corrugated cardbox and
paper board
Ferrous metal waste is processed in a local foundry to produce
reinforcing bars for use by the construction industry
Measures to Reduce Solid Waste
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.273)
The proportions of waste materials recovered in Hong Kong in
2000
New Way Chemistry for Hong Kong A-Level Book 4
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Government has done a lot on encouraging waste recycling,
e.g.
Provides recycling bins for plastic bottles, aluminium cans and
waste paper at around 200 public areas
Recycling bins for plastic bottles, aluminium cans and waste
paper
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.274)
Launched a 10-year Waste Reduction Framework Plan aiming to reduce
the amount of waste produced, to encourage waste recycling and
reuse, to promote greater efficiency and economy in the management
of waste, and to prolong the usable life of Hong Kong’s
landfills
Provides land for waste recycling industry
Hold various waste reduction campaigns with business sectors,
academics and green groups to increase the public awareness
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.274)
Programs such as the “Reusable Bag” campaign, the “Bring Your Own
Bad (BYOB)” campaign and the “Use Less Plastic Bags” campaign have
been launched to educate the public to minimize the use of plastic
bags
Co-operate with green groups to organize a series of programs to
enhance the awareness and participation of the public in using less
plastic bags
Reduce, Reuse or Recycling of Plastics
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.275)
The recycled paper produced by local recyclers is used to make
corrugated paper and duplex papercard for packaging
A variety of waste paper, including high-value waste paper (e.g.
white paper and computer printout), old corrugated cardboard,
newspaper and other mixed paper, are being exported to overseas
countries for recycling
Reuse and Recycling of Paper
New Way Chemistry for Hong Kong A-Level Book 4
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46.5 Pollution Control in Hong Kong (SB p.275)
Metal waste can be recycled by smelting used metals and using them
again
e.g.
(1) ferrous metal waste is processed in a local foundry to produce
reinforcing bars for use in the construction industry
(2) Al, Pb, Fe, Cu, Ag, Sn, Zn and Au can also be recycled
Reuse and Recycling of Metals
New Way Chemistry for Hong Kong A-Level Book 4
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Reasons:
(1) These mineral resources are limited but the demand is
increasing
(2) Pollution can be reduced and energy is saved
46.5 Pollution Control in Hong Kong (SB p.275)
Collecting aluminium cans
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