part 5 fossil fuels and carbon compounds
TRANSCRIPT
Part V Fossil Fuels and Carbon Compounds/P.1
Part V Fossil Fuels and Carbon Compounds
I. Fossil Fuels
Coal, Petroleum and Natural Gas are fossil fuels. They are so called because they were formed from
the remains of plants and animals that lived millions of year ago. All fossil fuels have one thing in
common – hydrocarbons (CxHy)
A. Origin of Fossil fuels
a. Coal
Plants Coal
Pressure (from overlying layers)heatbacterial actionmillions of year
b. Petroleum and Natural Gas 天然氣天然氣天然氣天然氣
Pressure (from overlying layers)heatbacterial actionmillions of year
Plants and
Marine animalsNatural Gas and Petroleum
Part V Fossil Fuels and Carbon Compounds/P.2
Petroleum (also called crude oil 原油原油原油原油 or just oil) is a complex mixture consisting mainly of hydrocarbons.
However, compounds containing sulphur, nitrogen and oxygen combined with carbon and hydrogen are also
found.
Natural gas is also a mixture mainly of hydrocarbons. It consists mainly of methane CH4 甲烷甲烷甲烷甲烷, small amount
of ethane C2H6 乙烷, propane C3H8 丙烷 and butane C4H10 丁烷.
B. Changing Petroleum into Useful Substances
a. Refining of Petroleum 石油的精煉石油的精煉石油的精煉石油的精煉
1. Petroleum (crude oil) is a mixture of hundreds of hydrocarbons. It is not suitable for direct use as a fuel and as
raw material in chemical manufacture. In oil refining, the complex mixture of hydrocarbons is separated into
less complex mixtures which are more useful.
2. Fractional distillation 分餾 can be used because the hydrocarbons have different boiling points. In general,
a hydrocarbon with larger molecules has a higher boiling point.
Part V Fossil Fuels and Carbon Compounds/P.3
3. Fractional distillation separates crude oil into several groups of hydrocarbons with different boiling point.
These groups or simpler mixtures are called fractions.
Fraction Number of carbon
atoms per molecule of
hydrocarbons
Boiling point
range (oC)
Uses
Refinery gases 煉油氣 1 – 4 below 40
as gaseous fuel; raw materials for
manufacturing chemicals*
Petrol & Naphtha 汽油及石腦油 5 – 10 40 – 170
as fuel for cars; manufacturing town gas;
raw materials for manufacturing
chemicals*
Kerosene 煤油(火水) 10 – 14 170 – 250
as fuel for aircraft; domestic fuel
Gas oil (diesel oil) 氣油(柴油) 14 – 25 250 – 350
as fuel for heavy vehicles and factories
Fuel oil
above 25 over 350
as fuel for ships and power stations
Lubricating oil,
wax and bitumen 瀝青
as lubricating oil for machines; making
candles; surfacing roads and roofs
An oil fraction consisting of hydrocarbon molecules with more carbon atoms has a higher boiling point range.
* Petroleum fractions are used as raw materials to produce different chemicals in the petrochemical
industry. These chemicals can be made into many useful products, such as alcohols, plastics, detergents,
food additives, cosmetics etc.
Part V Fossil Fuels and Carbon Compounds/P.4
b. Fractional distillation of crude oil in laboratory
Fraction
Properties 1 2 3 4
Boiling point range Room temperature
to 100cC
100 – 150oC 150 – 200
oC 200 – 250
oC
Volatility (Ease of
evaporation) Evaporates quickly
Evaporates slowly
Colour Colourless Very pale yellow Yellow Brown
Viscosity黏(滯)度 Non-viscous
(flows easily)
Fairly viscous
Flammability Very easy to burn
Difficult to burn
Colour and
sootiness of flame
Yellow with blue
edges; non-sooty
Yellow / orange;
Slightly sooty Orange; sooty Orange; very sooty
The above results indicate that:
Fraction with a lower boiling point range Fraction with a higher boiling point range
lighter in colour darker in colour
less viscous more viscous
easier to evaporate (or more volatile) more difficult to evaporate (or less volatile)
more flammable less flammable
burns with a cleaner flame burns with a sootier flame
Part V Fossil Fuels and Carbon Compounds/P.5
Classwork
The diagram below shows a tower used to separate petroleum into fractions.
a. Name the process that is used to separate the petroleum into fractions.
b. Name a product at each of the outlets A, B, C and D.
c. Suggest one use of each of the products stated in (b).
d. Apart from the difference in boiling points, state five other properties in which you would expect
products at the different outlets to differ from one another.
e. Give one reason to account for the differences in properties of products at the different outlets.
f. Draw a labeled diagram to show how you could obtain similar products in the laboratory on a
test-tube scale.
(HKCEE 1992)
C. Using petroleum and natural gas
a. Use of petroleum
Refined petroleum has three main uses:
(i) As fuels
An energy source for heating, electricity and transportation. At present, petroleum supplies about
37.5% of the world’s energy needs.
(ii) As lubricants
(iii) As a source of hydrocarbons to manufacture other useful chemicals.
b. Uses of Natural gas
Unlike petroleum, most natural gas is burnt directly to produce energy. The rest is used to produce useful
chemicals, Natural gas burns with a clean blue flame, causing little pollution.
c. Petroleum resource is running out
Petroleum resource is limited and non-renewable. Most of it would run out within 60 years.
Part V Fossil Fuels and Carbon Compounds/P.6
II. Consequences of using Fossil Fuels
A. Burning of fuels
1. There is usually a temperature change when a chemical reaction occurs.
An exothermic reaction is one that gives out heat.
An endothermic reaction is one that takes in heat.
(i) Examples of exothermic reactions:
1. Combustion reactions e.g. C(s) + O2(g) → CO2(g)
2. Precipitation reactions e.g. Ag+(aq) + Cl-(aq) → AgCl(s)
3. Displacement reactions e.g. Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
4. Acid-alkali neutralizations.
(ii) Examples of endothermic reactions:
1. Cracking of oil fractions.
2. Thermal decomposition of calcium carbonate.
CaCO3(s) → CaO(s) + CO2(g)
2. ∆∆∆∆H Notation and Energy Level Diagram
(i) The total energy stored in a substance is called the heat content (symbol H) of the substance.
The heat change (∆∆∆∆H) during a reaction is the difference between the total heat content of products
(Hp) and that of reactants (Hr).
∆∆∆∆H = Hp - Hr
Heat change is measured in kilojoules (kJ mol-1
).
Part V Fossil Fuels and Carbon Compounds/P.7
(ii) Exothermic reactions
During an exothermic reaction, the temperature of products rises above initial temperature. The heat
energy produced is lost to the surroundings, the ∆H value is negative.
(iii) Endothermic reaction
In an endothermic reactions, the temperature of products falls below the initial temperature. The
system gains energy from the surroundings and the ∆H value is positive.
3. Complete and incomplete combustion
(i) A hydrocarbon (CxHy), when burnt completely in plenty of air, forms carbon dioxide and water as the
only products. Very little soot (unburnt carbon particles) is produced. The flame us thus blue, with a
high temperature.
CxHy + (x + y/4) O2(g) → xCO2(g) + y/2 H2O(l)
(ii) If oxygen supply is poor, the combustion of hydrocarbons would be incomplete. The flame
temperature is thus lower. Carbon monoxide and carbon are formed at the same time. Since a lot of
soot is produced, the flame is yellow or orange and black smoke can be seen.
Part V Fossil Fuels and Carbon Compounds/P.8
4. Dangers associated with use of household fuels
a. Carbon monoxide poisoning
1. Sources of carbon monoxide
(i) Town gas contains a mixture of gases including hydrogen (49%), carbon monoxide (3%), methane
(28.5%) and carbon dioxide (19.5%). If there is a gas leakage, carbon monoxide will diffuse into the
air.
(ii) Liquefied petroleum gas (LPG) contains mainly propane and butane liquefied under pressure.
(iii) Carbon monoxide (CO) is produced if the fossil fuels are burnt incompletely.
2. Effect of carbon monoxide
Carbon monoxide is a highly dangerous gas because it is toxic, yet colourless and odourless.
Carbon monoxide combines readily with haemoglobin in blood to form a stable cherry red
compound called carboxyhaemoglobin.
The normal function of haemoglobin to carry oxygen from lungs to other body tissues would thus
be hindered.
b. Fire and explosion
A mixture of flammable gases and air can be a source of danger. A small flame or spark may ignite the mixture,
causing a fire or even an explosion.
Part V Fossil Fuels and Carbon Compounds/P.9
c. Precautions in using household fuels
1. Ensuring that gas-burning appliances are installed and regularly checked by a qualified technician.
2. If you smell gas or suspect of a leak, you must
� turn off the main gas valve
� extinguish all flames nearby
� open windows and door wide
� do not operate any electrical switches or appliances
� do not use a telephone or mobile phone in your home
� do not press the doorbell of an adjacent flat
� inform the gas company or Fire Services Department if necessary
3. Ensure there is an adequate supply of fresh air for gas-burning appliances. Otherwise, carbon monoxide
will be produced as a result of incomplete combustion.
Part V Fossil Fuels and Carbon Compounds/P.10
B. Environmental problems associated with fossil fuels
1. Major air pollutants from cars, factories, incinerators and power stations
In a city, air pollution is mainly caused by motor vehicles and industrial machinery. Both motor vehicles and
industrial machinery require the burning of fuel to produce the energy needed. The major problems associated
with the burning of fuels are:
(i) Incomplete combustion
(ii) Presence of impurities
a. Carbon monoxide (CO)
(i) Carbon monoxide is produced whenever a hydrocarbon fuel is burned incompletely. Carbon
monoxide is colourless, odourless and very poisonous gas.
(ii) In low concentration, CO makes a person feel dizzy, headache and irritable.
In high concentration, it will cause unconsciousness and even death.
(iii) Some people died in stopped private cars when enjoying air-conditioning due to inhaling the carbon
monoxide formed.
b. Unburnt Hydrocarbons
Car exhausts contain a mixture of unconsumed hydrocarbons. Some hydrocarbons such as benzene may cause
cancer. Hydrocarbons are also one of the main causes of the formation of photochemical smog 光化學煙霧光化學煙霧光化學煙霧光化學煙霧.
c. Suspended particulates / Dark Smoke
(i) Incomplete burning of hydrocarbon produces dark smoke which contains mainly carbon particles.
(ii) The suspended carbon particles may enter the lung and cause serious lung diseases such as
tuberculosis and lung cancer.
(iii) The dark smoke in air also causes the reduction of visibility and solar radiation. It is also related to
the formation of photochemical smog.
Part V Fossil Fuels and Carbon Compounds/P.11
d. Nitrogen Oxides NOx
(i) Nitrogen oxides are formed during the burning of fuel in car engines and power station furnaces.
High temperature causes the chemical combination of oxygen and nitrogen in air.
(ii) Nitrogen oxides NOx (e.g. nitrogen monoxide NO, nitrogen dioxide NO2) are poisonous. They irritate
and attack the respiratory tracts and the lung.
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)
nitrous acid nitric acid
(iii) Nitrogen oxides together with hydrocarbon and smoke produce photochemical smog. Photochemical
smog occurs as a brown haze and causes reduced visibility, eye and bronchial irritation, and the damage
to plants and animals.
e. Lead Compounds #
(i) To increase the efficiency of burning, oil companies have added a lead compound, tetraethyl lead
(TEL), to the petrol used by motor vehicles.
(ii) The TEL may react with oxygen in the air to form lead compounds. They may enter the lung of animals
and men and cause serious lung diseases.
(iii) Lead compound are cumulative poisons. They stay and accumulate in the body. They have harmful
effects on red blood cells and brain cells. Lead compounds also have neuropsychological effects.
Children are more easily affected. Lead is also associated with heart attacks, strokes and
hypertension.
f. Sulphur Dioxide
(i) Factories and power stations burn either coal or low-grade petroleum, which both contain sulphur.
The smoke may contain sulphur dioxide SO2.
(ii) Incinerators burn a lot of rubbish, much of it being paper containing sulphur compounds.
(iii) The effects of sulphur dioxide are similar to nitrogen oxides. It irritates respiratory tracts and reduces
the normal functions of the lung. In high concentration, it may cause cancer and death. It is also another
cause of acid rain.
SO2(g) + H2O(l) → H2SO3(aq) sulphurous acid
Part V Fossil Fuels and Carbon Compounds/P.12
2. Acid Rain
a. What causes acid rain?
(i) Normally, rainwater's pH value is about 5.6. It is slightly acidic because carbon dioxide in the air
reacts with rainwater to form carbonic acid.
CO2(g) + H2O(l) → H2CO3(aq)
(ii) Air pollutants such as oxides of sulphur and nitrogen emitted from power plants, various factories
and motor cars react with rainwater to form acids that lower the pH value of rainwater. This gives
rise to acid rain.
SO2(aq) + H2O(l) → H2SO3(aq)
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)
b. Environmental problems associated with Acid Rain
1. Acid rain can damage to plants, including crops and forests.
2. Water in many rivers and lakes has become acidic due to acid rain. This results mainly from the inflow of
acidic water containing poisonous metal ions. Fish and water plants cannot survive in acidic water.
3. Acid rain has bad effects on common building materials: limestone, marble, sandstone, cement and concrete.
All these materials contain calcium carbonate, to a greater or lesser extent.
CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O(l)
Acid rain has caused great damage to many status and monuments.
Metals are also attacked by acid rain. Metal objects corrode faster when rain water is more acidic.
Part V Fossil Fuels and Carbon Compounds/P.13
3. The Global Greenhouse Effect 地球溫室效應地球溫室效應地球溫室效應地球溫室效應
a. Greenhouse and Greenhouse Effect
(i) In a greenhouse on a sunny day, sunlight penetrates through the glass. The heated plants and things
inside the greenhouse give out infrared radiation. Most of this radiation , when striking on the glass, is
reflected back into the greenhouse, which is thus kept warm.
(ii) Energy from the sun falls on the Earth. The solar energy is either absorbed by the Earth or reflected
back into space. About half of the solar energy is absorbed, warming the atmosphere and the Earth's
surface.
(iii) The Earth's surface re-radiates most of the absorbed energy (mainly as infrared radiation).
*Carbon dioxide, water vapour and a few other gases (methane, chlorofluorocarbons CFCs, nitrogen
oxides and ozone) absorb some of this infrared energy and hold it back. They act as the glass in a
greenhouse.
*They are also called as greenhouse gases.
Part V Fossil Fuels and Carbon Compounds/P.14
b. Global Warming 全球增溫全球增溫全球增溫全球增溫 Due to enhanced Greenhouse Effect
(i) The greenhouse effect would be constant if the greenhouse gases remained in their normal
concentrations.
(ii) However, the natural balance is being disturbed by a rapid increase of carbon dioxide concentration
through increased burning of fossil fuels. This results in enhanced greenhouse effect, causing a rise in
the Earth's surface temperature. The increase in temperature due to enhanced greenhouse effect is
known as global warming.
(iii) The global warming will melt many of ice caps at the North Pole and South Pole. Average sea level
will rise, causing disastrous flooding in low-lying coastal areas.
(iv) The Earth's climate (e.g. rainfall) will possibly change. Some regions will suffer drought and crops
will fail. Other regions will have more frequent storms and flooding. Storms and floods will cause
economic loss.
Part V Fossil Fuels and Carbon Compounds/P.15
4. Methods of reducing air pollution
a. Cutting Down Pollutants From Motor Car
(i) By using Unleaded Petrol 無鉛汽油無鉛汽油無鉛汽油無鉛汽油 in motor cars, lead emission into the air can be greatly
reduced.
The Hong Kong government has introduced unleaded petrol since 1991. In 1999, the government
banned the sale of leaded petrol.
Internet Search: The greenhouse effect and global warming
Search Hint
1. How does carbon dioxide cause the greenhouse effect?
2. What is global warming?
3. What are the consequences if the temperature of the Earth’s surface rises?
4. What measures have been taken to tackle the problem?
Reference websites
1. An education website of global warming
http://www.globalwarming.org/brief/student.htm
2. U.S. Environmental Protection Agency: Global Warming’s Homepage
http://www.epa.gov/gobalwarming
3. Commonwealth Scientific and Industrial Research Organization
http://www.dar.csiro.au/information/greenhouse.html
Part V Fossil Fuels and Carbon Compounds/P.16
(ii) Use of Catalytic Converter 催化轉化器催化轉化器催化轉化器催化轉化器 on the exhaust pipe of a motor car.
1. The catalytic converter is a stainless steel cylinder containing a honeycomb structure coated
with a catalyst (usually platinum).
2. In the first half of the catalytic converter,
2CO(g) + 2NO(g) platinum
→ 2CO2(g) + N2(g)
poisonous gases harmless gases
3. In the second half of the converter, hydrocarbons and any remaining carbon monoxide are
oxidized to carbon dioxide and water.
2CO(g) + O2(g) platinum
→ 2CO2(g)
2C8H18(l) + 25O2(g) platinum
→ 16CO2(g) + 18H2O(l)
4. The catalytic converter can work efficiently only on unleaded petrol. This is because the catalyst
is easily 'poisoned' (made ineffective) by lead.
Part V Fossil Fuels and Carbon Compounds/P.17
b. Cutting Down Pollutants From Industry
(i) Minimizing sulphur dioxide emission
Sulphur dioxide emission can be minimized by burning fuels of low sulphur content.
(ii) Use of Scrubbers 滌氣器滌氣器滌氣器滌氣器 to take away the sulphur dioxide from the waste gases after burning of coal.
In the scrubbers, the waste gases are sprayed by jets of limewater before they reach the chimneys. The
limewater dissolves soluble gases (mainly sulphur dioxide) and washes away smoke and dust.
Dry scrubbing: CaCO3(s) → CaO(s) + CO2(g)
CaO(s) + SO2(g) → CaSO3(s)
Wet scrubbing: CaO(s) + H2O(l) → Ca(OH)2(aq)
Ca(OH)2(aq) + SO2(g) → CaSO3(s) + H2O(l)
These products are washed away as a slurry 淤漿 – a mixture of solids and water.
Part V Fossil Fuels and Carbon Compounds/P.18
(iii) Removing particulates
Particulates from waste gases can be removed by using Electrostatic Precipitator 靜電沉積器靜電沉積器靜電沉積器靜電沉積器. The
gases are passed through a strong electric field where particulates become negatively charged. The
charged particulates are collected on positively charged plates.
5. The role of the government in controlling air pollution
a. Legislation
The Environmental Protection Department (EDP) ensures the implementation of air pollution control laws
e.g. taxi operators are encouraged to replace diesel taxis with those operated on LPG.
b. Monitoring and Investigation
The EDP has set up monitoring stations to monitor concentration of air pollutants throughout Hong Kong.
c. Planning
The government has to make sure that possible environmental problem are considered during the planning
stage of future developments.
Part V Fossil Fuels and Carbon Compounds/P.19
Classwork
1. For environmental reasons, the Hong Kong government has launched a plan for taxis to switch from using
diesel to using LPG.
a. Both LPG and diesel are petroleum products. State the origin of petroleum.
b. With reference to their chemical constituents, explain why LPG is a cleaner fuel than diesel.
c. State one problem that may occur in the initial stage in launching this plan.
(HKCEE 2001)
2. Carbon dioxide constitutes about 0.03% of the atmosphere. Over millions of years, the concentration of carbon
dioxide in the atmosphere has remained almost constant because of a number of processes.
a. Suggest one process by which carbon dioxide is added to the atmosphere.
b. Suggest one process by which carbon dioxide in the atmosphere is consumed.
c. Carbon dioxide is one of the greenhouse gases in the atmosphere.
(i) Explain why carbon dioxide can cause the greenhouse effect.
(ii) State the importance of the greenhouse gases in the atmosphere to living things on Earth.
(iii) Increasing the concentration of the greenhouse gases in the atmosphere leads to global warming.
State one harmful effect of global warming.
(HKCEE 2000)
3. The illustration below shows the exhaust from a motor car using unleaded petrol:
a. Explain why the exhaust contains carbon monoxide
b. (i) Write two chemical equations for the formation of acid rain from nitrogen oxides.
(ii) State one undesirable effect of acid rain.
c. State one health hazard associated with particulates.
d. Suggest one other pollutant that may be found in the exhaust.
e. Suggest a device that can be installed in the motor car to reduce the emission of carbon monoxide
and nitrogen oxides.
(HKCEE 1999)
Part V Fossil Fuels and Carbon Compounds/P.20
III. Energy Crisis and Alternative Sources of Energy
A. Renewable and non-renewable energy sources
Fossil fuels are examples of non-renewable energy sources. Once used, they are gone forever – we cannot
wait for hundreds of millions years for them to form again.
On the other hand, solar power, hydroelectric power, tidal power, wind power, geothermal power and
power from biomass are never used up. These are renewable energy sources. In general, renewable energy
sources cause fewer environmental problems.
B. Energy crisis
Fossil fuels are being used up rapidly. It has been estimated that the known reserves for oil and natural gas will
all be used up by the year 2070. Coal deposits are more plentiful, but they are expected to last for less than 200
years.
C. Alternative Energy Sources
a. Nuclear Power
(i) In a nuclear reaction, uranium 鈾鈾鈾鈾 is used as the 'fuel' and to release energy by nuclear fission.
(ii) When a neutron collides with a uranium-235 nucleus, it causes the nucleus to split into two smaller
nuclei.
During the splitting, heat energy and more neutrons are released. These neutrons collide with other
uranium nuclei and start a chain reaction. The heat energy is used to heat water and produce steam,
which turns turbines and thus generates electricity.
Part V Fossil Fuels and Carbon Compounds/P.21
(iii) Limitations:
1. Reaction safety is a concern. Nuclear reactors produce powerful radiation which can kill in large
doses. Even though nuclear power plants are designed to contain radiation, accidents still have
occurred.
2. Nuclear reactors produce lots of radioactive waste. There is no perfect way to dispose these wastes
since they remain dangerous for thousands of years.
The Daya Bay Nuclear Power Station (near Hong Kong) was started to operate in 1993.
Part V Fossil Fuels and Carbon Compounds/P.22
b. Solar Power
(i) Solar energy is the radiant heat and light energy given out by the Sun.
(ii) Solar energy is unlimited and costs nothing. However, it is not cheap to trap and use solar energy at
present.
(iii) Limitations:
1. Solar panels are expensive to build
2. Most of the solar energy is collected during the summer. However, energy is needed to a greater
extent during winter. We need to develop efficient ways of storing solar energy until it is needed.
Part V Fossil Fuels and Carbon Compounds/P.23
c. Hydroelectric Power
(i) The potential energy from falling water can be changed to kinetic energy and is used to drive
turbines which generate electricity.
(ii) Countries with heavy rainfall and mountainous ground are ideal for hydroelectric power.
Once the hydroelectric station is built, the cost of electricity can be fairly cheap.
(iii) Limitations:
1. Building a huge dam is very expensive.
2. It may mean flooding a pretty, populated valley with water.
3. Wildlife habitats and farming areas may also be affected.
Part V Fossil Fuels and Carbon Compounds/P.24
d. Tidal Power
(i) A dam is built across a bay where high tide and low tide vary by more than 10 meters. When the
gates are open and the tide comes in, seawater fills the reservoir behind the dam. Every time the tide
comes in or goes out, the turbines turn and generate electricity.
(ii) Limitations:
1. The dam, turbines and generators are expensive to build.
2. The generator can only generate electricity twice a day.
3. Interfering with the tides may affect the ecology of the area.
4. There are not many bays with such a large tidal range.
e. Wind Power
(i) Many countries, such as USA, Sweden and Denmark, have large windmills. As the wind pushes the
blades around, the generator spins to generate electricity. Wind energy is a clean and renewable energy.
(ii) Limitations:
1. Wind does not always blow. We need alternative supplies for “still” days.
2. If the wind blows too hard, windmills may be severely damaged.
3. Thousands of windmills are needed to provide enough electricity.
Part V Fossil Fuels and Carbon Compounds/P.25
f. Geothermal Power
(i) Heat energy which comes from deep within the Earth is called geothermal energy.
(ii) Geothermal power stations are found in some countries like New Zealand, USA and Japan. In Iceland,
many building and even swimming pools are heated with geothermal energy.
(iii) Limitations:
1. Geothermal energy must be found near the Earth’s surface. Drilling deep wells is expensive and
causes pollution.
2. Geothermal wells release hydrogen sulphide and sulphur dioxide gases, which are poisonous.
3. The water may contain toxic substances. A geothermal power plant has to plan for safe disposal of
cooled wastewater.
g. Power from Biomass
(i) Biomass means the organic matter (plant and animal materials) and waste substances that come from
them., which can be changed by biotechnology into more useful and valuable fuels.
For example, alcohol can be produced from fermentation of sugar obtained from sugar cane. Many
plants produce vegetable oils. Alcohol and vegetable oils are fuels.
(ii) Plants give out biogas (about 65% methane) when they rot in the absence of air. The methane can be
collected-it is a good fuel.
Part V Fossil Fuels and Carbon Compounds/P.26
(iii) Limitations:
It reduces the amount of manure and crop residues which can be used as fertilizers.
Classwork
“Fossil fuels” such as petroleum and coal constitute the world’s major source of energy. However, many countries
have been developing alternative energy sources.
a. Why are petroleum and coal called “fossil fuels”?
b. Give two reasons why it is necessary to develop alternative energy sources.
c. Nuclear power is used as an alternative to fossil fuels in many countries. Suggest one advantage and one
disadvantage of using nuclear power.
d. Suggest one energy source, other than nuclear power, that can be used as an alternative to fossil fuels.
(HKCEE)
Part V Fossil Fuels and Carbon Compounds/P.27
IV. Introducing Organic Chemistry
� The hydrocarbons in fossil fuels are examples of organic compounds. The protein, carbohydrates and
lipids present in our bodies, the fuels we burn and many things we use (e.g. plastics, detergents) are all
organic compounds. In fact, organic compounds are carbon compounds.
Then, organic chemistry is the study of carbon compounds.
� Carbon forms a very large number of compounds (over 4 000 000). This is far more than the number of
compounds of all other elements put together (less than 100 000).
Carbon can form so many compounds because:
1. Carbon atoms can join with other carbon atoms to form chains and rings.
2. Carbon atoms can form multiple bonds (double bond and triple bond).
3. Carbon atoms can combine with many other elements such as hydrogen, oxygen, chlorine,
nitrogen, sulphur, and even metals.
Part V Fossil Fuels and Carbon Compounds/P.28
A. Structural Formula
1. A molecular formula does not show the structure of a molecule very well but structural formula can show how
the atoms are joined to one another in the molecule.
2. A structural formula is only a two-dimensional representation of an actually three-dimensional molecule. It
does not show the actual molecular shape, e.g. butane C4H10
Ball and stick model Space filling model
Part V Fossil Fuels and Carbon Compounds/P.29
3. We may also write the structural formula of a compound in a condensed form.
In this form, single bonds are omitted (except those joining anything other than hydrogen atoms to the main
carbon chain). However, carbon-carbon multiple bonds (that is C=C or C≡C) must be written.
Part V Fossil Fuels and Carbon Compounds/P.30
Classwork
Write condensed structural formulae for the following compounds:
Part V Fossil Fuels and Carbon Compounds/P.31
B. Saturated Hydrocarbons and Unsaturated Hydrocarbons
A hydrocarbon in which all the carbon atoms are connected to each other by single bonds is called a saturated
hydrocarbon.
A hydrocarbon that has one or more double (C=C) or triple bonds (C≡C) between the carbon atoms is called an
unsaturated hydrocarbon.
CH3CH
2CH
2CH
3 CH
3CH=CHCH
3
a saturatedhydrocarbon
an unsaturated hydrocarbon
C. Homologous Series
1. We can see that the four hydrocarbons present in natural gas are in fact related. Each one of them differs from
the next be a –CH2- group. Their molecular formulae can all be summarized by the same general formula
CnH2n+2 (“n” represents the number of carbon atoms). We say that these hydrocarbons belong to the same
homologous series.
2. A homologous series is a family of compounds all having the same general formula and with members
differing from the next by a –CH2- unit.
Part V Fossil Fuels and Carbon Compounds/P.32
D. Functional Group 官能基官能基官能基官能基
1. Ethane (CH3CH3) and ethanol (CH3CH2OH) have similar molecular structures except one hydrogen atom in
ethane is replaced by a hydroxyl group (–O–H) in ethanol. However, they have very different properties.
CH
H
H
C
H
H
H CH
H
H
C
H
H
O H
ethane ethanol
Property Ethane Ethanol
State at room temperature and pressure Gas Liquid
Reaction with sodium No reaction Reacts to give hydrogen gas
2. The hydroxyl group (-OH) in ethanol modifies the properties of the ethane skeleton. The hydroxyl group is an
example of a functional group.
3. A Function Group is an atom, or a group of atoms, which determines most of the properties of an
organic compound.
4. Some common functional groups:
(i) C=C double in Alkene
(ii) hydroxyl group (-OH) in Alkanol
(iii) carboxyl group (-COOH) in Alkanoic acid
Part V Fossil Fuels and Carbon Compounds/P.33
E. Naming of Organic Compounds
Organic compounds are usually named systematically by IUPAC system of naming.
(IUPAC stands for International Union of Pure and Applied Chemistry)
a. Naming of Alkanes by IUPAC System
1. Naming of Straight-chain Alkanes
(i) To name straight-chain alkanes, use a prefix to show how many carbon atoms are in the straight chain,
followed by the suffix –ane.
Number of carbon
atoms
Condensed structural formula
of alkane
Prefix of alkane Name of alkane
1 CH4 meth- methane
2 CH3CH3 eth- ethane
3 CH3CH2CH3 prop- propane
4 CH3(CH2)2CH3 but- butane
5 CH3(CH2)3CH3 pent- pentane
6 CH3(CH2)4CH3 hex- hexane
7 CH3(CH2)5CH3 hept- heptane
8 CH3(CH2)6CH3 oct- octane
9 CH3(CH2)7CH3 non- nonane
10 CH3(CH2)8CH3 dec- decane
(ii) Alkyl groups
Alkyl groups are derived from alkanes by removal of a hydrogen atom. They are often represented by the
symbol R-. They are named by replacing the suffix –ane of the parent alkanes by –yl.
Alkane CnH2n+2 Alkyl group CnH2n+1−−−− (or R−−−−)
CH4 methane CH3− methyl
CH3CH3 ethane CH3CH2− ethyl
CH3CH2CH3 propane CH3CH2CH2− propyl
Classwork
a. Name the following alkyl groups:
(i) CH3CH2CH2CH2- (ii) CH3(CH2)4CH2-
b. Write the condensed structural formulae for the following alkyl groups:
(i) pentyl (ii) heptyl
Part V Fossil Fuels and Carbon Compounds/P.34
2. Naming of Branched-chain alkanes
Branched-chain alkanes are named by considering them as straight-chain alkanes with hydrogen atoms
replaced by other atoms or groups (substituents). The substituents here are alkyl groups. Thus the IUPAC name
for a branched-chain alkane consists of two parts.
(i) the prefixes which indicate the alkyl group substituents
(ii) the “root”, which indicates the parent hydrocarbons
The IUPAC rules of naming are illustrated below:
a. Step 1: Find the longest continuous carbon chain in the compound
The name of this main carbon chain is the “root” of the name.
In the example here, the longest chain has six carbon atoms. The root is therefore “hexane”.
b. Step 2: Recognize the substituents
Number the carbon atoms (1, 2, 3 onwards) in the chosen main chain, starting from the end such that the
smallest value is given to the lowest numbered substituent.
In the example here,
In direction (i), the lowest numbered substituent is attached to C-3, whereas in direction (ii), it is C-2.
Hence direction (ii) is chosen.
Part V Fossil Fuels and Carbon Compounds/P.35
c. Step 3: Name each substituent and state the number of the carbon atom to which the substituent is
attached.
The number is placed in front of the name, followed by a hyphen.
In the example, they are named as 2-methyl, 2-methyl, 3-ethyl and 4-methyl
If several substituents are the same, they should be grouped together and named with a multiplying prefix.
Number of substituents Multiplying prefix
2 di-
3 tri-
4 tetra-
5 penta-
All the numbers of the attaching carbon atoms have to be written down, separated by commas.
In the example, three methyl groups are grouped together and named as 2,2,4-trimethyl.
d. Step 4: Arranged the substituents
The substituents should be arranged in alphabetical order, separated by hyphens. Then join them as
prefixes to the root.
Note: The multiplying prefix does not count in the alphabetical order.
Part V Fossil Fuels and Carbon Compounds/P.36
Classwork
1. Give the IUPAC names of the following compounds:
a. b.
2. Write the structural formulae of the following compounds:
a. 2,2,3-trimethylpentane b. 3,3-diethyl-2-methylhexane
3. Naming of halogen-substituted alkanes
The IUPAC rules for naming alkanes described above can be applied to halogeno-substituted alkanes. The
substituents fluoro (F-), chloro (Cl-), bromo (Br -) and iodo (I-) are also named as prefixes.
Classwork
1. Name the following compounds by IUPAC system:
a. b.
2. Write the structural formulae of the following compounds:
a. 1,1,1-trichloroethane b. 2,3-dibromo-1-florobutane
Part V Fossil Fuels and Carbon Compounds/P.37
b. Naming of Alkenes
Alkenes have the general formula CnH2n. They are named using the same general rules as those described for
alkanes, but using the suffix –ene instead of –ane. Thus ethene and propene are the names of the first two
members in alkenes series.
Alkene Molecular formula Condensed structural formula
ethene C2H4 CH2=CH2
propene C3H6 CH3CH=CH2
but-1-ene C4H8 CH3CH2CH=CH2
pent-1-ene C5H10 CH3CH2CH2CH=CH2
hex-1-ene C6H12 CH3CH2CH2CH2CH=CH2
For an alkene having four or more carbon atoms in the basic chain, a number is included in the name to
indicate position of the double bond.
e.g.
The name for the compound is given below.
Part V Fossil Fuels and Carbon Compounds/P.38
Classwork
1. Name the following compounds by IUPAC system:
a. b.
2. Write the structural formulae of the following compounds:
a. 2-methylbut-2-ene b. 2,3-dichlorobut-1-ene
Part V Fossil Fuels and Carbon Compounds/P.39
c. Naming of Alkanols
Alkanols have the general formula CnH2n+1OH, where n = 1, 2, 3, 4 etc
In naming an alkanol, the longest continuous carbon chain containing the hydroxyl group –OH is chosen. The
final "-e” of the corresponding alkane name is changed to “–ol”, e.g. methanol, ethanol etc.
Alkanol Condensed structural formula Corresponding alkane
methanol CH3OH methane CH4
ethanol CH3CH2OH ethane CH3CH3
propan-1-ol CH3CH2CH2OH propane CH3CH2CH3
butan-1-ol CH3CH2CH2CH2OH butane CH3CH2CH2CH3
For alkanols of three or more carbon atoms, a number has to be added before the suffix “-ol” to indicate the
position of the –OH group. e.g propan-1-ol, propan-2-ol, butan-1-ol etc
Names of some alkanols are given below:
Classwork
Name the following compounds by IUPAC system:
Part V Fossil Fuels and Carbon Compounds/P.40
d. Naming of Alkanoic Acids
Alkanoic acids have the general formula RCOOH, where R- is an alkyl group or hydrogen.
In naming an alkanoic acid, the longest continuous carbon chain containing the carboxyl group –COOH is
chosen. The final “-e” of the corresponding alkane name is changed to “-oic acid”.
Alkanoic acid Condensed structural formula Corresponding alkane
methanoic acid HCOOH methane CH4
ethanoic acid CH3COOH ethane CH3CH3
propanoic acid CH3CH2COOH propane CH3CH2CH3
butanoic acid CH3CH2CH2COOH butane CH3CH2CH2CH3
Names of some alkanoic acids are given below:
Classwork
1. Name the following compound by IUPAC system:
2. Write the structural formula for
a. 2-methylpropan-1-ol b. 2,2-dichlorobutanoic acid
Part V Fossil Fuels and Carbon Compounds/P.41
E. Structural Isomerism 結構同分異構結構同分異構結構同分異構結構同分異構
1. Structural Isomerism is the existence of two or more compounds with the same molecular formula but
different structures. The different compounds are Structural Isomers結構同分異構體結構同分異構體結構同分異構體結構同分異構體, which are said to be
isomeric with each other.
2. For example, butane and 2-methylpropane are different compounds having the same molecular formula
C4H10
Classwork
Give the structural formulae and names for all the structural isomers of
a. C3H7OH
b. C4H9Cl
Part V Fossil Fuels and Carbon Compounds/P.42
V. Alkanes 烷烴烷烴烷烴烷烴
Petroleum and natural gas contain hydrocarbons, most of which are alkanes. Alkanes are saturated
hydrocarbons with the general formula CnH2n+2.
A. Physical Properties of Alkanes
Name Molecular Formula Melting Point (oC) Boiling Point (
oC) State at room
temperature and
pressure
Methane CH4 -182 -162
gas Ethane C2H4 -183 -89
Propane C3H8 -190 -42
Butane C4H10 -138 -0.5
Pentane C5H12 -130 36
liquid Hexane C6H14 -95 69
� � � �
Heptadecane C17H36 22 292
Octadecane C18H38 28 308 solid
Nondecane C19H40 32 320
1. There is a gradual change of physical properties in the series.
The melting point, boiling point density and viscosity 黏度 increase with increasing molecular size (due to
greater van der Waals' forces).
2. The first four members of the series are colourless gases at room conditions. The C5 to C17 alkanes are
colourless oily liquids, while higher members are waxy solids.
3. Alkanes are insoluble in water. All liquid alkanes have density less than 1 g cm-3 and thus float on water. On
the other hand, alkanes are soluble in many non-aqueous solvents such as methylbenzene and
tetrachloromethane.
Part V Fossil Fuels and Carbon Compounds/P.43
B. Chemical Properties of Alkanes
All alkanes have similar chemical properties because of their similar structures.
Alkanes are saturated hydrocarbons. They show little reaction towards common chemical reagents. For
example, they do not react with acids, alkalis, dehydrating agents (H2SO4), oxidizing agents (e.g. KMnO4) or
reducing agents (e.g. Na, SO2).
a. Combustion
(i) In a good supply of oxygen, alkanes undergo complete combustion to give carbon dioxide and water,
and give out much heat.
The general equation for the complete combustion of alkanes (or other hydrocarbons) is:
C H xy
O xCOy
H Ox y + + → +( )4 2
2 2 2
(ii) If the oxygen supply is limited, incomplete combustion occurs. Consequently, the alkanes burn with a
yellow flame and produce soot. Carbon monoxide, carbon and water are produced.
(iii) Under ordinary conditions, complete combustion seldom takes place. In general, higher alkanes burn
less completely with more sooty flame. Carbon monoxide and unburnt carbon particles (as soot) would
also be produced.
b. Reaction With Halogens
(i) Alkanes react with bromine (in 1,1,1-trichloroethane) in diffuse sunlight, as shown by the
disappearance of the red-orange colour of bromine.
Note: 1. No reaction when in dark.
2. In direct sunlight, the reaction takes place very rapidly or may cause explosion.
Part V Fossil Fuels and Carbon Compounds/P.44
(ii) Methane also reacts with bromine in the presence of light.
(iii) The above reactions are called substitution reaction 取代反應
A SUBSTITUTION REACTION is a reaction in which an atom (or group of atoms) of
an organic molecule is replaced by another atom (or group of atoms).
(iv) In general, substitution reactions of alkanes consist of three steps, including
(1) initiation
(2) propagation
(3) termination
Part V Fossil Fuels and Carbon Compounds/P.45
(v) Example: monosubstitution of methane with chlorine
Step 1: Initiation
In this step, a type of very reactive species, called free radicals* (or radicals) are produced in the
reaction process.
*A free radical (or radical) is an atom or group of atoms with at least one unpaired electron. They are
highly reactive and exist only momentarily.
The Cl-Cl bond is broken by UV radiation (from diffuse sunlight) to give two chlorine radicals and
start the chain reaction.
x
xx
xx
xxClCl x
xx
xx
xxClCl
Cl Cl Cl
+
+chlorine radicalchlorine radical
Cl
Step 2: Propagation
(a) Each chlorine radical combines with a hydrogen atom to form a hydrogen chloride molecule and a
methyl radical.
xxx
x
x
CH
H
H
H + x
x
x
CH
H
H
HCl Cl+
C
H
H
HH Cl+ C
H
H
H H Cl+
methyl radical
Part V Fossil Fuels and Carbon Compounds/P.46
(b) Some methyl radicals then combine with chlorine atoms from another chlorine molecule to form
chloromethane and other chlorine radicals.
x
x
x
CH
H
H
+ x
xx
xx
xxClCl
x
x
x
CH
H
H
Clx
xx
xx
xxCl+
C
H
H
H Cl Cl + C
H
H
H Cl + Cl
Step 3: Termination
Some methyl radicals combine directly with chlorine radicals to form chloromethane.
Cl
x
x
x
CH
H
H
+x
x
x
CH
H
H
Cl
C
H
H
H + C
H
H
H Cl
Cl
Classwork
Name the products when methane reacts with excess chlorine in diffuse sunlight.
Part V Fossil Fuels and Carbon Compounds/P.47
C. Cracking of Petroleum 裂解作用裂解作用裂解作用裂解作用
a. Greater Demand Than Supply For Some Distilled Oil Fractions
(i) Petroleum (or Crude Oil) is refined by fractional distillation into different fractions. But the supply
for petrol, kerosene and gas oil cannot meet the greater demand.
(ii) To produce more petrol, cracking of the heavy fractions are necessary.
(iii) Fractions with high boiling point ranges may be cracked. In the process, large alkane molecules are
broken down into smaller alkane molecules, together with alkene molecules.
(iv) Cracking is the breaking down of larger hydrocarbon molecules with heat and/or a catalyst to
produce smaller hydrocarbon molecules.
Fractions Supply Demand
Refinery gases 5% 5%
petrol 10% 25%
naphtha 5% 5%
kerosene 20% 25%
diesel oil 15% 35%
fuel oil and lubricating oil 45% 5%
Part V Fossil Fuels and Carbon Compounds/P.48
b. Catalytic Cracking催化裂解作用催化裂解作用催化裂解作用催化裂解作用
During the cracking process, the heavy fractions are heated in the absence of air (otherwise they will burn)
aluminium oxide mixed with silicon(IV) oxide as catalyst.
c. Cracking of medicinal paraffin 藥用藥用藥用藥用石石石石蠟油蠟油蠟油蠟油 in laboratory
Broken pieces of unglazed porcelain 素瓷片 are heated strongly. The vapour of medicinal paraffin is cracked
on the hot catalytic surface of porcelain. The products are lower alkanes and alkenes, the gaseous portions
being collected over water.
* Broken pieces porous pot 多孔瓷片多孔瓷片多孔瓷片多孔瓷片, pumice stone 浮石浮石浮石浮石 or aluminium oxide may also be used as the
catalyst.
Part V Fossil Fuels and Carbon Compounds/P.49
d. Importance of cracking
1. To produce extra petrol.
2. As a source of alkenes. (Alkenes are good starting points for preparing a great variety of organic chemicals, e.g.
alkanols and plastics)
Part V Fossil Fuels and Carbon Compounds/P.50
VI. Alkenes 稀烴稀烴稀烴稀烴
Alkenes are unsaturated hydrocarbons, having the general formula CnH2n.
Alkene Molecular formula Condensed structural formula
ethene C2H4 CH2=CH2
propene C3H6 CH3CH=CH2
but-1-ene C4H8 CH3CH2CH=CH2
but-2-ene C4H8 CH3CH=CHCH3
pent-1-ene C5H10 CH3CH2CH2CH=CH2
A. Physical Properties of Alkenes
Name Structural Formula Melting Point
(oC)
Boiling Point
(oC)
State at room
temperature and
pressure
Ethene CH2=CH2 -169 -104
Gas Propene CH3CH=CH2 -185 -47
But-1-ene CH3CH2CH=CH2 -185 -6
Pent-1-ene CH3CH2CH2CH=CH2 -138 30 Liquid
Hex-1-ene CH3CH2CH2CH2CH=CH2 -140 63
There is also a gradual change of physical properties of alkenes as the length of the carbon chain in the
molecules increases.
B. Chemical Properties of Alkenes
Alkenes are unsaturated, they are much more reactive than alkanes.
a. Combustion
Alkenes burn in excess oxygen to form carbon dioxide and water.
2CH3CH=CH2(g) + 9O2(g) → 6CO2(g) + 6H2O(l)
In ordinary air, the oxygen present is insufficient for complete combustion. Alkenes therefore burn with a
luminous, smoky flame due to unburnt carbon particles being formed.
Part V Fossil Fuels and Carbon Compounds/P.51
b. Addition Reactions 加成反應加成反應加成反應加成反應
(i) Reaction with halogens
When an alkene reacts with bromine in 1,1,1-trichloroethane, the red-orange colour of bromine disappears
rapidly. During the reaction, a bromine atom is added to each of the doubly-bonded carbon atoms.
An ADDITION REACTION is a reaction in which two or more molecules react to give a single molecule.
Classwork
Bromine (in tetrachloromethane) is added separately to hex-1-ene and hexane. The red-orange colour of bromine is
discharged in both cases. Compare and contrast the two reactions.
Part V Fossil Fuels and Carbon Compounds/P.52
(ii) Reaction with potassium permanganate solution
Alkenes rapidly decolourize an acidified solution of potassium permanganate.
The purple permanganate ion MnO4- is reduced to almost colourless manganese(II) ion Mn2+. The alkene is
oxidized to a diol 二醇二醇二醇二醇. This is an addition reaction, two -OH groups being added across the double bond.
c. Test for Alkenes
1. The orange solution of bromine dissolved in an organic solvent becomes colourless quickly when shaken with
an alkene.
2. The purple solution of acidified potassium permanganate becomes colourless quickly when shaken with an
alkene.
Part V Fossil Fuels and Carbon Compounds/P.53
Classwork
In the cracking process, large hydrocarbon molecules in petroleum fractions are broken down into smaller
molecules. One example is illustrated by the following equation:
C10H22 →→→→ A + B
where A is a saturated hydrocarbon containing 8 carbon atoms and B is an unsaturated hydrocarbon.
a. Write the molecular formula of A.
b. Draw the structural formula of B.
c. Suggest a chemical test to distinguish B from A. State the expected observation.
Exercises
1. Crude oil is a mixture consisting mainly of alkanes. Fractional distillation of crude oil gives different petroleum
fractions. The table below lists the length of carbon chain of the alkanes in some of the fractions.
Fraction Length of carbon chain
Petrol / Naphtha C5 – C10
Kerosene C11 – C18
Diesel C18 – C25
X C20 – C34
a. Describe the principle underlying the fractional distillation of crude oil.
b. (i) Explain why the global demand for petrol is greater than that for kerosene.
(ii) Cracking kerosene can produce petrol. State the conditions required for the cracking process.
c. Give one use of fraction X in cars.
(HKCEE 2000)
Part V Fossil Fuels and Carbon Compounds/P.54
2. The following experimental set-up is used to crack medicinal paraffin.
a. What is cracking?
b. What is the purpose of the broken porous pot?
c. Why is the wool soaked with medicinal paraffin NOT heated directly?
d. Why should the first tube of gas collected be discarded?
e. Is the gaseous product soluble or insoluble in water? Explain your answer.
f. At the end of the experiment, should the student remove the delivery tube from the water first or should he
remove the heating first? Explain your answer.
g. Which one of them, the medicinal paraffin or gas G, has a smaller relative molecular mass? Explain your
answer.
Part V Fossil Fuels and Carbon Compounds/P.55
VII. Addition Polymers
A. Introduction
a. Different Kinds of Plastics
There are many different kinds of plastics. Some common ones are: polythene, polyvinyl chloride (PVC),
polystyrene, perspex, nylon, urea-methanal and phenol-methanal
b. Where Do Plastics Come From?
Petroleum is the most important raw material used in the production of plastics. About 4% of petroleum is
eventually turned into plastics.
Plastics are made mainly from ethene and other alkenes, which are obtained by cracking oil fractions like
naphtha and gas oil.
c. What Plastic are?
Plastics are polymers. Polymers consist of very large molecules, made by joining many small molecules
(monomers) together.
For example, under special conditions, ethene molecules can join together to form polythene:
Part V Fossil Fuels and Carbon Compounds/P.56
d. Why Plastics are so Useful?
Plastics have the following properties:
� Plastics are usually strong but light.
� They usually have no reactions with air, water, acids, alkalis and most other chemicals.
� They are good insulators of heat and electricity.
� They can be moulded easily into any shape.
� They are usually transparent and clear.
� They can be dyed.
� They can be flexible.
B. Polymers and Polymerization
A POLYMER is a compound consisting of very large molecules formed by many small molecules joined
together repeatedly.
POLYMERIZATION is the process of joining together many small molecules repeatedly to form very
large molecules.
In polymerization, the compounds whose molecules join together repeatedly are called monomers.
Even in the same polymer sample, the macromolecules present do not have the same size. For example,
polythene may be represented as [ CH2-CH2 ] n , where n ranges from about 1000 to 30000.
Part V Fossil Fuels and Carbon Compounds/P.57
C. Addition Polymerization
a. What is Addition Polymerization?
ADDITION POLYMERIZATION is a reaction in which monomer molecules join together to form
polymer molecules, without elimination of small molecules.
The monomer molecules involved must contain carbon-carbon double bonds. They undergo repeated
addition reactions among themselves to form an addition polymer.
Structure of an addition polymer can be expressed in terms of its repeating unit.
A REPEATING UNIT is the smallest part of a polymer molecule, by repetition of which the whole
polymer structure can be derived
In our case here, the repeating unit is:
which is derived for one monomer molecule.
Part V Fossil Fuels and Carbon Compounds/P.58
b. Addition Polymers
1. Polythene
n
nEquation:
Example: Write an equation to show the polymerization of propene. Name the polymer formed.
2. Polystyrene
n
nEquation:
Part V Fossil Fuels and Carbon Compounds/P.59
Laboratory preparation
Equal volumes of styrene and kerosene are heated for about one hour. Kerosene acts as a solvent and catalyst.
3. Perspex
Write an equation to show the polymerization of methyl 2-methylpropenoate
Part V Fossil Fuels and Carbon Compounds/P.60
4. Polyvinyl Chloride (PVC)
Write an equation to show the polymerization of chloroethene.
Classwork
1. A polymer is represented by the following structure:
Give the structural formula and IUPAC name of the monomer for this polymer.
2. The flow diagram below shows the key stages involved in the production of polyvinyl chloride (PVC) pipes
from petroleum.
a. Name the process for obtaining heavy oil from petroleum in stage I.
b. Name the two main processes involved in the production of unsaturated compound A from heavy oil in stage II.
c. Write the chemical equation for the formation of PVC from its monomers.
Part V Fossil Fuels and Carbon Compounds/P.61
Uses and properties of some common addition polymers
Name Properties Uses of polymer
Polythene
PE
Low density polythene LDPE
� light
� flexible
� low-melting
High density polythene HDPE
� tougher
� higher melting
� more transparent than LDPE
films for packaging, wrapping
plastic bags, squeeze bottles,
toys
Thick plastic bottles, buckets,
food boxes, toys
Polystyrene
PS
Polystyrene
� transparent
� brittle
� hard
Expanded Polystyrene
� white
� extremely light solid foam
“see-through” containers, milk
bottles
disposable cups, packaging
material of delicate articles and
electrical appliances
Polyvinyl
chloride
PVC
PVC
� stiff
� brittle
PVC with plasticizer
� more flexible
Pipes and bottles
floor tiles, coverings of electrical
wires, raincoats, shower curtains
Perspex
� strong
� rigid
� highly transparent
glass substitute, contact lenses,
aircraft windows
Part V Fossil Fuels and Carbon Compounds/P.62
D. Relating the Structures of Plastics to Their Thermal Properties
a. A Thermoplastic is a plastic which can be softened by heating and hardened by cooling, the process being
repeatable any number of times.
e.g. polythene, polystyrene, perspex, PVC
b. A thermoplastic consists of separate, long flexible polymer chains. These chains are tangled, holding each other
in place by weak intermolecular forces.
When heated, the chains vibrate more vigorously, becoming further apart. The intermolecular forces are
weakened, and the chains can slide over each other easily. The plastics thus softens and melts.
When cooled, the long chains have less energy. They become closer and attract each other more. The plastic
thus hardens.
When reheated, the plastic object melts again.
Part V Fossil Fuels and Carbon Compounds/P.63
E. Environmental issues related to the use of plastics
1. Plastic waste disposal problems
(i) Most plastics are non-biodegradable
Plastics cannot be decomposed by bacteria. Often plastic wastes are buried in landfill sites. They remain
there for a long time.
(ii) Burning plastics gives off poisonous gases
Burning plastics will produce toxic carbon monoxide.
Burning PVC will produce hydrogen chloride gas.
(iii) Plastic waste in the sea poses direct danger to marine lives
Small fishes die when digestive tracts are clogged by fragments of plastic bags they ingest.
Sea animals are suffocated to death by plastic bags.
2. Solutions to plastic waste disposal problems
(i) Making biodegradable plastics
(ii) Use of alternative materials
Paper, glass and other materials can be used instead of plastics, e.g. paper bags instead to plastic bags
(iii)Recycling of plastic waste
3. Recycling of Plastics
Recycling of plastics is a possible solution to the plastic waste disposal problem. The recycling includes the
following forms:
a. Direct recycling
This applies only to thermoplastics. the plastics in the waste are separated, cleaned, ground to powder and
remoulded into new plastic items. The regenerated plastics usually have deteriorated properties due to
repeated processing. the success of this method depends on the collection of clean and uncontaminated
plastic waste, which is the most difficult step.
b. Recycling of energy
The energy obtained from burning plastic wastes in incinerators can be used for heating or generating
electricity.
Part V Fossil Fuels and Carbon Compounds/P.64
c. Recycling of chemicals
If plastics are heated in air, they burn to form mainly carbon dioxide and water (from hydrocarbons). Some
plastics may produce choking gas when heated in air.
However, if plastics are heated in the absence of air to about 700oC, the molecules would break down to
form smaller molecules. The process is called pyrolysis.
Pyrolysis is employed for recycling of plastics because the process does not require the separation of the
various types of plastics.
A mixture of common plastics such as polythene, polypropene and polystyrene when pyrolysed, would
give hydrocarbons such as methane, ethene, propene and benzene. These hydrocarbons could be separated
out by distillation and used as the starting materials for other chemicals including plastics.
At present, the process is still at an experimental stage and has to prove its economic viability. The
following is a schematic diagram for the process.
Part V Fossil Fuels and Carbon Compounds/P.65
4. Recycling of plastics is important:
a. it protects the environment by reducing the amount of plastic waste;
b. it conserves raw materials since many plastics are made from non-renewable petroleum;
c. it might save money when petroleum becomes more expensive.
5. Problems with recycling
a. it is difficult to separate plastics from other waste;
b. it is difficult to separate different plastics;
c. recycled plastics lose their original properties;
d. it is difficult to remove additives in plastics;
e. the process is uneconomical