Part 5 Fossil Fuels and Carbon Compounds

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


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.


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.


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


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.


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

).


(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.


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.


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.


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.


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


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.


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.


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.


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


(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.


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.


(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.


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)


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.


(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.


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.


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.


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.


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.


(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)


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.


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


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.


Classwork

Write condensed structural formulae for the following compounds:


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.


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


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


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.


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.


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.


a. 1,1,1-trichloroethane b. 2,3-dibromo-1-florobutane


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.


Classwork

1. Name the following compounds by IUPAC system:

a. b.


a. 2-methylbut-2-ene b. 2,3-dichlorobut-1-ene


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:


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


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


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.


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.


(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


(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


(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.


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%


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.


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)


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.


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.


(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.


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)


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.


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:


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.


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.


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:


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


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.


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


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.


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.


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.


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


6. International plastic coding system for recycling

7. Plastics: Good or Bad?

Documents

Part 5 Fossil Fuels and Carbon Compounds