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Topic 5 – Fossil Fuels
Formation of fossil fuels
Coal: Millions of years ago, plants on the earth surface died and covered by sand and mud. In
time, heat, pressure and bacterial reaction gradually change the dead plant into coal.
Petroleum and natural gas: Millions of years ago, marine plants and animals died and covered
by sand and mud. In time, heat pressure and bacterial reaction gradually change the dead bodies
into petroleum and natural gas.
Fractional distillation of petroleum
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Petroleum is a mixture of many different kinds of hydrocarbons.
Fractional distillation is to separate them for different purposes.
Petroleum is first heated to about 400oC in a furnace. Compounds with low boiling point will
vapourize, while others remain liquid. This mixture is then pumped into a fractionating tower.
Different compounds will be collected at different levels.
Fraction
Boiling point
Number of carbons in Uses
each molecule
Refinery gases <40
1-4
Making Liquefied Petroleum Gas
(LPG)
Petrol
5-10
Fuel for cars
40-170
Naphtha
Kerosene
Gas oil
Manufacturing town gas
170-250
250-350
10-14
14-25
Fuel for aircraft
Diesel fuel
Fuel oil
Fuel for ship
Lubricating oil
and waxes
Bitumen
Lubricating oil for machines;
making candles
Surfacing roads and roofs
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>350
Above 25
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Different air pollutants
Pollutants
Source
Effect
Remedies
NOx
Direct combination of N2
and O2 at high temp in car
engine / in lightning.
Acid rain, irritant to
respiratory system, forms
smog
Install catalytic
converter
SO2
Combustion of S containing
fuel
Acid rain, irritant to
respiratory system Use
fuel with lower S content
use of scrubber
CO
Incomplete combustion of
fossil fuel
Toxic, lowers the oxygen
carrying capability of
blood by forming stable
carboxyhaemoglobin with
haemoglobin,
causes dizziness or even
dealth
Install catalytic
converter,
increase the amount
of O2 in the
combustion process
Particulates
Incomplete combustion of
fossil fuel
Irritant to respiratory
system
Install electrostatic
precipitator in
chimney, Install
filtering system,
Fine tune the engine
regularly.
Unburnt
hydrocarbon
Incomplete combustion of
petroleum
products
Carcinogenic, toxic
forms smog Install
catalytic converter
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Homologous series
Members in the same homologous series have:
1. Same functional group
2. Differs from next by a CH2 unit
3. Have Gradual change in physical properties.
4. Have similar chemical properties.
Name of homologous series Functional group
Example
Alkane (CnH2n+2)
-----
CH3CH2CH3
Alkene (CnH2n)
Alkyne (CnHn-2)
> C=C <
—C≡C—
CH2=CH2
H—C≡C—H
Alkanol
—O—H
CH3CH2OH
—NH2
CH3CH2NH2
Alkanal
Alkanone
Alkanoic acid
Amide
Anime
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Properties of alkanes
Physical properties:
The melting point and boiling point rise as the number of carbon atoms in the alkanes increase.
Chemical properties:
Combustion: alkanes burn completely with sufficient oxygen supply.
alkane + oxygen → carbon dioxide + water
e.g. CH4 + 2O2 → CO2 + 2H2O
Substitution reaction: hydrogen atoms can be replaced by chlorine or bromine.
Replacement of chlorine is faster.
A substitution reaction is a reaction in which an atom (or a group of atoms) of a molecule is
replaced by another atom (or a group of atoms).
CH4 + Cl2 → CH3Cl + HCl
CH3Cl + Cl2 → CH2Cl + HCl
CH2Cl + Cl2 → CHCl3 + HCl
CHCl3 + Cl2 → CCl4 + HCl
(chloromethane)
(dichloromethane)
(trichloromethane)
(tetrachloromethane, carcinogenic)
Condition: Heat or ultraviolet light
In general, substitution reactions of alkanes consist of three steps
1. initiation (how the reaction gets started)
2. propagation (how the reaction proceeds)
3. termination (how the reaction stops)
Free radicals, a very reactive species, are involved in the reaction process.
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What is cracking?
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Cracking is breaking of large molecules into smaller molecules.
In a cracking process, a very long alkane is broken down into a long alkane and a small
alkene.
Methods of cracking
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Heat is required in cracking process, so the process is also called thermal cracking.
However, cracking is usually carried out at a lower temperature and pressure with catalyst of
Al2O3 + SiO2 and the process is called catalytic cracking.
Thermal cracking: 700oC + 30 atm
Catalytic cracking 500oC + 1 atm + catalyst
Importance of cracking
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cracking breaks a long alkane into a shorter alkane and a small alkene.
This converts the less demanded heavy oil to petrol to meet the demand.
The unsaturated alkene is more reactive than alkane. It is used as the raw material of
production of a lot of chemicals (e.g. plastic).
Making of town gas
Town gas is made from catalytic cracking of naphtha. Liquid naphtha is heated into vapour. It is
then mixed with steam and pass through a hot nickel catalyst at 700oC. An odourant is also add to
make town gas leakage can be detected easily.
C5H12 + 5H2O → 5CO + 11H2
C5H12 + 10H2O → 5CO2 + 16H2
2CO + H2 → CO2 + CH4
CO + H2O → CO2 + H2
Heat change in reaction
We can find the heat change by this formula: ΔH = Hp – Hr. (ΔH = heat change, Hp = heat content
of the product, Hr = heat content of reactant)
Exothermic reaction is a reaction that gives out heat energy (ΔH is negative). Endothermic
reaction is a reaction that takes in heat energy (ΔH is positive).
Burning of petrol
Complete combustion: 2C8H18 + 25O2 → 16CO2 + 18H2O
Incomplete combustion (insufficient oxygen): 2C8H18 + 17O2 → 16CO + 18H2O
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Addition reaction of alkenes
Addition reaction is a reaction in which two or more compound react to give a single product.
React with bromine:
When an alkene is shaken with bromine in 1,1,1-trichloroethane, its orange colour will disappear.
React with acidify potassium permanganate solution
When an alkene is shaken with acidify potassium permanganate solution, its purple colourless will
disappear.
Thermoplastic and thermosetting plastic
Hardness
Thermoplastic
Soft
Thermosetting plastic
Hard
Flexibility
Flexible
Non-flexible
Ease to melt
Melt upon heating
Do not melt upon heating
Structure
Separated polymer chains.
Polymer chains with crosslinks forming 3-dimension
network of bond.
Examples
Polythene, polystyrene,
polyvinyl chloride, perspex,
nylon
Urea-methanal
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Polymerization
Addition polymerization is a process of repeatly joining together many monomers to form
polymers. No elimination smell molecule occurs.
Be careful with the term ‘addition polymerization’, NOT ‘additional polymerization’.
Polythene
Low density polythene (LDPE):
Light, flexible, low melting point.
High density polythene (HDPE):
Tougher, higher melting point,
transparent
Polystyrene
Polystyrene:
Transparent, brittle, hard
Expended polystyrene:
White, extremely light.
Polyvinyl chloride (PVC)
PVC:
Stiff, brittle
PVC with plasticizer:
Flexible.
Perspex
Perspex:
Strong, rigid, highly transparent.
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Condensation polymerisation
Condensation polymerisation is a process which repeatly joining together monomers to form
polymers. Small molecules (e.g. H2O, HCl, NH3) are always eliminated.
Nylon
Nylons are polyamides which contain the amide linkage.
Nylons are formed by condensation polymerisation.
We make nylon-6-6 in laboratory by adding aqueous solution of hexane-1,6-diamine in 1M
sodium hydroxide solution to a solution of hexanedioyl dichloride in 1,1,1-trichloroethane. They
are immiscible, A layer of nylon will form at the surface between them.
Nylon has high melting point (200oC) and tensile strength.
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Polyester
Polyesters contain the ester linkage.
Polyesters are formed by condensation polymerization.
Similar to formation of ester, A dioic acid and a diol condenses together with elimination of water
molecule to form an ester molecule.
Since each monomer has two function groups, it is capable to condense repeatedly to form a
very long chain polymer.
Pollution problems associated with disposal of plastic waste
Pollution problems:
1. Most plastic are non-biodegradable.
2. Burning of plastic give off poisonous gases such as hydrogen chloride and chlorine. (Can use
scrubbers to remove)
3.
Plastic waste in sea poses directly danger the marine lives.
(a) Small fishes die when their digestive tracts are clogged by fragments of plastic bags.
(b) Sea animals suffocated to death by plastic bags.
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Alkanols
Alkanols can be represented by: R—O—H (R is alkyl group, O—H is hydroxyl group)
Preparation of ethanol
In laboratory: we preparation ethanol by fermentation in laboratory.
1. Add a spoonful of ‘living’ yeast to a dilute solution of glucose in a conical flask.
2. Leave it for a few days. The maximum concentration of ethanol by fermentation is 18%,
because the yeast dies at this concentration.
3. To increase the concentration, we can use fraction distillation.
In industry: we pass the ethene with steam over a catalyst of phosphoric acid at 300oC and 65
atm.
CH2=CH2 + H2O → CH3CH2OH
Uses of alkanols
1.
Alkanols as fuels
e.g. CH3CH2OH + 3O2 → 2CO2 + 3H2O
2. Alkanols as solvents (it dissolve oil and it is miscible with water)
3.
4.
5.
Alkanols in alcoholic drinks (all alcoholic drinks contain alkanols)
Production of vinegar (vinegar is a dilute solution of ethanoic acid)
Production of ester
Properties of alkanols
Physical properties:
Boiling point: boiling point increase as the number of carbon in the molecule increase.
Solubility: the solubility decreases as the length of the alkyl group increase. It is because the
hydroxyl group tends to mix with water. When the length alkyl group increases, the
influence of hydroxyl will decrease.
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Chemical properties:
1. Reaction with Concentrated hydrochloric acid
HCl + CH3CH2OH →CH3CH2Cl + H2O
2.
Oxidation reactions
When using acidify potassium dichromate as oxidizing agent:
When using acidify potassium permanganate as oxidizing agent:
When we heat alkanols with oxidizing agent in a reflux condenser, ethanoic acid will form. We
can separate it from the mixture by distillation.
3.
Esterification (condensation reaction)
1.
2.
3.
4.
Add 2 cm3 ethanol, 1 cm3 ethanoic acid and 3 drop of conc. sulphuric acid (catalyst) into
a test tube.
Warm the test tube in warm water bath for 5 minutes.
Pour the mixture into cold water.
An insoluble layer of ester forms on the water, ethanoic acid and ethanol dissolve in the
water.
Naming of ester: name R1 first
e.g. methyl ethanoate for
e.g. ethyl ethanoate for
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