Download - CH1 Products From Crude Oil
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Cracking
Short alkane chains from Fractional Distillation (used e.g. for petrol) are in much
greater demand than long chains.
This means that after fractionation, there are long chains left over when all theshort chains have been sold.
To make most efficient use of the crude oil, these long
chains are cracked in the refinery.
Cracking produces SHORTER CHAIN ALKANES plusmolecules called ALKENES
The shorter chain alkanes help satisfy the demand
for short chain fractions to be used as fuels.
Alkenes are important starting points for manyproducts made in the chemical industry e.g. plastics,
pharmaceuticals.
Cracking is a THERMAL DECOMPOSITION reaction
the long chains are heated until vapourised and the
vapour is then passed over a hot catalyst.A catalytic cracking plant
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What are alkenes ?
Recall that the alkanes are a family of hydrocarbons.
The alkenes are also a family of hydrocarbons, but they have a double bond
between two of the carbon atoms. This means that
i) they are unsaturated
ii) they have a general formula Cn
H2n
Having a double bond means that alkenes are more reactive than alkanes which is
why they are so useful as starting points for making other chemical compounds.
Here are displayed formulae for some alkenes:
H H HH | | |
C = C C C H
H | |
H H
H H
C = C
H H
H HH | |
C = C C H
H |
HEthene C2H4 Propene C3H6 Butene C4H8
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Cracking reactions
The number of carbon and hydrogen atoms always has to add up to the same on
both sides of the equation always check to make sure this is so !
e.g. octane hexane + ethene
C8H18 C6H14 + C2H4
decane heptane + propene
C10H22 C7H16 + C3H6
Sometimes a chain may crack twice two alkenes will be formed
e.g. decane propane + butene + propene
C10H22 C3H8 + C4H8
+ C3H6
Hydrogen can also be made as a product of cracking
e.g. hexane hexene + hydrogen
C6H
14 C
6H
12+ H
2
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Telling an alkane from an alkene
The test for a compound which contains one (or more) double bond between carbon
atoms is to shake the compound with bromine water
If the compound is unsaturated (contains a double bond) then the bromine water
reacts with the alkene. The bromine water turns from orange to colourless. We say
that the alkene decolourises bromine water.
If the compound is saturated (no double bonds) the bromine water cant react with
it, and the bromine waterremains orange.
Testing an alkane and an
alkene with bromine water
After adding bromine water and
shaking
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The alcohol (called ethanol) in both of these products has
been produced by a chemical reaction (its the same
alcohol we might find in wine, or beer). While the alcohol is
the same, the reactions are very different.
You will need to be able to compare the two routes by
which ethanol is made.
Industrial ethanol
made by the
reaction of
ethene withsteam
Ethanol as an
alternative fuel
made by the
fermentation of
plant matter
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Making ethanol from ethene
When heated and treated with a catalyst, ethene reacts with steam to produce
ethanol. The equation for the reaction is:
H H H H
| | | |
C = C + H-OH H C C H
| | | |
H H H OH
ethene + water ethanol
C2H4 + H2O C2H5OH
The ethene is obtained from cracking of long chain fractions from crude oil. It istherefore from a non-renewable source.
The reaction goes at high temperature, so there are high energy costs
The reaction can carry on as a continuous process, so large amounts of ethanol
can be made relatively quickly this way.
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Making ethanol by fermentation
Plant matter containing sugars (build up by the plant during photosynthesis) can be
fermented using yeast. The yeast organisms convert the sugars into ethanol and
carbon dioxide.
sugars ethanol + carbon dioxide
C6H12O6 2 C2H5OH + 2 CO2
The process uses plant matter which can be
regrown continuously, so its from a renewable
source
The process does not require high temperatures
low energy costs
The yeast dies from ethanol poisoning, so the
process has to be stopped and the ethanolextracted and purified before starting over again
with fresh yeast. This is called a batch process.
The reaction is slow. It takes a long time to make
ethanol by this route compared to the industrial
route.
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Plastics are made from molecules which have extremely long chains of carbonatoms. We call these molecules polymers. They are made by linking together
huge numbers of much smaller molecules to form the chain.
Alkenes are suitable molecules to use for these polymerisation reactions.
Common plastic materials such as
polythene
polypropylene
PVC
Perspex
are all polymers made from alkenes
Manufacturing large polythene bags
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A polymer molecule is made by reacting together many smaller molecules to link
together into a chain. These smaller molecules which form the chain are called
monomers.
Polymers are named by taking the monomer name, putting it in brackets, andadding poly in front.
e.g. The polymer made from ethene is
called poly(ethene) known by the
common name polythene
Alkenes are one type of monomer which link togetherto form polymers. The reaction is called addition
polymerisation, because the monomers add on to one
another to form the chain, and no other product is formed.
It would be impossible to draw the whole of a polymer
molecule its too long ! Fortunately a polymer chain
contains the same arrangement of atoms repeated over
and over again. Instead we can draw a repeat unit, or
a section of the polymer.
Products made from
poly(ethene)
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Look at these monomers, and see how the polymer repeat unit and structure are
drawn using DISPLAYED FORMULAE:
Monomer Repeat Unit Section (showing 3 repeats)
H H H H H H H H H H
| | | | | | | | | |
C = C C C - C C C C C C -
| | | | | | | | | |
H H H H n H H H H H H
ethene poly(ethene)
H H
H C| / H
C = C
| |
H H
H H
H C| / H
C C
| |
H H n
propene poly(propene)
H CH3 H CH3 H CH3| | | | | |
- C C C C C C -
| | | | | |
H H H H H H
poly(ethene)
poly(propene)
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Points to remember when drawing polymers:
1) The polymer is formed by breaking open the double bond and using it to join to
monomer molecule either side. This means that every repeat unit is a section of
the chain two carbon atoms long.
2) The double bond is broken when the polymer is made, leaving just single bonds.
There are no double bonds in the repeat unit or section
Balanced equations:Chains are different lengths, made from differing numbers of monomers. The best we
can do is to say there were n monomers joined together to make the chain.
The balanced equation therefore is written as: (note where the n s are)
Cl H
| |
C = C
| |
H H
Cl H
| |
C C
| |
H H
n
n
n monomers join
together to form a
polymer which is n
repeats long
chloroethene poly(chloroethene)
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AdvantagesAdvantagesandand
disadvantagesdisadvantagesofofusing products fromusing products from
crude oil as fuels or ascrude oil as fuels or as
raw materialsraw materials
Fossil fuels contain a little sulphur
which, when burned, forms sulphur
dioxide this causes acid rain.
Crude oil is a non-renewableresource, taking millions of years
to form it is a finite resource.
Products from crude oil provide
valuable starting points forchemicals such as life-saving
drugs and high-tech polymers.
The economies of the
industrialised world are
dependent on oil supplies.
Rapid transport and
available electrical power
underpins our society.The main gas implicated in Global
Warming is carbon dioxide one of the
products of burning fossil fuels.
Fuels such as petrol and
diesel are easy to transport,
relatively safe to use,
Alternative fuels are not yet
sufficiently safe, cost effective or
capable of meeting the massive
energy needs of modern society, but
fossil fuels are available in sufficient
supply to meet current needs.
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At first when a new polymer was discovered, Scientists thought long and hard about
what jobs these polymers could do. Some polymers, such as the adhesive on Post-It
notes, were happy accidents !
Modern scientists understand polymers and how to make them very well. Now
polymers are designed with specific properties to do well-defined jobs. There might
be called Designer Polymers.
Polymers designed for specific jobs include:
Kevlar - designed for body armour- very strong but lightweight
Lycra - sports-wear
- very flexible but strong
Goretex - breathable fabric- lets sweat out but prevents
rain getting in
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Designer polymers have made an impact in medical applications too:
Smart Materials are materials which
change in response to changes
around themselves.
Shape memory polymers can change
their shape in response to heat.
Stitches which can pull the edges of a
wound together at body-temperature
are one example.
Hydrogels contain cross-links which trap
water, making wound dressings which are
moist and sterile possible, as well as being
useful for soft contact lenses.
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Remember !
Polymers are made from monomers such as alkenes
Alkenes are made by cracking reactions, along with
short chain alkanes the starting material is long-chainalkanes
These long chain alkenes come from fractional
distillation of crude oil
Therefore polymers of this kind are from non-renewable sources, and are using up our crude oil
reserves.
RECYCLING helps to reduce the rate at which crude oil
is used up to make polymers. BUT recycling is labour
intensive, and the different polymers all have to besorted. Therefore recycling plastics is not very cost
effective.
Many people cant be bothered to save and sort plastic
waste for recycling.
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Many polymers are not bio-degradable.This means that they will not be broken
down in nature by micro-organisms.
This causes problems for disposal of waste polymers:
- because they dont bio-degrade they can take up valuable landfill
- they may give offtoxic fumes when burnt
- plastic waste is unsightly and can be dangerous to wildlife, if animals
or birds swallow the plastic, or get trapped in waste objects
- recycling takes a lot of effort to collect the waste
- to recycle the plastics have to be sorted into different types, which is
labour intensive and an unpleasant job- for these reasons recycled plastics can be more expensive than just
making new polymers using products from crude oil
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What are scientists doing to solve polymer disposal problems ?
Designing polymers which are bio-degradable,
or photo-degradable (broken down slowly bysunlight),
or water-soluble.
Developing ways of burning plastic waste as a fuel
but without the toxic emissions, e.g. by using
scrubbers to remove these gases.