Download Section 5b and c: crude oil and synthetic polymers Fractional

Section 5b and c: crude oil and synthetic polymers
Fractional distillation
5.6 recall that crude oil is a mixture of hydrocarbons
5.7 describe how the industrial process of fractional distillation separates crude oil into fractions
5.8 recall the names and uses of the main fractions obtained from crude oil: refinery gases,
gasoline, kerosene, diesel, fuel oil and bitumen
5.9 describe the trend in boiling point and viscosity of the main fractions
Crude oil is a mixture of many compounds, mainly hydrocarbons (mainly alkanes), which is why it is not
useful and why it needs to be separated using fractional distillation. Remember that the main difference
between all these hydrocarbons is their number of carbon and hydrogen atoms and therefore their
(molecular) mass. As the boiling points of compounds depends on their mass, all the different
hydrocarbons in crude oil have different boiling points.
Principles of fractional distillation
The crude oil or petroleum is heated before it is added into the fractionating column. When the heated
crude oil enters the fractionating column or tower, most of the compounds in the crude oil boil or
vapourise to form a vapour mixture that rises up the fractionating column. Within this fractionating column
the temperature decreases as you go higher. As the vapour mixture rises, some of the compounds
condense into the liquid state as the temperature in the column drops below their boiling point. The
different compounds in the vapour mixture are removed from the fractionating column at different heights.
As it is too difficult to separate each compound separately from the mixture, the compounds are removed
in groups of similar boiling points; such groups are called fractions. The higher up the column a fraction is
collected, the lower the boiling point of its compounds. Some of the compounds do not condense at all
and make up the fraction that leaves the fractionating column at the top as a vapour or gaseous mixture.
Some of the compounds have boiling points higher than the temperature at the bottom of the fractionating
tower; these compounds do not vapourize at all and are collected as liquids at the bottom of the column.
Each liquid collected from the fractionating tower is separated further.
The diagram and table below show where the various fractions are collected and what they are used for.
fraction
uses
viscosity
A = refinery
gases
camping fuels
gases
B = gasoline
fuel in cars
very runny
C =kerosene
oil stoves and aircraft
fuel (aviation)
fuel in diesel engines
quite runny
D = diesel
lowest
runny
E = fuel oil
lubricants and
making waxes and
polishes
quite thick
F = bitumen
making roads
high
viscosity
slow to pour
Section 5 b and c: crude oil and synthetic polymers
boiling
point
highest
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Complete the following passage using the words below. The number in brackets indicate the number of
times a word should be used.
fraction(2)
carbon (2)
coolest
boil
volatile (2)
hydrocarbons
liquids
condense
fractionating tower
lower (2)
Crude oil is a mixture of ___________________ which ___________ at different temperatures.
The many hydrocarbons in crude oil are different because they have different numbers of
_______________atoms; this could be anything from 1 to well over 25.
The lower the number of ____________ atoms in a hydrocarbon the _____________ its boiling point; the
more _______________ the hydrocarbon is.
A group of hydrocarbons with similar numbers of carbons and therefore similar boiling points is
called a ___________ . Each _______________ has its own range of boiling points.
The fraction with the lowest boiling points goes to the top of the _________________________
where it is the ________________ and is removed as a gas. The other fractions also rise as
vapour but _________________ lower down the tower and are removed as ____________ .
The higher up the fractionating tower, the ______________ the number of fractions in the vapour
mixture and the richer the vapour is in more _________________ fractions.
Recall that as you go up the fractionating column, the fractions:

have lower boiling points as they have a lower relative molecular mass/fewer number of carbon
atoms; fractions with lower boiling points are collected higher up the column

have a lower viscosity (=less thick) i.e. they are ‘runnier’.
The trends can be explained by the fact that the smaller the molecules the weaker the intermolecular
forces between the molecules.
Which fraction does not condense during fractional distillation? Give a reason.
Which fraction does not vapourize at all? Give a reason.
Section 5 b and c: crude oil and synthetic polymers
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Combustion
5.10 recall that incomplete combustion of fuels may produce carbon monoxide and explain that carbon monoxide is
poisonous because it reduces the capacity of the blood to carry oxygen
5.11 recall that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to
react, forming nitrogen oxides
Burning of gasoline, kerosene and diesel produces carbon dioxide and water as shown by the word
equation below:
gasoline + oxygen  carbon dioxide + water
Sometimes there is insufficient oxygen and incomplete combustion occurs; in that case carbon monoxide
and water are produced. Incomplete combustion also produces a black solid called soot which is really
carbon.
Carbon monoxide is poisonous. When you breath in CO it combines with haemoglobin in the blood and
stops the haemoglobin from taking up oxygen (which it should be doing) reducing the capacity of the
blood to carry oxygen.
The burning of fossil fuels, in particular in car engines, at high temperatures in car and plane engines
cause the nitrogen in the air to react with the oxygen in the air and form nitrogen oxides which are acidic
oxides and form acid rain.
nitrogen + oxygen  nitrogen oxide
Catalytic cracking
5.12 recall that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly
and fewer short-chain hydrocarbons than required
5.13 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking,
using silica or alumina as the catalyst and a temperature in the range of 600–700°C
The fractions obtained from crude oil are mostly alkanes which are not so useful as they are saturated
and therefore quite unreactive.
However, as we already know, the shorter-chain alkanes are used as fuels because they are volatile and
therefore are set alight easily/very flammable. The longer-chain alkanes in crude oil are even less useful
as they are less volatile and catch fire less easily.
However, these longer-chain alkanes can be converted into more useful shorter-chain alkanes (fuels) and
alkenes (starting chemicals for the plastics industry) by a process which is called catalytic cracking.
longer-chained molecules (=decomposition) into shorter
Cracking is the breaking up of
molecules using heat and a catalyst.
longer-chain alkane  shorter-chain alkane + alkene
or
longer-chain alkane  alkene + hydrogen (this happens usually with shorter alkanes)
or
longer-chain alkane  alkane + alkene + alkene
Examples: i) hexane  benzene + hydrogen : C6H14  C6H6 + 4H2
ii) decane  pentane + propene + ethene : C10H22  C5H12 + C3H6 + C2H4
Section 5 b and c: crude oil and synthetic polymers
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heating
a catalyst
Cracking involves
(600-700 C) and using
(= conditions!!!!!!!!!!!!)
The alkane is heated to a vapour and this vapour is passed over a heated catalyst as shown below.
In the laboratory the cracking of an
alkane can be carried out using the
apparatus on the left.
Synthetic polymers
5.14 recall that an addition polymer is formed by joining up many small molecules called monomers
5.15 draw the repeat unit of addition polymers, including poly(ethene), poly(propene) and poly(chloroethene)
5.16 deduce the structure of a monomer from the repeat unit of an addition polymer
Polymerisation refers to a reaction during which many small molecules join together to form a large
chained molecule.
The product is a polymer (poly = many; mer = unit) which is a very large molecule made from many
separate small identical molecules that have chemically joined together. To polymerise = to join together.
Examples of polymers are polythene, PVC, protein, fats, plastics, and nylon
The small molecules which are chemically joined together and therefore repeated in the polymer are
called monomers. Each polymer chain is made from thousands of monomer units.
Examples of monomers are ethane, glucose, amino acids, …
There are two types of polymerization reactions: addition and condensation polymerization.
Addition polymerization
This type of polymerization can only be carried out by using unsaturated monomers, i.e. alkenes, as
during this reaction the double bond breaks open and the spare electrons are used to make bonds
between the different monomer molecules as shown below. The monomers are added together.
Section 5 b and c: crude oil and synthetic polymers
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An important example of an addition polymer is poly(ethene) which is made when a large number of
identical molecules, ethene molecules, react with each other in the following conditions:



high pressure;
high temperature;
catalyst.
During this reaction, the double bond pops open which frees up electrons to make new covalent bonds
between the different ethene molecules as shown below.
Addition polymerisation changes unsaturated monomers into saturated polymers without the production of
any by-product.
Write a symbol equation showing the addition polymerization of ethene to form polythene. Polythene is
called an addition polymer:
Draw a structural equation showing the addition polymerization of ethene. Show clearly the repeated unit.
Draw a structural equation showing the formation of poly(propene).
Draw a structural equation showing the formation of poly(chloroethene).
Section 5 b and c: crude oil and synthetic polymers
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Nylon: condensation polymerization
5.17 recall that nylon is a condensation polymer
5.18 understand that the formation of a condensation polymer is accompanied by the
release of a small molecule such as water or hydrogen chloride
5.19 recall the types of monomers used in the manufacture of nylon
5.20 draw the structure of nylon in block diagram format
The formation of nylon is an example of a condensation polymerization reaction. Nylon which is a
synthetic (= man made) molecule is made from monomers which are not unsaturated as they do not have
double bonds between carbon atoms; as a result an addition polymerization is not possible.
To free up electrons to make new covalent bonds to connect a large number of monomers, atoms or
groups of atoms need to be removed first at either end of the monomers so that there are spare electrons
to make the connections to other monomers. The atoms which have been removed make a new small
molecule such as water or hydrogen chloride.
This is what happens in condensation polymerization.
Condensation polymers such as nylon (a polyamide) are formed from two different types of monomers. In
the case of nylon one type is a diamine which is a compound with an amine group at either end of the
molecule whilst the other type is a dicarboxylic acid which is a compound with an acid group at either end.
Examples of such monomers are:
diaminohexane (= diamine)
hexanedioic acid (= dicarboxylic acids)
In the box below show how the condensation polymerization occurs.
In the box below draw the repeated unit of a nylon polymer obtained from polymerizing the above
monomers.
Section 5 b and c: crude oil and synthetic polymers
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