Section 3.1.6 Alkanes

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• Section 3.1.6 Alkanes
• know that alkanes are saturated hydrocarbons
• know that petroleum is a mixture consisting
  mainly of alkane hydrocarbons
• understand that different components (fractions)
  of this mixture can be drawn off at different
  levels in a fractionating column because of the
  temperature gradient
• understand that cracking involves the breaking of
  CC bonds in alkanes
• know that thermal cracking takes place at high
  pressure and high temperature and produces a high
  percentage of alkenes (mechanism not required)
• know that catalytic cracking takes place at a
  slight pressure, high temperature and in the
  presence of a zeolite catalyst and is used mainly
  to produce motor fuels and aromatic hydrocarbons
  (mechanism not required)
• understand the economic reasons for the cracking
  of alkanes (e.g. ethene used for poly(ethene)
  conversion of heavy fractions into higher value
  products)
• know that alkanes are used as fuels and
  understand that their combustion can be complete
  or incomplete and that the internal combustion
  engine produces a number of pollutants (e.g. NOx,
  CO and unburned hydrocarbons)
• know that these pollutants can be removed using
  catalytic converters
• know that combustion of hydrocarbons containing
  sulfur leads to sulfur dioxide that causes air
  pollution and understand how sulfur dioxide can
  be removed from flue gases using calcium oxide
• know that the combustion of fossil fuels
  (including alkanes) results in the release of
  carbon dioxide into the atmosphere
• know that carbon dioxide, methane and water
  vapour are referred to as greenhouse gases and
  that these gases may contribute to global warming

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Alkanes and isomersim
Alkanes with four or more carbon atoms display
structural isomerism because the carbon chain may
be either straight or branched.
2-methylbutanebranched chain
pentanestraight chain
The naming of alkanes depends on whether they are
straight or branched.
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Trends in boiling points
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Trends in boiling points
The boiling point of straight-chain alkanes
increases with chain length due to increasing van
der Waals forces between molecules.
As the length of the chain increases, so does its
surface area, and so the van der Waals forces are
stronger.
Branched-chain alkanes have lower boiling points
because the chains cannot pack as closely
together. There are fewer points of contact
between molecules so the van der Waals forces are
weaker.
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Crude oil and alkanes
Crude oil is a mixture composed mainly of
straight and branched chain alkanes.
It also includes lesser amounts of cycloalkanes
and arenes, both of which are hydrocarbons
containing a ring of carbon atoms, as well as
impurities such as sulphur compounds.
The exact composition of crude oil depends on the
conditions under which it formed, so crude oil
extracted at different locations has different
compositions.
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Fractional distillation
What is distillation?
Distillation is a method of separating mixtures
usually liquids - based on differences in the
boiling points of the components of the mixture.
What is fractional distillation?
The process in which the components of a mixture
are separated into groups or fractions of
different boiling points ranges.
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Fractional distillation
a fractionating column
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Fractional distillation
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Uses of the fractions
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Uses of the fractions
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Uses of the fractions
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Uses of the fractions
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Fractions and their uses
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C2.5.2
Basics of Organic Chemistry
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C2.5.2 Targets
By the end of these two pages you should be able
to

• describe the process of cracking
• recall that when alkanes are cracked, mixtures of
  alkanes and alkenes are formed
• explain the differences between alkanes and
  alkenes
• describe how bromine water is used to show if
  something is an alkane or an alkene
• explain how ethene can be reacted with water to
  make ethanol.

H
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Cracking of Hydrocarbons
Crude oil is separated into its parts by
FRACTIONAL DISTILLATION
Hydrocarbons, e.g. alkanes and alkenes, are
extracted from....
CRUDE OIL
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Cracking of Hydrocarbons
TOO LITTLE!
TOO MUCH!
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Cracking of Hydrocarbons
Heavy oil
Petrol
PETROL
HEAVY FUEL
PETROL
HEAVY FUEL
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Cracking of Hydrocarbons
Ethene
Octane
Decane
Smaller molecules
C2H4
aluminium oxide
500ºC
Large molecules
C8H18
C10H22
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Cracking of Hydrocarbons
decane
C10H22
octane
C8H18
ethene
C2H4
SATURATED contains only single bonds
UNSATURATED contains a double bond
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Cracking of Hydrocarbons
Cracking reaction summary
C10H22(g) 
  C8H18(g)     C2H4(g)
saturated saturated
unsaturated
The (g) is called a state symbol and stands
for gas
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Fractions and their uses
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Supply and demand
Demand for lower boiling point (short chain)
fractions is greater than the proportion in crude
oil.
Crude oil contains more higher b.p. (longer
chain) fractions, which are in lower demand and
less economically valuable...
So, theres a shortage of shorter chain fractions
and a surplus of longer chain ones.
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What is cracking?
Cracking is a process that splits long chain
alkanes into shorter chain alkanes, alkenes and
hydrogen.
C10H22 ? C7H16 C3H6
Cracking has the following uses

• it increases the amount of gasoline and other
  economically important fractions

• it increases branching in chains, an important
  factor for petrol

• it produces alkenes, an important feedstock for
  chemicals.

There are two main types of cracking thermal and
catalytic.
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Cracking
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Cracking
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Cracking
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Cracking
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Cracking
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Cracking
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Thermal v Catalytic cracking
Catalytic cracking has several advantages over
thermal cracking

• it produces a higher proportion of branched
  alkanes, which burn more easily than
  straight-chain alkanes and are therefore an
  important component of petrol

• the use of a lower temperature and pressure mean
  it is cheaper

• it produces a higher proportion of arenes, which
  are valuable feedstock chemicals.

However, unlike thermal cracking, catalytic
cracking cannot be used on all fractions, such as
bitumen, the supply of which outstrips its demand.
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Other products from cracking
Alkenes such as ethene are always produced in
cracking. They are an important feedstock for use
in the chemical industry, particularly in the
production of polymers.
Arenes such as benzene are also produced during
catalytic cracking. Benzene is added in small
quantities to petrol as a replacement for the
lead compounds. It too is now the subject of
health concerns, and its use is being reduced.
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Complete combustion
Complete combustion
In excess oxygen, short chain alkanes can undergo
complete combustion
alkane oxygen ? carbon dioxide water
For example
propane oxygen ? carbon dioxide water
C3H8(g) 5O2(g) ? 3CO2(g) 4H2O(g)
The combustion of alkanes is a highly exothermic
process. This makes them good fuels because they
release a relatively large amount of energy per
gram of fuel.
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Incomplete combustion
If oxygen is limited then incomplete combustion
will occur
alkane oxygen ? carbon monoxide water
alkane oxygen ? carbon water
For example
propane oxygen ? carbon monoxide water
C3H8(g) 3½O2(g) ? 3CO(g) 4H2O(g)
propane oxygen ? carbon water
C3H8(g) 2O2(g) ? 3C(s) 4H2O(g)
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The internal combustion engine carbon
Alkanes with chain lengths of 510 carbon atoms
are used as fuels in internal combustion engines.
This releases carbon dioxide into the atmosphere
nonane oxygen ? carbon dioxide water
C9H20(g) 14O2(g) ? 9CO2(g) 10H2O(g)
Although modern internal combustion engines are
more efficient than in the past, incomplete
combustion still occurs
nonane oxygen ? carbon monoxide water
2C9H20(g) 19O2(g) ? 18CO(g) 20H2O(g)
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The internal combustion engine nitrogen
The temperature in an internal combustion engine
can reach over 2000 C. Here, nitrogen and
oxygen, which at normal temperatures dont react,
combine to form nitrogen monoxide
N2(g) O2(g) ? 2NO(g)
Nitrogen monoxide reacts further forming nitrogen
dioxide
2NO(g) O2(g) ? 2NO2(g)
Nitrogen dioxide gas reacts with rain water and
more oxygen to form nitric acid, which
contributes to acid rain
4NO2(g) 2H2O(l) O2(g) ? 4HNO3(aq)
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The catalytic converter
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The catalytic converter
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The catalytic converter
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The catalytic converter
CO, NOx, HC
CO2, N2, H2O
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The catalytic converter
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Sulphur contamination of fossil fuels
Sulphur is found as an impurity in crude oil and
other fossil fuels. It burns in oxygen to form
sulphur dioxide
S(s) O2(g) ? SO2(g)
Sulphur dioxide may be oxidized to sulphur
trioxide
2SO2(g) O2(g) ? 2SO3(g)
Both of these oxides dissolve in water forming
acidic solutions
SO2(g) H2O(l) ? H2SO3(aq)
SO3(g) H2O(l) ? H2SO4(aq)
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What is acid rain?
Acid rain is caused by acidic non-metal oxides
such as sulfur oxides and nitrogen oxides
dissolving in rain water.
Rain water is naturally acidic because carbon
dioxide dissolves in it, forming weak carbonic
acid.
Sulphfur and nitrogen oxides form more acidic
solutions, which can damage trees and affect
aquatic life in lakes and rivers.
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Removing sulfur dioxide pollution
Sulphur dioxide emissions from vehicle fuels such
as petrol and diesel are reduced by removing
nearly all of the sulphur impurities from the
fuel before it is burnt.
Removing the sulphur from coal before it is burnt
is not practical. Instead, the acidic sulphur
oxides are removed from the waste gases using a
base such as calcium oxide.
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Carbon dioxide in the atmosphere
Burning fossil fuels releases carbon dioxide into
the atmosphere.
Fossil fuels are being burned faster than they
are being formed, which means that their
combustion leads to a net increase in the amount
of atmospheric carbon dioxide.
It has been suggested that increases in the
amount of carbon dioxide and other greenhouse
gases may be responsible for apparent changes to
the climate.
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Greenhouse gases
Carbon dioxide, water vapour and methane have
been described as the main greenhouse gases.
This is because these have been suggested as the
gases responsible for the majority of the
greenhouse effect.
The greenhouse effect is a theory that has been
suggested to explain apparent rises in the
average temperature of the Earth.
Increasing the amount of any of the greenhouse
gases traps more heat energy from the Sun in the
Earths atmosphere, raising the average
temperature.
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END