1 Topic 6 – National Chemistry Summary Notes Fuels Fuels and Combustion Coal, oil, gas and wood can all be used as fuels. These fuels have energy-rich chemical bonds which were created using the energy from the SUN. The energy is released when the fuel burns. When fuels burn they react with OXYGEN. This may also be referred to as OXIDATION. Scientists use the word COMBUSTION to describe the process of burning. Combustion is an example of an exothermic reaction i.e. one which releases energy. e.g. the combustion of carbon: C (s) + O 2 (g) CO 2 (g) (balanced) An ENDOTHERMIC reaction is one in which energy is taken in. The Fire Triangle The three things needed for burning are a fuel, oxygen and heat. These make up the three sides of the fire triangle. If you take away one side of the triangle, it will collapse. This means that if you deprive a fire of either, fuel, oxygen or heat, then it will go out. A FUEL is a substance which gives out ENERGY when it burns. Any compound with STORED energy can be considered as a fuel. LI 1 LI 2
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Topic 6 – National Chemistry Summary Notes
Fuels
Fuels and Combustion
Coal, oil, gas and wood can all be used as fuels. These fuels have energy-rich
chemical bonds which were created using the energy from the SUN. The energy
is released when the fuel burns.
When fuels burn they react with OXYGEN. This may also be referred to as
OXIDATION. Scientists use the word COMBUSTION to describe the process
of burning.
Combustion is an example of an exothermic reaction i.e. one which releases
energy.
e.g. the combustion of carbon:
C (s) + O2 (g) CO2 (g) (balanced)
An ENDOTHERMIC reaction is one in which energy is taken in.
The Fire Triangle
The three things needed for burning are a fuel, oxygen and heat. These make up
the three sides of the fire triangle.
If you take away one side of the triangle, it will collapse. This means that if you
deprive a fire of either, fuel, oxygen or heat, then it will go out.
A FUEL is a substance which gives out ENERGY when it burns.
Any compound with STORED energy can be considered as a fuel.
LI 1
LI 2
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Fossil Fuels
Coal, oil and natural gas are the three main fuels used in the UK today.
They are called the FOSSIL FUELS because they were formed from
once LIVING THINGS.
Fossil fuels such as oil and gas are examples of HYDROCARBON compounds
with minor impurities (coal is mostly just carbon). Hydrocarbons are compounds
made up of hydrogen and carbon only.
Even although there are large amounts of coal, oil and gas in the world today,
they will eventually run out. For this reason fossil fuels are called FINITE
resources because supplies are limited.
Formation of Coal
The formation of coal started around 300 million years ago. It is made from
dead plants and fern like trees that became buried in swampy land.
Pressure and temperature gradually made coal.
Formation of Oil & Natural Gas
Crude oil and natural gas came from microscopic sea animals and plants that
died and became covered by sediment. Again pressure and temperature over
millions of years changed the once living organisms into crude oil and natural gas.
Uses of Fossil Fuels
The fossil fuels are a useful reserve of fuels and are used to satisfy the
demands of an energy-dependent world e.g.
Electricity: The most common use of fossil fuels is the generation of electricity.
Fossil fuels like coal run most of the power plants in the world that generate
electricity.
Transportation: Most modes of transport are powered by coal and petrol.
Though hybrid and electric vehicles have made inroads into our lives, fossil fuels
still power millions of vehicles.
Industrial production: Fossil fuels, especially coal and oil, power most of the
industries throughout the world.
Kitchens: Nearly one-third of world's population uses fossil fuels like coal and
biomass, (such as wood and dung) for cooking and heating.
Heating: During winters, fossil fuels serve as excellent heat generators.
LI 3
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Fractional Distillation of Crude Oil
The oil which comes out our oil wells is a black, sticky liquid with a very
unpleasant smell. It is called CRUDE OIL. It contains a rich mixture of valuable
chemicals called HYDROCARBONS.
The problem for chemists is how to separate the hydrocarbons in crude oil.
However, each hydrocarbon has its own BOILING POINT. If the crude oil is
slowly heated then each different hydrocarbon will boil and turn into a GAS at
different temperatures. The different gases can be collected separately. When
the gases cool they CONDENSE and turn into LIQUIDS. Each separate liquid is
called a FRACTION.A fraction is defined as a mixture of hydrocarbons with
similar boiling points. This whole process is called FRACTIONAL
DISTILLATION.
The first fractions which are obtained consist of GASES then they become
light coloured LIQUIDS and after that darker in colour liquids and thicker. The
final fraction which is left is known as BITUMEN RESIDUE and is the sticky
material used for building roads.
Fractional Distillation in the Lab
The set-up below shows how fractional distillation can be carried out in the lab.
LI 4
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The purpose of the air condenser is to cool the gases into liquids to allow for
collection.
The first fractions are volatile, light in colour, runny and highly flammable.
The later fractions are darker in colour, viscous and not so flammable.
definitions - volatile =
flammability =
viscosity =
evaporates easily
how easily a substance catches fire
how thick a liquid is
Fraction Boiling
Range
(ºC)
Number of
Carbon
Atoms per
Molecule
Colour Viscosity Flammability Speed of
Evaporation
Gases
Petrol
Naphtha
Kerosene (paraffin)
Gas oils (diesel)
Residue
-106–20
20 - 65
65 - 180
180–250
250–350
> 350
1 - 4
5 - 6
6 - 11
9 – 15
15 – 25
> 25
Gases
Clear, light coloured liquid
Pale yellow
Dark yellow
Dark Brown
LOW
RUNNY LIQUID
HIGHLY VISCOUS
(very thick)
Catches fire
easily
Catches fire with difficulty
Does not
catch fire
FAST
SLOW
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Uses of Fractions
FRACTION USE
Refinery Gas Cooking, heating
Petrol Car fuel
Naphtha Chemicals e.g. plastics, solvents
Kerosene Jet fuel
Diesel (gas oils) Heavy transport, some cars
Lubricating oils Engine oil, candle wax
Bitumen Building roads, roofing
The following saying can help us to remember the order and names of the
fractions obtained from the fractional distillation of crude oil.
Giant Planes Need Kerosine During Our Boarding
G
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Liquefied petroleum
gas
Chemicals
Petrol for vehicles
Jet fuel, paraffin
for heating and
lighting
Diesel fuels
Lubricating oils,
waxes and polishes
Fuels for
ships,
factories
and central
heating
Bitumen for roads
and roofing
Fractions
decreasing in
density and
boiling point
Crude oil
Fractions
increasing
in density
and
boiling
point
Fractional Distillation in Industry
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Properties of Crude Oil Fractions
The fractions obtained from crude oil have different properties because they
contain hydrocarbons of different sizes. The simplest way to think about the
size of a hydrocarbon is to count the number of carbon atoms in the molecule.
a) Boiling Points
The first fractions that are distilled have low boiling points. This is because
they consist of small molecules.
There are small forces of attraction that exist between molecules. These
forces are bigger and stronger for large molecules. It takes more energy to
separate large molecules and therefore larger molecules have higher boiling
points.
b) Viscosity (thickness)
The fractions containing bigger molecules have a higher viscosity. This is
because they are made of long chains of atoms which tend to get tangled up and
clump together.
c) Flammability
Small hydrocarbons are more flammable than larger molecules i.e. they catch
fire more easily. This is because small molecules can react more quickly with
oxygen in the air.
d) Ease of Evaporation
Small molecules evaporate more easily than larger molecules since they have
much lower boiling points.
LI 5
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Combustion of a Hydrocarbon
This apparatus can be used to show the combustion products of a hydrocarbon.
The burning wax candle produces gases which are drawn through the apparatus
by the pump.
Results
1. The limewater turns milky indicating the presence of carbon dioxide gas.
2. The drops of condensation in the u-tube boil at 100ºC and freeze at 0⁰C. This
indicates the presence of water.
Complete Combustion
hydrocarbon + oxygen carbon dioxide + water
e.g. complete combustion of methane gas - CH4
CH4 (g) + O2 (g) CO2 (g) + H2O (l) (unbalanced)
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) (balanced)
Hydrocarbons burn in a plentiful supply of oxygen to produce
CARBON DIOXIDE and WATER. This is called
COMPLETE COMBUSTION.
LI 6
u-tube
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Incomplete combustion
Often when a fuel burns there is not enough oxygen to produce carbon dioxide
gas (CO2). Instead, the gas carbon monoxide, (CO) is formed. Carbon monoxide
is a poisonous gas. In large amounts it can kill by destroying the blood’s ability
to carry oxygen.
hydrocarbon + limited carbon monoxide + water
oxygen
Catalytic Converters
In car engines, catalytic converters can be used to minimise the output of
carbon monoxide. This device fits on to the exhausts of cars and converts
harmful gases into harmless ones.
However, they have no beneficial effect on carbon dioxide levels which
contribute to GLOBAL WARMING.
Fuels and Pollution
POLLUTANT
MAIN SOURCE EFFECT
Carbon monoxide Vehicle engines and
industry
Poisonous
Sulphur dioxide Burning fossil fuels in
power stations
Forms acid rain
Hydrocarbons Burning fuels in vehicles
and factories
Irritating, toxic
compounds formed in air
Oxides of nitrogen (NOx)
Vehicle engines Forms acid rain
Lead compounds
Used to be added to
petrol to make it burn
more smoothly
Toxic and can cause brain
damage
In a limited supply of oxygen, hydrocarbons burn to produce
carbon monoxide gas. This is called INCOMPLETE COMBUSTION.
LI 7
LI 8
LI 9
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Global Warming and the Greenhouse Effect
The increasing level of carbon dioxide in the atmosphere is a cause for concern.
As the amount of carbon dioxide in the atmosphere increases more of the suns
energy is retained. This causes the average temperature of the earth’s surface
to rise. This is known as the GREENHOUSE EFECT. Scientists still argue about
the effect this will have but it is thought some parts of the world will warm up
by around 4⁰C whilst other parts may cool down. This may well cause flooding in
certain areas due to an increase in the levels of rivers and seas.
Burning fuels for heating, for cars and other vehicles and for making electricity
in power stations all produces carbon dioxide which adds to the greenhouse
effect.
How much CO2 does
your lifestyle emit
each year?
LI 10
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Reducing Carbon Dioxide Emissions
Alternative Energy Sources
Fossil fuels are in short supply and will eventually run out, for this reason they
are called NON RENEWABLE sources of energy.
Other methods of generating electricity need to be developed. These include
using the sun’s energy, wind, water and wave action. These methods provide an
unlimited supply and they will not run out. They are called RENEWABLE or