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Combustion, fuels and hydrocarbons
When a substance burns,
it is said to combust.
Combustion is a rapid
reaction between a
substance and oxygen
that releases heat and
light energy.
A fuel is a substance that reacts with oxygen (combusts) to
release useful energy.
Many fractions obtained from crude oil are used as fuels
because they contain hydrocarbons that burn easily and
release a large amount of useful energy.
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Complete combustion of hydrocarbons
Plenty of air is needed to provide enough oxygen for a
hydrocarbon fuel to burn completely.
The blue flame of a gas hob
or a Bunsen burner is an
example of complete
combustion of a hydrocarbon
(in this case, natural gas).
What are the products of the
complete combustion of a hydrocarbon?
hydrocarbon
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+
carbon
oxygen 
dioxide
+
water
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Complete combustion of propane
Propane is a hydrocarbon used in camping gas. What is the
equation for its combustion?
propane
C3H8
+
+
carbon
oxygen 
dioxide
5O2

3CO2
+
+
water
4H20
How would the equations change if butane was used?
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Products of combustion
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Incomplete combustion of hydrocarbons
If there is a shortage of air (oxygen), incomplete combustion
of hydrocarbons takes place.
Instead of producing just
carbon dioxide and water,
incomplete combustion
also produces carbon
monoxide and/or carbon
(soot). It also releases less
energy than complete
combustion.
Carbon monoxide is a poisonous gas because it reduces the
ability of blood to carry oxygen.
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Alternative fuels
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Alternative fuels
How soon will we be using alternative fuels?
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The need for alternative fuels
Most vehicles in the world use petrol or diesel as a fuel.
These are produced from crude oil, a fossil fuel.
Although fossil fuels are convenient
sources of energy, they are very
polluting, and will one day run out.
As a result, some people have
already begun using alternative
fuels to power their vehicles, such
as biofuels and hydrogen.
Why is it important to develop and use these fuels before oil
supplies run out?
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What are biofuels?
Biofuels are renewable fuels
produced from plant material,
such as agricultural crops.
Two types of biofuel used in
vehicles are bioethanol and
biodiesel.
They can be safely combined
with normal petrol or diesel
and used in conventional
engines to reduce levels of
harmful emissions without
causing engine damage.
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What is bioethanol?
Bioethanol is an alcohol produced by the natural fermentation
of the carbohydrates (such as starch) in sugar beet/cane or
wheat crops.
‘Flexi-Fuel’ vehicles, fitted with modified fuel injection
systems, can run on E85 fuel (85% bioethanol, 15% petrol),
which cuts carbon dioxide emissions by 70% compared to
normal petrol-engine cars.
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What is biodiesel?
Biodiesel is produced by chemically reacting vegetable oils
or animal fats with alcohol and a catalyst. The process can
be completed in 12 hours.
Biodiesel can be mixed with
conventional diesel, which
significantly reduces
emissions, especially toxic
hydrocarbons, particulates
and carbon monoxide.
There are few garages in the UK that sell biodiesel. Homemade fuels, usually from waste vegetable oils, are heavily
taxed.
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Advantages of biofuels
What are some of the advantages of using biofuels?
 Biofuels are carbon neutral:
the carbon released during
combustion comes from the
carbon dioxide the plants took
in when they were growing.
 Storage, transport and
distribution costs are low as
biofuels can be handled in the
same way as conventional fuels.
 By-products of production, such as pressed seedcake, can
be burnt in power stations instead of fossil fuels or used a
animal feed.
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Disadvantages of biofuels
What are some of the disadvantages of using biofuels?
 Although biofuels themselves produce relatively little when
combusted, their production needs energy from fossil fuels.
 There are few UK producers of biofuels, and only small
quantities of fuel are made. Biofuels therefore need to be
imported, mainly from Brazil and South-East Asia.
 The high demand for
land to plant biofuel
crops can lead to
deforestation and
habitat loss, for
example in Malaysia.
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What are hydrogen fuel cells?
Hydrogen fuel cells are electrochemical devices like batteries,
which produce electricity by converting hydrogen and oxygen
into water.
A big advantage of
hydrogen fuel cells is
that they do not create
any pollution: the only
emission they produce
is water vapour.
The chemicals inside fuel cells are constantly replenished, so
unlike batteries, fuel cells never run flat or need recharging.
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Disadvantages of fuel cells
What are some of the disadvantages of fuel cells?
 The production, storage and distribution of hydrogen for
use in fuel cells relies heavily on energy from fossil fuels.
 Pure hydrogen is expensive and highly flammable.
One way around these
problems is to replace
hydrogen with a more
convenient substance.
Methanol, natural gas
and propane have all
shown potential for use
in fuel cells.
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Opinions on alternative fuels
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Glossary (1/2)
 biodiesel – A biofuel made by chemically reacting plant
oils or animal fats with alcohol and a catalyst.
 bioethanol – A biofuel made from the natural fermentation
of the carbohydrates in plants such as sugar cane/beet and
wheat.
 biofuel – A renewable fuel made from biological material,
such as plant matter.
 combustion – A rapid reaction between a substance and
oxygen that produces heat and light energy.
 complete – The type of combustion that only produces
carbon dioxide and water.
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Glossary (2/2)
 fuel – A substance that provides useful energy when it
burns.
 fuel cell – A device that produces electricity from a
chemical reaction, e.g. between hydrogen and oxygen.
 hydrocarbon – A molecule containing only hydrogen and
carbon.
 incomplete – The type of combustion that produces
carbon monoxide and/or soot in addition to carbon dioxide
and water.
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Anagrams
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Multiple-choice quiz
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