Higher Chemistry
Unit 2(a)
Fuels
Petrol
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The gasoline fraction of crude oil is the main source
of petrol.
The gasoline fraction is obtained by the fractional
distillation of crude oil. The fractions are separated
according to their boiling point ranges.
The petrol fraction contains straight chain
hydrocarbon compounds which do not perform well
as a fuel in a petrol-burning engine.
Petrol engines versus diesel engines
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In a petrol engine, fuel is mixed with air,
compressed by pistons and ignited by sparks
from spark plugs.
In a diesel engine, however, the air is
compressed first, and then the fuel is injected.
Because air heats up when it's compressed,
the fuel ignites.
Reforming
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Because petrol from the gasoline fraction contains
only straight chain hydrocarbon compounds it autoignites and so does not burn efficiently.
To prevent auto-ignition (also known as ‘knocking’)
aromatic, branched and cyclic hydrocarbons are
added.
Reforming of the molecules in the naphtha fraction
is used to produce aromatic, cyclic and branched
chain hydrocarbons. These compounds are then
added to the compounds from the gasoline fraction
to produce petrol for use in petrol engines.
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Reforming alters the arrangement of atoms in
molecules. It may also alter the number of carbon
and/or hydrogen atoms in the molecule.
e.g. Straight chain – branched chain
CH3CH2CH2CH2CH2CH2CH2CH3
↓
CH3C(CH3)2CH2CH(CH3)CH3
Straight chain – aromatic [see Unit 2(b)]
C6H14 → C6H6 + 4H2
Straight chain – cyclic
C6H14 → C6H12 + H2
The removal of hydrogen from a molecule is
also known as dehydrogenation.
Volatility
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The volatility of a substance is the measure
of how quickly it evaporates. If a substance
has a high volatility it evaporates easily.
The hydrocarbon blend in petrol is adjusted in
summer and winter to ensure that it is volatile
enough for the temperatures.
In low temperatures more high-volatility
compounds are included like butane.
In higher temperatures more low volatility
compounds are included.
Octane rating
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Petrol's octane rating is a measurement of the fuel's
ability to resist engine knocking.
Knock occurs when the fuel-air mix in the cylinder
explodes instead of burning in a controlled way. This
shockwave moves within the combustion chamber,
and creates a metallic 'pinging' sound.
Knocking is inefficient and can cause damage to the
engine.
An octane rating is often referred to as an 'antiknock index'. If fuel has a high octane number, it will
have a higher resistance to engine knocking.
Lead and unleaded petrol
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Leaded petrol was banned from use in 2001
due to the harmful effects of the lead
compounds to the environment.
Leaded petrol contained the lead compound,
lead tetra-ethyl, which acted as an antiknocking agent.
Unleaded petrol contains aromatic, cyclic and
branched hydrocarbon compounds to prevent
auto-ignition.
Alternative fuels
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Currently, the majority of our fuels are derived
from fossil fuels which are finite and so are
going to run out.
Fuels from fossil fuels also contain carbon
which forms carbon dioxide when the fuel is
burned. Carbon dioxide cause global
warming.
Alternative renewable fuels include ethanol,
methanol, biogas and hydrogen.
Ethanol
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Ethanol, C2H5OH, is produced by fermentation of
sucrose from sugar cane.
Gasohol is a lead-free petrol containing between 10
– 20% ethanol.
Both ethanol and gasohol have a high octane rating.
Although ethanol contains carbon and so produces
carbon dioxide on burning, the sugar cane it is made
from takes in carbon dioxide during its growth.
Ethanol is therefore neutral in its contribution to
global warming.
Methanol
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Methanol is currently used as a fuel in racing
cars.
Advantages:
burns more efficiently than petrol so less
CO produced.
contains no carcinogens.
less volatile and explosive than petrol.
car engine requires little modification.
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Disadvantages
methanol absorbs water forming immiscible
layers which become corrosive to car
engines.
methanol is toxic.
less energy is produced than from the same
volume of petrol so bigger fuel tanks are
required.
methanol is made from synthesis gas which
is made from methane – a fossil fuel. This
increases greenhouse gases.
Biogas
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Biogas is produced when animal and
vegetable matter decay in the absence of
oxygen.
Biogas is a mixture of methane (approx.
60%) and carbon dioxide.
Sewage works can now be powered by the
biogas produced in the plant and the biogas
produced from decaying matter in landfill
sites can be used in local heating schemes.
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The burning of biogas does not contribute to
global warming since it is produced by the
decay of plant material which used up CO2
during photosynthesis.
Hydrogen
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Hydrogen is being seen as the new fuel of
the future when fossil fuels run out.
Hydrogen burns efficiently to produce only
water and no CO2 which contributes to global
warming.
Hydrogen is made by the electrolysis of
water.
Renewable energy sources such as solar
power are being investigated as the best way
to provide the energy required for
electrolysis.
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The major problem is the difficulty for a small
car to carry enough of it in liquid form (it must
be kept below -250oC) in well-insulated tanks.
This adds considerable weight to the vehicle.
Fuel cells are used on board spacecraft.
The fuel cell uses hydrogen and air in the
reverse of electrolysis to form water and
electrical energy.
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