Standard Grade Chemistry
Summary Notes
Topic 5: Fuels
Learning Outcomes
General
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A fuel is a chemical which burns giving out energy
Combustion is a reaction of a substance with oxygen, giving out energy
The test for oxygen is that it relights a glowing splint
The main components of air are oxygen and nitrogen in the proportion 1:4
An exothermic reaction is one in which energy is released
A finite resource is non-renewable. It will run out. Fossil fuels are finite
resources.
Coal, oil and natural gas are fossil fuels formed from ancient living things.
All fossil fuels are formed from the remains of living things which have been
buried underground over millions of years.
The pollution formed by the burning of fossil fuels includes sulphur dioxide,
nitrogen dioxide, soot and unburned hydrocarbons.
Carbon monoxide and carbon (soot) are formed by incomplete combustion,
because of a lack of oxygen.
Catalytic converters in car exhausts change harmful gases into harmless
gases.
Reducing the fuel-to-air ratio (increasing the air-to-fuel ratio) in car engines
means more complete combustion and less pollution.
The chemical compounds which are found in oil and natural gas are mainly
hydrocarbons.
A hydrocarbon is a compound which contains hydrogen and carbon only.
The test for carbon dioxide is that it turns lime water milky.
The test for water is that it freezes at 0 oC and boils at 100oC.
Crude oil is a mixture of compounds.
These compounds have specific melting and boiling points.
A fraction is a group of compounds with boiling points within a specific range.
Fractional distillation is the process used to separate crude oil into
fractions.
Fractional distillation separates crude oil into fractions depending on their
boiling points.
Flammability is the ease of which a fuel catches fire.
Viscosity is the thickness of a liquid.
The fractions in order of increasing boiling point are : fuel gases < petrol <
naptha < kerosene < gas oil < fuel oil < lubricating oils < bitumen.
As the boiling points of the fractions increases, their molecular size
increases.
As the molecular size of the fraction increases, their viscosity increases.
As the molecular size of the fraction increases, their flammability
decreases.
The balanced equation for a chemical reaction has the same number of atoms
of each element on each side.
Credit
Crude oil
fraction
fuel gases
gasoline
naphtha
kerosene
gas oil
fuel oil
lubricating oils
bitumen
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
Number of carbon
atoms
C1 to C4
C4 to C10
C6 to C12
C9 to C16
C15 to C25
C20 to C36
C34 to C70
> C70
Uses of fractions
portable stoves and heaters
petrol for cars
to make chemicals
paraffin and fuel for jet aircraft
diesel fuel for lorries and trains
domestic and industrial heating
machinery
tar for roads and roofs
The smallest molecules require less energy to separate and therefore
evaporate, so have lower boiling points.
The larger molecules require more energy to separate and therefore
evaporate, so have higher boiling points.
Fuels
A fuel is a chemical which burns giving out energy.
The type of chemical reaction which takes place when a fuel is burned is called
combustion. Combustion is the reaction of a substance with oxygen, giving out
energy. The fuel itself and oxygen are two of the three requirements necessary
for a combustion reaction to take place. A source of ignition, often a spark or a
little heat, is the third requirement. The three requirements are sometimes said
to form the three sides of the ‘Fire Triangle’.
Combustion reactions, where energy is given out during the reaction are called
exothermic reactions. Most of the energy released is in the form of heat
energy.
A good fuel is one which is cheap and readily available, which burns easily to
produce lots of energy and which produces little or no pollution.
A fuel burned in pure oxygen will burn better than the same fuel burned in air.
Air is composed of about 1/5th oxygen and 4/5th nitrogen. Although air contains
the oxygen necessary for combustion, the lower percentage of oxygen in air
means fewer collisions and a slower reaction. The test for oxygen gas is that it
will relight a glowing splint.
Many different fuels are forms of the element carbon or are compounds
containing carbon. Charcoal which is commonly used as a fuel on a barbecue, is a
form of the element carbon and is a good fuel. Natural gas is the gas we use in
our homes, schools and factories and is one of the commonest fuels in use today.
Natural gas is 96% methane. Methane is the simplest hydrocarbon and has the
chemical formula CH4. Hydrocarbons are compounds which contain hydrogen and
carbon only.
When the charcoal burns, the carbon atoms combine with oxygen to form carbon
dioxide. Carbon dioxide gives a cloudy/ milky colour when tested with lime
water.
Methane can be burned in the laboratory using a Bunsen burner. Because
methane contains carbon and hydrogen atoms, the oxygen required for
combustion can combine with both types of atom to form two different
products.
The carbon atoms combine with oxygen to form carbon dioxide and the
hydrogen atoms combine with oxygen to form water.
The lime water turns milky indicating that carbon dioxide has formed, and the
colourless liquid collected was found to boil at 100oC and freeze at 0oC. This
is the test for water.
Formula Equations
During a chemical reaction the atoms which make up the reactants get
rearranged to make the products.
In the combustion of methane, carbon dioxide and water are formed as
products.
methane
+
oxygen

carbon dioxide
+
water
+
H2O
This word equation can be rewritten as a formula equation.
CH4
+
O2

CO2
However, the formula equation does not fully represent what happened in the
reaction. It would appear that 2 atoms of hydrogen have been ‘lost’ during the
reaction. This is not the case.
Atoms can neither be ‘lost’ or ‘gained’ during a chemical reaction, only changed
from one substance to another. Therefore all the atoms which make up the
reactants should be found in the products.
The above formula equation is not balanced and must be balanced so that it can
truly represent what happens in the reaction.
Fossil Fuels
Natural gas is an example of a fossil fuel and is often found with crude oil,
another fossil fuel.
Crude oil is one of the most important sources of fuel in the world today. It is a
mixture of many different chemical compounds, mostly hydrocarbons, which
are found in fuels such as petrol, diesel and fuel oil.
Crude oil and natural gas are termed fossil fuels because they were formed
from things that were living millions of years ago. Coal is a third very
important fossil fuel.
Formation Of Crude Oil
Crude oil was formed from the remains of dead sea plants and animals. The
dead remains were covered by layers of sediment and over millions of years
they were affected by heat, pressure and bacteria and converted into crude
oil.
Natural gas and crude oil are finite resources. The word ‘finite’ means that they
cannot be replaced and will run out in time. Although there are greater
reserves of coal, supplies of all three of these very important fossil fuels are
eventually going to run out. This is such a major problem that it has been called
the ‘Energy Crisis’.
One thing that could be done is to use energy sources that are not going to run
out. These are called renewable sources e.g. wind power, wave power, solar
power etc. They are called alternative energy sources, because they are
alternatives to the fossil fuels which are running out.
Crude Oil
Crude oil is one of the most important sources of fuels in the world today
because it is a mixture of many different chemical compounds, mostly
hydrocarbons, which is found in these fuels.
Before these hydrocarbons can be used they have to be separated. This is done
in large scale oil refineries, as part of the whole process of refining the crude
oil. The separation of crude oil into its useful parts (or fractions) is called
fractional distillation.
Other parts of the refining process involve fractions being converted to others
and purifying of products.
A fraction collected will be a group of compounds with boiling points within a
range of temperatures.
Crude Oil Fractions
At an oil refinery, the crude oil, which is a mixture of hydrocarbons is
separated into fractions in a large tower called a fractionating column.
The crude oil is continuously fed into the bottom of the tower and
heated.
The components of the mixture evaporate and pass into the tower and
rise up. As they rise, they cool at different rates (due to their different
boiling points) and are constantly tapped off at different levels where
they condense.
The fractions collected through fractional distillation have many
different uses depending on their properties. Properties such as
flammability and viscosity must be considered.
Flammability is a measure of how easily a substance catches fire or
ignites.
Viscosity is a measure of the thickness and stickiness of a liquid. A
viscous liquid is one of high viscosity which due to its thickness and
stickiness will not flow easily.
Typical fractions obtained through fractional distillation of crude oil are:
Fraction
Number of
carbon
atoms in
typical
molecule
1 to 4
Boiling
point
range
(oC)
fuel gas
(refinery
gas)
gasoline / 5 to 12
naphtha
up to
20oC
kerosene 10 to 16
(paraffin)
120oC
to
240oC
240oC
to
350oC
gas oil
14 to 30
residue
25+
20oC to
120oC
350oC +
Flammability
Viscosity
& colour
Uses
more
difficult
to
ignite
becoming
thicker
and
darker
in
colour
bottled
gas
petrol,
making
chemicals
aviation
fuel,
paraffin
diesel
fuel,
lubricating
oil, central
heating
fuel
waxes,
bitumen
for roads
and roofs
The fraction with the lowest boiling point range is the most easily
evaporated. The fraction with the highest boiling point range is the most
difficult to evaporate.
It can also be seen from the table that the fractions with the lowest
boiling point ranges are the most flammable, the least viscous and the
lightest in colour.
It can be noted that the fractions with the bigger molecules have the
greater boiling point ranges. It is more difficult for the bigger, heavier
molecules to move fast enough and escape from the liquid.
For similar reasons, the bigger molecules also have greater viscosities as
the molecules are more likely to get tangled together.
Smaller molecules which are more likely to evaporate are more
flammable.
Coal
Coal is still burned extensively as a source of energy, particularly in power
stations.
Burning coal to obtain energy is however a rather wasteful way of using it as it
has many other uses e.g. as a source of coke for use in blast furnaces in the
steel industry or to produce a wide range of chemicals for making disinfectants,
insecticides, wood preservatives etc.
Coal like crude oil and natural gas is also a fossil fuel.
Formation of coal
Coal is formed from the remains of dead vegetation (plants and trees). The
dead remains were covered by layers of sediment and over millions of years
they were affected by heat, pressure and bacteria and converted into coal.
Oxides Of Carbon
When methane is burned, carbon dioxide and water are formed as products. In
fact when any hydrocarbon is burned completely only carbon dioxide and
water are produced.
hydrocarbon +
oxygen

carbon dioxide
+
water
However, when hydrocarbons burn there is not always a sufficient supply of
oxygen to ensure complete combustion. Unless you use a very efficient burner,
you will probably get some unburned carbon produced as soot. This can happen
with the combustion reactions in a Bunsen burner when it is used with the air
hole closed. You may have seen the sooty deposits on the bottom of beakers you
have heated. You can also see soot in the flame itself. The sootier the flame,
the more unburned carbon is being produced.
Another possible product of the incomplete combustion of carbon, or a fuel
containing carbon is carbon monoxide. This can occur in boilers for gas central
heating systems if the boiler is not properly ventilated.
The following table compares the properties of carbon dioxide ad carbon
monoxide.
chemical formula
appearance
pH
combustion reaction
lime water
dangers
carbon dioxide
carbon monoxide
CO2
colourless, odourless,
tasteless gas
slightly acidic
does not burn
turns lime water milky
can cause suffocation in
very large volumes
also thought to
contribute to Global
Warming through its
action as a Greenhouse
Gas
CO
colourless, odourless, tasteless
gas
neutral
burns to form CO2
no effect
very dangerous
extremely poisonous, combines
with haemoglobin in the blood
stopping oxygen transport,
leading to death through
oxygen starvation
Uses Of Carbon Dioxide
Carbon dioxide is commonly used as a fire extinguisher. It is very
effective for this purpose as it does not burn. The gas is heavier than air
which causes it to sink and lie over the fire like a blanket preventing the
necessary oxygen reaching the fire thus smothering the fire.
Solutions of carbon dioxide in water have a pleasant taste – soda water.
Most fizzy drinks are made by dissolving carbon dioxide in them at high
pressure.
Solid carbon dioxide ‘dry ice’ is used for refrigerating ice-cream, meat
and soft fruits. Dry ice is also used for producing smoke effects in
concerts.
Carbon dioxide is the gas used by green plants in the process known as
photosynthesis.
Pollution
Soot and carbon monoxide produced through the incomplete combustion of
carbon containing fuels contribute to pollution in the air around us. Several
other pollutants also result from burning fossil fuels.
Quite often hydrocarbon fuels from crude oil contain small amounts of
compounds containing the element sulphur. When the fuel is burned the sulphur
impurities cause problems.
The gas produced in the burning of sulphur is sulphur dioxide. It is a choking gas
causing irritation to the nose, throat and eyes.
Burning hydrocarbon fuels e.g. in power stations, can result in the poisonous gas
being released into the air, causing damage to health and property. Sulphur
dioxide also dissolves in rain water contributing to what is called acid rain.
The sulphur compounds can be removed from the fuels at the refinery. However,
although this reduces the amount of air pollution, it does increase the cost of
the fuel.
Car engines can contribute to pollution of the air in other ways. Pollutant gas are
produced in the cylinders of the car engines by the high energy sparks from the
spark plugs.
The spark of the engine produces enough energy to get the reaction between
nitrogen and oxygen from the air started. The ‘starting up’ energy is called the
activation energy of the reaction.
The nitrogen and oxygen in the air react together to form oxides of nitrogen
such as brown nitrogen dioxide gas. This is an acidic gas which dissolves in rain
water and also contributes to acid rain.
Acid rain results from sulphur dioxide and nitrogen dioxide dissolving to make
rain water acidic. The acid rain causes damage to crops, forests and life in
rivers, lakes and lochs.
Some types of petrol used in cars still contain lead compounds. The lead
compounds were added to the fuel to allow it to burn more smoothly, but end up
in the air as another dangerous pollutant.
Unburned hydrocarbons can also be passed out of the car’s exhaust system and
contribute to air pollution.
Summary of possible air pollutants from cars
 carbon monoxide
 sulphur dioxide
 oxides of nitrogen
 lead
 unburned hydrocarbons
Car Design
Car designers have to find ways to minimise the amount of pollution coming from
cars. Car manufacturers now build engines which operate with much improved air
to fuel ratios to ensure that sufficient oxygen is available to burn the fuel
more completely. Improving the combustion of the fuel in the engine through
improved air to fuel ratios goes some way to reducing the amount of pollutants
given out.
To further reduce the amount of pollutants in the exhaust gases a catalytic
converter is fitted within a special exhaust system. The catalytic converter is a
box on the exhaust system which consists of a steel or ceramic honeycomb
containing small amounts of transition metal elements. This helps change most of
the carbon monoxide, nitrogen oxides and unburned hydrocarbons into less
harmful carbon dioxide, nitrogen and water.
Most modern cars now use unleaded petrol. This means that for most modern
cars lead is no longer a pollutant. It is essential that cars fitted with catalytic
converters use unleaded petrol, otherwise the lead can poison the catalyst and
stop it working.