Fuels
Fuels are substances
which release usable energy either through an oxidation-reduction reaction with
an oxidiser or through nuclear fission or fusion. Fuels are our main source of
energy and are used in various ways – for example they are used to power cars
and many other forms of transport, to provide the heat for cooking and to
generate electricity.
There are various
types of oxidation-reduction reactions involving fuels. The simplest of these
are burning or combustion reactions where the fuel is burnt in oxygen. In this
case oxygen is the oxidiser. All combustion reactions are exothermic. That is
to say they release energy, mainly in the form of heat. Most electric power
stations use the burning of fuel in air to produce steam which is in turn used
to generate electricity.
Explosion is a
special type of combustion in which the fuel is exploded to release mechanical
energy. This is most often used in car engines. In fuel cell reactions a fuel
reacts in an electrochemical cell releasing electrical energy directly. Using
simple combustion reactions to indirectly produce electricity wastes as much as
70% of the energy released. Fuel cells, however, are much more efficient and
many consider them the future for generating electricity. A number of car manufacturers
have expressed an interest in developing fuel cells to power electric cars.
Hereon in only
fuels which release energy by burning in oxygen will be considered. When selecting
fuels to use there are many factors to consider – the waste products of burning
most fuels are harmful to the environment, some fuels are more expensive than
others and the energy output from fuels varies.
Various fuels will
now be considered and their advantages and disadvantages in a number of
circumstances made clear.
Hydrogen
At room
temperature hydrogen is a diatomic gas. This makes it difficult to store as in
its gaseous form it leaks readily and cannot be held in an open container.
Additionally unless it is held under high pressure it has a relatively low
density. Where it is being used as a fuel hydrogen is usually held under very
high pressure. This allows large amounts of hydrogen to be held in a small area
but makes storage more complex. Hydrogen reacts as follows:
2H2 + O2 → 2H2O
Hydrogen + Oxygen → Water
It can be seen
that hydrogen burns in oxygen to produce water alone; this means that no
harmful substances are released into the environment.
∆Hcөdiatomic
hydrogen = -285kJ mol-1
The molar mass of
diatomic hydrogen is 2.02g mol-1
Burning 2.02g (=
2.02 x 10-3kg) diatomic hydrogen emits releases 285kJ energy.
The energy density
of hydrogen = 285 ÷ 2.02 x 10-3kg = 141000 kJ kg-1. (3s.f.)
Ethane
Ethane is a gas at
room temperature and so has the same storage problems as hydrogen. Ethane
combusts completely as follows:
2C2H6 + 7O2 → 4CO2 + 6H2O
Ethane + Oxygen → Carbon
Dioxide + Water
As can be seen
this reaction produces carbon dioxide which in large quantities is harmful to
the environment and is a contributor to global warming. More dangerously if
this reaction is not sufficiently ventilated then the release of carbon
monoxide can result:
2C2H6 + 3O2 → 4CO + 6H2O
Ethane + Oxygen → Carbon
Monoxide + Water
Carbon monoxide is
toxic - it has a higher affinity for haemoglobin than oxygen and as a result
prevents the blood from carrying vital oxygen around the body. Worryingly to
warn of suffocation the body detects high concentrations of carbon dioxide
rather than low concentrations of oxygen. If carbon monoxide is bound to the
haemoglobin in the body the carbon dioxide concentration remains roughly
constant and the oxygen concentration in the blood can fall dramatically. The
body does not detect this and does not take actions to avoid further carbon
monoxide inhalation and suffocation results. Due to its almost undetectable
nature special care must be taken that ethane is oxidised completely and that
the reaction is well ventilated.
∆Hcөethane
= -1423 kJ mol-1
The molar mass of
ethane is 30.07g mol-1
Burning 30.07g (=
3.007 x 10-2kg) ethane emits releases 1423kJ energy.
The energy density
of ethane = 1423 ÷ 3.007 x 10-2kg = 47320 kJ kg-1. (4s.f.)
Butane
Butane is also a
gas at room temperature. It reacts with oxygen as follows:
C3H8 + 7O2 → 3CO2 + 4H2O
Butane + Oxygen → Carbon
Dioxide + Water
As with ethane and
indeed all the alkanes carbon dioxide and water are the end products and
incomplete combustion results in the production of toxic carbon monoxide.
∆Hcөbutane
= -2877 kJ mol-1
The molar mass of
ethane is 58.12g mol-1
Burning 58.12g (=
5.812 x 10-2kg) butane emits releases 2877kJ energy.
The energy density
of butane = 2877 ÷ 5.812 x 10-2kg = 49500 kJ kg-1. (4s.f.)
Octane
Octane is a liquid
at room temperature and therefore does not have the storage constraints
associated with gasses. It combusts as follows:
2C8H18 + 25O2 → 16CO2 + 18H2O
Octane + Oxygen → Carbon
Dioxide + Water
Again, being an
alkane, carbon dioxide and water are the products of the complete combustion of
octane and incomplete combustion results in the production of carbon monoxide.
∆Hcөoctane
= -5400 kJ mol-1 (2 s.f.)
The molar mass of octane
is 114.2g mol-1
Burning 114.2g (= 1.142
x 10-1kg) octane emits releases 5400kJ energy.
The energy density
of octane = 5400 ÷ 1.142 x 10-1kg = 47000 kJ kg-1 (2
s.f.)
Ethanol
Ethanol is a
liquid at room temperature and so can be stored easily. It burns in oxygen as
follows:
C2H6O + 3O2 → 2CO2 + 3H2O
Ethanol + Oxygen → Carbon
Dioxide + Water
As with the
alkanes carbon dioxide and water result from the complete combustion of
ethanol. Unfortunately incomplete combustion can also result inn the production
of carbon monoxide.
∆Hcөethanol
= -1368 kJ mol-1
The molar mass of
ethanol is 46.07g mol-1
Burning 46.07g (= 4.607
x 10-2kg) ethanol emits releases 1368kJ energy.
The energy density
of ethanol = 1368 ÷ 4.607 x 10-2kg = 29690 kJ kg-1 (4
s.f.)
Which is best?
Octane is the best
fuel in terms of energy produced per mole, however, in practical terms this
means next to nothing.
Hydrogen has by
far the best energy density and is the ‘cleanest’ in terms of products of
combustion. Being a gas it is difficult to store. I feel that hydrogen would be
suitable where a lot of fuel is required and the weight to energy ratio of the
fuel is important. Although the example is rather clichéd hydrogen would make a
good rocket fuel.
In many
applications it works out inefficient to store a gas for use as a fuel. In
circumstances where a liquid fuel should be used octane has the best energy
density.
Ethanol, although
having a relatively low energy density, is useful as it can be produced by
processes such as fermentation which do not rely on the fossil fuels are
rapidly running out.
In conclusion
there is no individual ‘best’ fuel, some properties of fuels are more important
in some situations than in others. As man’s supply of fossil fuels runs out and
the awareness of environmental issues rises different fuels will become more
frequently used.
Node your homework even if it makes for dull and incomplete nodes like this one :-P