C EN ERGY
Introduction
All societies depend on energy resources.
We extract energy from sunlight, plants,
petrochemicals, wind, water, and other sources
and convert it to forms that are useful to us;
however with each conversion the quality is
degraded as some of the available energy is
dispersed or converted to heat. C onverting
energy from one form to another in the
world around us results from potential and
kinetic energy changes at the molecular level.
Exothermic reactions can release potential energy
and raise the kinetic energy of the surrounding
molecules. The usefulness or quality of the
energy becomes lessened the more it is dispersed.
C.1 Energy sources
Understandings
 A useul energy source releases energy at




a reasonable rate and produces minimal
pollution.
The quality o energy is degraded as heat
is transerred to the surroundings. Energy
and materials go rom a concentrated into a
dispersed orm. The quantity o the energy
available or doing work decreases.
Renewable energy sources are naturally
replenished. Non-renewable energy sources
are f nite.
Energy density is energy released rom uel/
volume o uel consumed.
The e ciency o an energy transer is
expressed as useul energy output/
total energy input  100%.
Applications and skills
 Discuss the use o dierent sources o
renewable and non-renewable energy.
 Determine the energy density and speciic
energy o a uel rom the enthalpies o
combustion, densities and the molar mass
o uel.
 Discuss how the choice o uel is inluenced
by its energy density or speciic energy.
 Determine the eiciency o an energy
transer process rom appropriate data.
Nature of science
 Use theories to explain natural phenomena
energy changes in the world around us result
rom potential and kinetic energy changes at
the molecular level.
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C
ENERGY
Energy sources: quality and efciency
What makes a good energy source? It needs not only to contain a large
quantity o potential energy but also or this potential energy to be
released or converted, at a reasonable rate, to a useul orm with minimal
pollution and unwanted products. I the conversion is too ast a large
quantity o the energy is dispersed, while i it is too slow it is not useul.
The combustion o glucose is an exothermic reaction:
C 6 H 1 2 O 6 ( s) + 6O 2 ( g)  6C O 2 ( g) + 6H 2 O ( l)  H = - 2 803 kJ
The same amount o energy is released when glucose is burnt in a bomb
calorimeter as is released by its oxidation in the human body. The slower
rate o oxidation in the body allows the energy to be converted to a
useul orm whereas the rapid oxidation o combustion disperses the
energy too quickly, lowering its quality.
reaction chamber
(bomb) ignition
wire thermal
insulation
stirrer
electronic
thermometer
water
combustible material + oxygen
Figure 1 Combustion in a bomb calorimeter is rapid, resulting in the potential energy
being dispersed
The International Energy
Agency is an autonomous
organization that works to
ensure reliable, afordable, and
clean energy or its member
countries and beyond.
The distribution o available
energy among the particles o a
material is known as entropy. The
more diferent ways the energy
can be distributed, the higher the
entropy, and the less energy is
available to do useul work.
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The term quality o energy can have dierent meanings. Energy
companies, or example, may consider the cost per unit energy more
important than the efciency o its conversion. The efciency o producing
electricity rom burning coal averages approximately 3 0% worldwide. This
means that 3 0% o the available thermal energy produced rom burning
coal becomes electricity. There are also by-products including greenhouse
gases and pollutants. Nevertheless, according to the International
Energy Agency the cost o obtaining electricity rom coal is 7% less than
rom gas and 1 9% less than rom nuclear sources.
All energy conversions undergo some orm o quality degradation as
some o the energy is dispersed as heat. The energy and materials in the
original source change rom a concentrated to a dispersed orm and the
energy available to do useul work diminishes.
The more the quality o energy is degraded, the less efcient the uel is:
useul output energy
efciency o energy transer = __  1 00%
total input energy
C.1 En ErGy sOurCEs
Worked example
a) C ompare the efciency o coal, oil, or gas or
use in home heating. Table 1 gives some typical
efciencies or the conversions in the process.
Coveio
Coal
Oil
Ga
extraction o raw
material
processing to a
usable orm
transporting the uel
to a power station
chemical potential
energy to electricity
in a power plant
transmission o
electricity and
conversion to heat in
the house
0.67
0.35
0.72
0.92
0.88
0.97
0.98
0.95
0.95
0.35
0.35
0.35
0.90
0.90
0.90
Table 1 The efciency o some energy conversions in
the generation, distribution, and use o electrical energy
b) S uggest a reason why the efciencies o the
last two conversions are the same or each
energy source.
Solution
a) We need to combine (multiply) the efciencies
o all the processes rom extracting the uel to
converting electricity to heat in the home.
The efciency o coal as a uel is:
efciency = 0.67  0.92  0.98  0.3 5  0.90
= 0.1 9 or 1 9% efcient
81 % o the chemical potential energy available
in coal is dispersed and is not used in heating
the house.
You should be able to veriy or yoursel that
oil is only 9% and natural gas 2 1 % efcient.
b) Electricity is generated in the same way in
each power plant. The uel boils water to
produce steam; this turns turbines which
generate electricity. The energy losses are
approximately 65 % in each case: only 3 5 %
o the initial chemical potential energy is
converted to useul electrical energy. This
electrical energy is transported to the house
and converted to heat in a heater in the same
way irrespective o the initial uel source. In
this example, it is assumed that gas needs
to be used to generate electricity, and then
electricity is transmitted to homes. However,
gas can be transported to homes and burned
in gas urnaces ( table 2 ) , which increases the
efciency rom 2 1 % to about 5 6% .
Device
Eeg
taomatio
Efciec
electric
heater
electrical  thermal
nearly
100%
battery
chemical  electrical
~90%
home gas
urnace
(boiler)
chemical  thermal
~85%
home oil
urnace
(boiler)
chemical  thermal
~65%
home coal
urnace
(boiler)
chemical  thermal
~55%
solar cell
light  electrical
~15%
incandescent
light bulb
electrical  light
~5%
Table 2 The relative efciencies o some energy
conversions
Energy density and specifc energy
The energy density is a useul measure o the quality o a uel, that
compares the energy released per unit volume o uel:
energy released rom uel
energy density = ___
volume o uel consumed
Note that because the
defnitions o energy density
and specifc energy are energy
released per unit mass/
volume, these quantities do
not have a negative value.
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C
ENERGY

One kilogram of coal burnt
in a power plant can power
a 100 W light bulb for about
4 days.

energy released rom uel
specifc energy = ___
mass o uel consumed
One kilogram of natural gas
can power a 100 W bulb for
about 6 days.

In a similar way the specifc energy is the energy contained per unit
mass o a uel:
One kilogram of uranium-235
releasing energy in a nuclear
reactor can power a 100 W
bulb for 140 years.
Worked example
The standard enthalpy o combustion o carbon is - 3 94 kJ mol - 1 . The
density o anthracite, one o the purest coals, is 2 2 67 kg m - 3 . Use this
inormation along with the relative atomic mass o carbon to calculate
the energy density and specifc energy o this orm o coal, assuming it
to be 1 00% carbon.
Solution
study tip
specifc energy = - 3 94 kJ mol - 1 /1 2 .01 g mol - 1 = 3 2 .8 kJ g - 1
You should be able to convert
energy densities to any units
required, such as kJ cm - 3 . See
if you can verify that 7.44 
10 7 kJ m - 3 is 74.4 kJ cm - 3 .
convert to kJ kg - 1 : 3 2 .8 kJ g - 1  1 000 g kg - 1 = 3 2 800 kJ kg - 1
The Intenational renewable
Enegy Agency (IRENA) , based
in Abu Dhabi, UAE, was founded
in 2009 to promote increased
adoption and sustainable use
of renewable energy sources
(bioenergy, geothermal
energy, hydropower, ocean,
solar, and wind energy) .
energy density = 32 800 kJ kg -1  2267 kg m -3 = 7 435 7600 kJ m -3
E xpressed in scientifc notation to 3 S F ( as the enthalpy o combustion
was given to 3 SF) this is 7.44  1 0 7 kJ m - 3 .
Renewable energy resources
S ome renewable or green energy resources include solar energy, wind
energy, biomass, water ( such as tides, currents, and waves) , geothermal
energy, and uel cells.
Geothermal energy is one o the more widely used commercial orms o
renewable energy resources. Although it has an efciency o only about
2 3 % , as with all energy resources it is important to consider not only the
efciency o conversion but also the cost per kilowatt-hour.
Figure 2 A thermal energy production plant in Iceland. Iceland generates 100% of its energy
from renewable resources
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C. 2 FO ss I l Fu E ls
Questions
D ecide whether each o the ollowing is true
or alse.
1
Ethanol is a uel produced rom plant products by
ermentation. It has a density o 789 g dm - 3 and
its enthalpy o combustion is - 1 367 kJ mol-1 .
2
a) The energy conversion in an automobile is
to convert heat to kinetic energy.
a) C alculate the energy density or ethanol.
b) The conversion o heat to electricity is usually
more ef cient than that o electricity to heat.
b) C alculate the specif c energy or ethanol.
c) Write a balanced equation or the
combustion o ethanol and state the
amount, in mol, o carbon dioxide produced
per mole o ethanol burned.
c) The f nal conversion step in most
commercial power plants is work ( kinetic
energy) to electricity.
d) Explain why this method is still considered
green chemistry even though it produces
carbon dioxide in the combustion reaction.
d) Nuclear energy is a renewable resource.
e) Green energy resources are sustainable,
renewable, and produce low pollution.
C.2 Foi fe
Understandings
 Fossil uels were ormed by the reduction o





biological compounds that contain carbon,
hydrogen, nitrogen, sulur and oxygen.
Petroleum is a complex mixture o hydrocarbons
that can be split into diferent component parts
called ractions by ractional distillation.
Crude oil needs to be re ned beore use. The
diferent ractions are separated by a physical
process in ractional distillation.
The tendency o a uel to auto-ignite, which
leads to knocking in a car engine, is related
to molecular structure and measured by
the octane number. The perormance o
hydrocarbons as uels is improved by the
cracking and catalytic reorming reactions.
Coal gasi cation and liqueaction are chemical
processes that convert coal to gaseous and
liquid hydrocarbons.
A carbon ootprint is the total amount o
greenhouse gases produced during human
activities. It is generally expressed in
equivalent tons o carbon dioxide.
Applications and skills
 Explain the eect o chain length and chain




branching on the octane number.
Write equations or cracking and reorming
reactions, coal gasiication and liqueaction.
Identiy various ractions o petroleum based
on volatility and uses, their relative volatility
and their uses.
Discuss advantages and disadvantages o
dierent ossil uels.
Calculate carbon dioxide production, when
diferent uels burn and determine carbon
ootprints or diferent activities.
Nature of science
 Scienti c community and collaboration  the
use o ossil uels has had a key role in the
development o science and technology.
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