Chemistry
& CHEMICAL REACTIVITY
SIXTH EDITION
John C. Kotz
SUNY Distinguished Teaching Professor
State University of New York
College at Oneonta
Paul M. Treichel
Professor of Chemistry
University of Wisconsin–Madison
Gabriela C. Weaver
Associate Professor of Chemistry
Purdue University
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About the Cover
What lies beneath the Earth’s surface? The mantle of the Earth
consists largely of silicon-oxygen based minerals. But about 2900 km
below the surface the solid silicate rock of the mantle gives way to
the liquid iron alloy core of the planet. To explore the nature of the
rocks at the core-mantle boundary, scientists in Japan examined
magnesium silicate (MgSiO3) at a high pressure (125 gigapascals)
and high temperature (2500 K). The cover image is what they saw.
The solid consists of SiO6 octahedra (blue) and magnesium ions
(Mg2+; yellow spheres). Each SiO6 octahedron shares the four O
atoms in opposite edges with two neighboring octahedra, thus forming a chain of octahedra. These chains are interlinked by sharing the
O atoms at the “top” and “bottom” of SiO6 octahedra in neighboring
chains. The magnesium ions lie between the layers of interlinked
SiO6 chains. For more information see M. Murakami, K. Hirose,
K. Kawamura, N. Sata, and Y. Ohishi, Science, Volume 304, page 855,
May 7, 2004.
The Chemistry of Fuels
and Energy Sources
Charles D. Winters
Gabriela C. Weaver
Supply and Demand: The Balance Sheet on Energy
E
nergy is necessary for everything we do. Look around you—
energy is involved in anything that is moving or is emitting
light, sound, or heat (Figure 1). Heating and lighting your home,
propelling your automobile, powering your portable CD
player—all are commonplace examples in which energy is consumed and all are, at their origin, based on chemical processes.
In this part of the text, we will examine how chemistry is fundamental to understanding and addressing current energy issues.
283
• With only 4.6% of the world’s population, the United States
consumes 25% of all the energy used in the world. This usage is equivalent to the consumption of 7 gallons of oil or 70
pounds of coal per person per day.
Two basic issues, energy consumption and energy resources, instantly leap out from these statistics. They form the basis for this
discussion of energy.
Charles D. Winters
Energy Consumption
Figure 1 Energy-consuming devices. Our lives would not be the same
without the heat and light in our homes and without our automobiles,
computers, cell phones, music players, stoves, and refrigerators.
Supply and Demand: The Balance
Sheet on Energy
We take for granted that energy is available and that it will always
be there to use. But will it? Recently, chemist and Nobel Prize
winner Richard Smalley stated that among the top 10 problems
humanity will face over the next 50 years, the energy supply ranks
as number one. What is the source of this dire prediction?
Information such as the following is often quoted in the
popular press:
• Global demand for energy has tripled in the past 50 years
and may triple again in the next 50 years. Most of the demand comes from industrialized nations.
• Fossil fuels account for 85% of the total energy used on our
planet. Nuclear and hydroelectric power each contribute
about 6% of the total energy budget. The remaining 3%
derives from biomass, solar, wind, and geothermal energygenerating facilities.
Methane hydrate, a potential fuel source. Methane, CH4, can be trapped in
a lattice of water molecules, but the methane is released when the pressure
is reduced. See Figure 6 on page 287.
Data indicate that energy consumption is related to the degree to
which a country has industrialized. The more industrialized a
country, the more energy is consumed on a per capita basis. Although some people express worries about the disproportionate
use of energy by developed nations, an equally serious concern is
the rate of growth of consumption worldwide. As a higher degree
of industrialization occurs in developing nations, energy consumption worldwide will increase proportionally. The rapid
growth in energy usage over the last half-century is strong evidence in support of predictions of similar growth in the next halfcentury.
One way to alter consumption is through energy conservation.
Energy conservation is a small part of today’s energy equation, although it has drawn greater attention recently (Figure 2). Some
examples where energy conservation is already important are described here:
• Aluminum is recycled because recycling requires only one
third of the energy needed to produce aluminum from
its ore.
• Light-emitting diodes (LEDs) are being used in streetlights
and compact fluorescent lights are finding wider use in the
home. Both use a fraction of the energy required for incandescent bulbs (in which only 5% of the energy used is
returned in the form of light; the remaining 95% is wasted as
heat).
• Hybrid cars offer twice the gas mileage available with conventional cars.
We can be sure that energy conservation will continue to
contribute to the world’s energy balance sheet. Science and technology can be expected to introduce a variety of new energysaving devices in coming years.
One of the exciting areas of current research in chemistry
relating to energy conservation focuses on superconductivity. Superconductors are materials that, at temperatures of 90–150 K,
offer virtually no resistance to electrical conductivity (see “The
Chemistry of Modern Materials,” page 642). When an electric
current passes through a typical conductor such as a copper wire,
some of the energy is inevitably lost as heat. As a result, there is
substantial energy loss in power transmission lines. Substituting a
superconducting wire for copper has the potential to greatly decrease this loss, so the search is on for materials that act as superconductors at moderate temperatures.
284
The Chemistry of Fuels and Energy Sources
Charles D. Winters
In addition, we have become accustomed
to an energy system based on fossil fuels. The
internal combustion engine is the result of
years of engineering. It is now well understood
and can be produced in large quantities
quickly and for a relatively low cost. The electric grid is well established to supply our buildings and roads. Natural gas supply to our
homes is nearly invisible. The system works
well.
But here is the root of the problem alluded to by Richard Smalley: Fossil fuels are
nonrenewable energy sources. Nonrenewable
resources are those in which the energy source
is used and not concurrently replenished.
Fossil fuels are the obvious example. Nuclear
energy is also in this category (although the
supply of nuclear fuels appears, for the moment, not likely to be used up in the conceivable future and breeder reactors can use
other, even more abundant sources to create
Figure 2 Energy-conserving devices. Energy efficient home appliances,
nuclear fuel). Conversely, energy sources that involve the sun’s
hybrid automobiles, and compact fluorescent bulbs all provide alternatives that
consume less energy than their conventional counterparts.
energy are renewable resources. These include solar energy and
energy derived from winds, biomass, and moving water. Likewise,
geothermal energy is a renewable resource.
Energy Resources
There is a limited supply of fossil fuels. No more sources are
being created. As a consequence, we must ask how long our fossil
On the other side of the energy balance sheet are energy resources,
fuels will last. Regrettably, there is not an exact answer to this quesof which many exist. The data cited earlier make it obvious that we
tion. One current estimate suggests that the world’s oil reserves
are hugely dependent on fossil fuels as a source of energy. The
will be depleted in 30–80 years. Natural gas and coal supplies are
percentage of energy obtained and used from all other sources is
projected to last longer. The estimated life of natural gas reserves
small relative to that obtained from fossil fuels. We rely almost enis 80–200 years, whereas coal reserves are projected to last from
tirely on gasoline and diesel fuel in transportation. Fuel oil and
150 to several hundred years. These numbers are highly uncertain,
natural gas are the standards for heating, and approximately 70%
however. In part, this is because the estimates are
of the electricity in the United States is generbased on guesses regarding fuel reserves not yet
ated using fossil fuels, mostly coal (Table 1).
Table 1 Producing Electricity
discovered; in part, it is because assumptions
Why is there such a dominance of fossil
in the United States
must be made about the rate of consumption in
fuels on the resource side of the equation? An
Coal
52%
future years.
obvious reason is that fossil fuels are cheap raw
Nuclear
21%
Despite our current state of comfort with
materials compared to other energy sources. In
Natural gas
12%
our energy system, we cannot ignore the fact that
addition, humans have made an immense inRenewable sources
7%
a change away from fossil fuels must occur somevestment in the infrastructure needed to distribPetroleum
3%
day. As supply diminishes and demand increases,
ute and use this energy. Power plants using coal
it will become necessary to expand the use of
or natural gas cannot be converted readily to acCombining heat and power*
5%
other fuel types. The technologies for doing so,
commodate another fuel. The infrastructure for
*Cogeneration facilities using fossil
and the answers regarding which alternative fuel
distribution of energy—gas pipelines, gasoline
fuels that yield both electricity and
types will be the most efficient and cost-effective,
dispensing for cars, and the grid distributing
heat. See Chemical and Engineering
can be provided by chemistry research.
electricity to users—is already set in place. Much
News, p. 21, February 23, 2004.
of this infrastructure may have to change if the
source of energy changes. Some countries already have energy
distribution systems that do not depend nearly as much as the
U.S. system on fossil fuels. For example, countries in Europe
Fossil fuels originate from organic matter that was trapped under
(such as France) make much greater use of nuclear power, and
the earth’s surface for many millennia. Due to the particular
certain regions on the planet (such as Iceland and New Zealand)
combination of temperature, pressure, and available oxygen, the
are able to exploit geothermal power as an energy source.
Fossil Fuels
285
Fossil Fuels
decomposition process from the basic compounds that constito 95%, with variable amounts of hydrogen, oxygen, sulfur, and
nitrogen being bound up in the coal in various forms.
tute organic matter resulted in the hydrocarbons that we extract
and use today: coal, crude oil, and natural gas—the solid, liquid,
Sulfur is a common constituent in some coals. The element
and gaseous forms of fossil fuels, respectively. These hydrowas incorporated into the mixture partly from decaying plants
carbons have varying ratios of carbon to
and partly from hydrogen sulfide, H2S, which
hydrogen.
is the waste product from certain bacteria. In
Fossil fuels are simple to use and relaaddition, coal is likely to contain traces of
Table 2 Energy Released by Combustion
tively inexpensive to extract, compared with
many other elements, including some that
of Fossil Fuels
the current cost requirements of other
are hazardous (such as arsenic, mercury, cadEnergy
sources for the equivalent amount of energy.
mium, and lead) and some that are not (such
Substance
Released (kJ/g)
To use the energy stored in fossil fuels, these
as iron).
Coal
29–37
materials are burned. The combustion
When coal is burned, some of the impuCrude petroleum
43
process, when it goes to completion, converts
rities are dispersed into the air and some end
Gasoline
hydrocarbons to CO2 and H2O (Section 4.2).
up in the ash that is formed. In the United
(refined petroleum)
47
States, coal-fired power plants are responsiThe heat evolved is then converted to meNatural gas
ble for 60% of the emissions of SO2 and 25%
chanical and electrical energy (Chapter 6).
(methane)
50
Energy output from burning fossil fuels
of mercury emissions into the environment.
varies among these fuels (Table 2). The heat
SO2 reacts with water and O2 in the atmoevolved on burning is related to the carbonsphere to form sulfuric acid, which conto-hydrogen ratio. We can analyze this relationship by considertributes (along with nitric acid) to the phenomenon known as
ing data on heats of formation and by looking at an example that
acid rain.
is 100% carbon and another that is 100% hydrogen. The oxidation of 1.0 mol (12.01 g) of pure carbon produces 393.5 kJ of
2 SO2(g) O2(g) ¡ 2 SO3(g)
heat or 32.8 kJ per gram.
SO3(g) H2O(/) ¡ H2SO4(aq)
C(s) O2(g) ¡ CO2(g)
¢ H° 393.5 kJ/mol C or 32.8 kJ/g C
Burning hydrogen to form water is much more exothermic, with
about 120 kJ evolved per gram of hydrogen consumed.
H2(g) 12 O2(g) ¡ H2O(g)
¢ H° 241.8 kJ/mol H2 or 119.9 kJ/g H2
Coal is mostly carbon, so its heat output is similar to that of
pure carbon. In contrast, methane is 25% hydrogen (by weight)
and the higher-molecular-weight hydrocarbons in petroleum
and products refined from petroleum average 16–17% hydrogen
content. Therefore, their heat output on a per-gram basis is
greater than that of pure carbon, but less than that of hydrogen
itself.
While the basic chemical principles for extracting energy
from fossil fuels are simple, complications arise in practice. Let
us look at each of these fuels in turn.
Because these acids are harmful to the environment, legislation
limits the extent of sulfur oxide emissions from coal-fired plants.
Chemical scrubbers have been developed that can be attached to
the smokestacks of power plants to reduce sulfur-based emissions. However, these devices are expensive and can increase the
cost of the energy produced from these facilities.
Coal is classified into three categories (Table 3). Anthracite,
or hard coal, is the highest-quality coal. Among the forms of
coal, anthracite has the highest heat content per gram and a low
sulfur content. Unfortunately, anthracite coal is fairly uncommon, with only 2% of the U.S. coal reserves occurring in this
form (Figure 3). Bituminous coal, also referred to as soft coal,
accounts for about 45% of the U.S. coal reserves and is the coal
most widely used in electric power generation. Soft coal typically
has the highest sulfur content. Lignite, also called brown coal because of its paler color, is geologically the “youngest” form of
coal. It has a lower heat content than the other forms of coal, often contains a significant amount of water, and is the least popular as a fuel.
Coal
The solid rock-like substance that we call coal began to form almost 290 million years ago, when swamp plants died. Decomposition occurred to a sufficient extent that the primary component
of coal is carbon. Describing coal simply as carbon is a simplification, however. Samples of coal vary considerably in their composition and characteristics. Carbon content may range from 60%
Table 3 Types of Coal
Type
Consistency
Sulfur Content
Heat Content (kJ/g)
Lignite
Very soft
Very low
28–30
Bituminous coal
Soft
High
29–37
Anthracite
Hard
Low
36–37
The Chemistry of Fuels and Energy Sources
© Tim Wright/Corbis
286
Figure 3 Anthracite coal. This form of coal has the highest energy
content of the various forms of coal.
Coal can be converted to coke by heating in the absence of
air. Coke is almost pure carbon and an excellent fuel. In the
process of coke formation, a variety of organic compounds are
driven off. These compounds are used as raw materials in the
chemical industry for the production of polymers, pharmaceuticals, synthetic fabrics, waxes, tar, and numerous other products.
Technology to convert coal into gaseous fuels (coal gasification) (Figure 4) or liquid fuels (liquefaction) has also been developed. These processes provide fuels that will burn more cleanly
than coal, albeit with a loss of 30–40% of the net energy content per
gram of coal along the way. As petroleum and natural gas reserves
dwindle, and the costs of these fuels increase, liquid and gaseous
fuels derived from coal are likely to become more important.
carbons may have anywhere from one carbon atom to 20 or more
such atoms in their structures, and compounds containing sulfur,
nitrogen, and oxygen may also be present in small amounts.
Petroleum goes through extensive processing at refineries to
separate the various components and convert less valuable compounds into more valuable components. Nearly 85% of the
crude petroleum pumped from the ground ends up being used
as a fuel, either for transportation (gasoline and diesel fuel) or
for heating (fuel oils).
The high temperature and pressure used in the combustion
process in automobile engines have the unfortunate consequence of also causing a reaction between atmospheric nitrogen
and oxygen that results in some NO formation. In a series of
exothermic reactions, the NO can then react further with oxygen
to produce nitrogen dioxide. This poisonous, brown gas is further oxidized to form nitric acid, HNO3, in the presence of water.
N2(g) O2 (g) ¡ 2 NO(g)
2 NO(g) O2 (g) ¡ 2 NO2 (g)
3 NO2 (g) H2O(/) ¡ 2 HNO3(/) NO(g)
¢ H°rxn 180.58 kJ
¢ H°rxn 114.4 kJ
¢ H°rxn 71.4 kJ
To some extent, the amounts of pollutants released can be
limited by use of catalytic converters. Catalytic converters are
high-surface-area metal grids that are coated with platinum or
palladium. These very expensive metals can catalyze a complete
combustion reaction, helping to combine oxygen in the air with
unburned hydrocarbons or other byproducts in the vehicle exhaust. As a result, the combustion products can be converted to
Natural Gas
Petroleum
Petroleum is a complicated mixture of hydrocarbons, whose molar masses range from low to very high (page 495). The hydro-
© Courtesy of Oak Ridge National Laboratory
Natural gas is found deep under the earth’s surface, where it was
formed by bacteria working on organic matter in an anaerobic
environment (in which no oxygen is present). The major component of natural gas (70–95%) is methane (CH4). Lesser quantities of other gases such as ethane (C2H6), propane (C3H8), and
butane (C4H10) are also present, along with other gases including
N2, He, CO2, and H2S. The impurities and higher-molecularweight components of natural gas are separated out during the
refining process, so that the gas piped through gas mains into
our homes is primarily methane.
Natural gas is an increasingly popular choice as a fuel. It
burns more cleanly than the other fossil fuels, emits fewer pollutants, and produces relatively more energy than the other fossil fuels. Natural gas can be transported by pipelines over land and
piped into buildings such as your home to be used directly to heat
ambient air, to heat water for washing and bathing, or for cooking.
Figure 4 Coal gasification plant. Advanced coal-fired power plants, such
as this 2544-ton-per-day coal gasification demonstration pilot plant, will
have energy conversion efficiencies 20% to 35% higher than those of conventional pulverized-coal steam power plants.
water and carbon dioxide (or other oxides), provided they land
on the grid of the catalytic converter before exiting the vehicle’s
tailpipe. Some nitric acid and NO2 inevitably remain in automobile exhaust, however, and they are major contributors to environmental pollution in the form of acid rain and smog. The
brown, acidic atmospheres in highly congested cities such as Los
Angeles, Mexico City, and Houston largely result from the emissions from automobiles (Figure 5). The pollution problems have
led to stricter emission standards for automobiles, and a high
priority in the automobile industry is the development of lowemission or emission-free vehicles. Another approach is provided
by the increasing popularity of hybrid vehicles, which use a combination of gasoline and electricity to run, thereby reducing the
gasoline consumption per mile.
Other Fossil Fuel Sources
287
©Reuters/Corbis
Fossil Fuels
Figure 5 Smog. The brown cloud that hangs over Santiago, Chile contains
nitrogen oxides emitted by millions of automobiles in that city. Other
compounds are also present, such as ozone (O3), nitric oxide (NO2), carbon
monoxide (CO), and water.
normal pressure and temperature) is about 165 times larger than
the volume of the hydrate.
If methane hydrate forms in a pipeline, is it found in nature
as well? In May 1970, oceanographers drilling into the seabed off
the coast of South Carolina pulled up samples of a whitish solid
that fizzed and oozed when it was removed from the drill casing.
They quickly realized it was methane hydrate. Since this original
a, John Pinkston and Laura Stern/U.S. Geological Survey/Science News,
11-9-96; c, Charles Fisher, The Pennsylvania State University
When natural gas pipelines were laid across the United States
and Canada, pipeline operators soon found that, unless water
was carefully kept out of the line, chunks of methane hydrate
would form and clog the pipes. Methane hydrate was a completely unexpected substance because it is made up of methane
and water, two chemicals that would appear to have little affinity
for each other. In methane hydrate, methane becomes trapped
in cavities in the molecular structure of ice (Figure 6). Methane
hydrate is stable only at temperatures below the freezing point of
water. If a sample of methane hydrate is warmed above 0° C, it
melts and methane is released. The volume of gas released (at
(a) Methane hydrate burns as
methane gas escapes from
the solid hydrate.
(b) Methane hydrate consists of a
lattice of water molecules with
methane molecules trapped in
the cavity.
(c) A colony of worms on an outcropping of
methane hydrate in the Gulf of Mexico.
Figure 6 Methane hydrate. (a) This interesting substance is found in huge deposits hundreds of feet down on the floor of the ocean. When a sample is
brought to the surface, the methane oozes out of the solid, and the gas readily burns. (b) The structure of the solid hydrate consists of methane molecules
trapped within a lattice of water molecules. Each point of the lattice shown here is an oxygen atom of a water molecule. The edges are O ¬ H ¬ O bonds. Such
structures are often called “clathrates” and are mined for substances other than methane. (c) An outcropping of methane hydrate on the floor of the Gulf of
Mexico. See E. Suess, G. Bohrmann, J. Greinert, and E. Lausch: Scientific American, pp. 76–83, November 1999.
288
The Chemistry of Fuels and Energy Sources
discovery, methane hydrate has been found in many parts of the
oceans as well as under permafrost in the Arctic. It is estimated
that 1.5 1013 tons of methane hydrate is buried under the sea
floor around the world. In fact, the energy content of this gas
may surpass that of all the other known fossil fuel reserves by as
much as a factor of 2! Clearly, this is a potential source of an
important fuel in the future. Today, however, the technology to
extract methane from these hydrate deposits is very expensive,
especially in comparison to the well-developed technologies used
to extract crude oil, coal, and gaseous methane.
There are other sources of methane in our environment.
For example, methane is generated in swamps, where it is called
swamp gas or marsh gas. Here, methane is formed by bacteria
working on organic matter in an anaerobic environment—
namely, sedimentary layers of coastal waters and in marshes. The
process of formation is similar to the processes occurring eons
ago that generated the natural gas deposits that we currently use
for fuel. In a marsh, the gas can escape if the sediment layer is
thin. You see it as bubbles rising to the surface. Unfortunately, because of the relatively small amounts generated, it is impractical
to collect and use this gas as a fuel.
In a striking analogy to what occurs in nature, the formation
of methane also occurs in human-made landfill sites. A great deal
of organic matter is buried in landfills. Because it remains out of
contact with oxygen in the air, this material is degraded by bacteria. In the past, landfill gases have been deemed a nuisance. Today, it is possible to collect this methane and use it as a fuel. In a
pilot plant at the Rodefeld Landfill site near Madison, Wisconsin,
a collection system for the methane produced in the landfill has
been set up. The gas is used to generate electricity that is sold
back to the local electric utility. In 2002, the methane gas collected at this facility was used to produce approximately 12 million kilowatt-hours of electricity, enough to power about 1700
homes for a year.
bustion-based energy production, with up to 60% energy conversion efficiency compared to 20–25% for electricity generation
from combustion.
Fuel cells are not a new discovery. In fact, the first fuel cell
was demonstrated in 1839, and fuel cells have been used in the
Space Shuttle. Fuel cells are currently under investigation for use
in homes and in automobiles.
The basic design of fuel cells is quite simple. Oxidation and
reduction take place in two separate compartments. [Recall the
definitions of oxidation and reduction (page 197): Oxidation is
the loss of electrons from a species, whereas reduction occurs
when a species gains electrons.] These compartments are connected in a way that allows electrons to flow from the oxidation
compartment to the reduction compartment through a conductor such as a wire. In one compartment, a fuel is oxidized, producing positive ions and electrons. The electrons move to the
other compartment, where they react with an oxidizing agent, typically O2. The spontaneous flow of electrons in the electrical circuit constitutes the electric current. While electrons flow through
the external circuit, ions move between the two compartments so
that the charges in each compartment remain in balance.
The net reaction is the oxidation of the fuel and the consumption of the oxidizing agent. Because the fuel and the oxidant never come directly in contact with each other, there is no
combustion and no loss of energy as heat. The energy of the reaction is converted directly into electricity.
Hydrogen is the fuel employed in the fuel cells on board the
Space Shuttle. The overall reaction in these fuel cells involves the
combination of hydrogen and oxygen to form water (Figure 7).
Hydrocarbon-based fuels such as methane (CH4) and methanol
e
Energy in the Future:
Choices and Alternatives
Fuel Cells
To generate electricity from the combustion of fossil fuels, the
energy is used to create high-pressure steam, which spins a turbine in a generator. Unfortunately, not all of the energy from
combustion can be converted to usable work. Some of the energy
stored in the chemical bonds of a fuel is lost as heat to the surroundings, making this an inherently inefficient process. A much
more efficient process would be possible if mobile electrons, the
carriers of electricity, could be generated directly from the chemical bonds themselves, rather than going through an energy conversion process from heat to mechanical work to electricity.
Fuel cell technology makes direct conversion of chemical
potential energy to electricity possible. Fuel cells are similar to
batteries, except that fuel is supplied from an external source
(Figure 7 and Section 20.3). They are more efficient than com-
Electrical energy output
e
e
Hydrogen
fuel
e
H2
H
H2
H
H H
Oxygen
from air
O2
H2O
H2O
Unused
fuel
ANODE
PROTON
EXCHANGE
MEMBRANE
2 H2 88n 4 H 4 e
Water
CATHODE
O2 + 4 H 4 e 88n 2 H2O
Figure 7 Hydrogen-oxygen fuel cell. The cell uses hydrogen gas, which
is converted to hydrogen ions and produces electrons. The electrons flow
through the external circuit and are consumed by the oxygen, which, along
with H+ ions, produces water.
289
Energy in the Future: Choices and Alternatives
(CH3OH) are also candidates for use as the fuel in fuel cells; for
these compounds the reaction products are CO2 and H2O. When
methanol is used in fuel cells, for example, the net reaction in
the cell is
2 CH3OH(/) 3 O2(g) ¡ 2 CO2(g) 4 H2O(/)
¢ H°rxn 727 kJ/mol CH3OH or 23 kJ/g CH3OH
Using heat of formation data (Section 6.8), we can calculate that the energy generated is 727 kJ/mol (or 23 kJ/g) of liquid methanol. That is equivalent to 200 watt-hours (W-h) of
energy per mol of methanol (1 W 1 J/s), or 5.0 kW-h per liter
of methanol. This means that oxidation of one liter of
methanol in a fuel cell could theoretically provide more than
5000 W of power over a 24-hour period, enough to keep about
70 standard desk lamps lit.
Prototypes of phones and laptop computers powered by fuel
cells have been developed recently. Small methanol cartridges
are used to fuel them. These devices are no bigger than a standard AA battery, yet they last up to 10 times longer than standard
rechargeable batteries.
Note, however, that fuel cells do not provide a new source of
energy. They require fuel to produce energy and are constructed
to use currently available fuels. The merits of fuel cells derive
from their greater efficiency of use and from their environmentally friendly nature.
Of course, there are many practical problems, including the
following as-yet-unmet needs:
• An inexpensive method of producing hydrogen
• A practical means of storing hydrogen
• A distribution system (hydrogen refueling stations)
Perhaps the most serious problem in the hydrogen economy
is the task of producing hydrogen. Hydrogen is abundant on
earth, but not as the free element. Thus, elemental hydrogen has
to be obtained from its compounds. Currently, most hydrogen is
produced industrially from the reaction of natural gas and water
by steam-reforming at high temperature (Figure 8).
Steam re-forming CH4(g) H2O(g) ¡ 3H2(g) CO(g)
¢ H°rxn 206.2 kJ/mol CH4
Hydrogen can also be obtained from the reaction of coal and water at high temperature (water gas reaction).
Water gas reaction C(s) H2O(g) ¡ H2(g) CO(g)
¢ H°rxn 131.3 kJ/mol C
Both reactions are highly endothermic, however, and both rely
on use of a fossil fuel as a raw material. This, of course, makes no
sense if the overriding goal is to replace fossil fuels.
Fuel
enters
A Hydrogen Economy
Predictions about the diminished supply of fossil fuels have led
some people to speculate about other alternative fuels. In particular, hydrogen, H2, has been suggested as a possible choice. The
term hydrogen economy has been coined to describe the overall
strategy using this fuel. As was the case with fuel cells, the hydrogen economy does not rely on a new energy resource; it merely
provides a different scheme for use of existing resources.
There are reasons to consider hydrogen an attractive option,
however. Oxidation of hydrogen yields almost three times as
much energy per gram as the oxidation of fossil fuels. Comparing
hydrogen combustion with combustion of propane, a fuel used
in some cars, we find that H2 produces about 2.6 times more heat
per gram than propane.
Exhaust
Ambient air
Combustion
chamber
Impurities
Hydrogen
to fuel cell
H2(g) 12 O2 (g) ¡ H2O(g)
¢ H°rxn 241.83 kJ/mol H2 or 119.95 kJ/g H2
C3H8(g) 5 O2 (g) ¡ 3 CO2 (g) 4 H2O(g)
¢ H°rxn 2043.15 kJ/mol C3H8 or 46.37 kJ/g C3H8
Another advantage of using hydrogen instead of a hydrocarbon
fuel is that the only product of H2 oxidation is H2O, which is environmentally benign.
Thus, for several reasons it is relatively easy to imagine hydrogen replacing gasoline in automobiles and replacing natural
gas in heating homes. It is similarly easy to imagine using hydrogen to generate electricity or as a fuel for industry.
Steam re-former
Hydrogen
purification
chamber
Figure 8 Steam re-forming. A fuel such as methanol (CH3OH) or a hydrocarbon and water are heated and then passed into a steam re-former chamber. There a catalyst promotes their decomposition to hydrogen and other
compounds such as CO. The hydrogen gas passes out to a fuel cell, and the
CO and unused carbon-based compounds are burned in a combustion chamber. A small unit may be suitable for a car or light truck.
290
The Chemistry of Fuels and Energy Sources
If the hydrogen economy is ever to take hold, the logical
source of hydrogen is water.
H2O(/) ¡ H2(g) 12 O2(g)
¢ H°rxn 285.83 kJ/mol H2O(/)
H2 gas
Metal hydride
Electrolyte
Metal
adsorbed
hydrogen
The electrolysis of water provides hydrogen but also requires
considerable energy. The first law of thermodynamics tells us that
we can get no more energy from the oxidation of hydrogen than
we expended to obtain H2 from H2O. Hence, the only way to obtain hydrogen in the amounts that would be needed is to use a
cheap and abundant source of energy to drive this process. A logical candidate is solar energy. Unfortunately, the technology to
use solar energy in this way has yet to become practical. Here is a
problem for chemists and engineers of the future to solve.
Hydrogen storage represents another problem to be solved.
A number of ways to accomplish this storage in a vehicle, in your
home, or at a distribution point have been proposed. An obvious
way to store hydrogen is as the gas under moderate conditions,
but this approach would be impractical because the volumes
occupied would be too large (Figure 9). In addition, storing hydrogen at high pressure or as a liquid (bp 252.87 ° C) would
require special equipment, and safety is a key issue.
One possibility known to chemists relies on the fact that certain metals will absorb hydrogen reversibly (Figure 10). When a
metal absorbs hydrogen, H atoms fill the holes, called interstices,
between metal atoms in a metallic crystal lattice. Palladium, for
example, will absorb up to 935 times its volume of hydrogen. This
hydrogen can be released upon heating, and the process of absorption and release can be repeated.
Another reversible system under study involves hydrogen
storage in carbon nanotubes (page 31). Researchers have found
that the carbon tubes absorb 4.2 weight percent of H2; that is,
they achieve an H : C atom ratio of 0.52 under a moderately high
Solid solution
a-phase
Hydride phase
b-phase
Figure 10 Hydrogen adsorbed onto a metal or metal alloy. Many metals
and metal alloys reversibly absorb large quantities of hydrogen. On the left
side of the diagram, H2 molecules are adsorbed onto the surface of a metal.
The H2 molecules can dissociate into H atoms, which form a solid solution
with the metal (a-phase). Under higher hydrogen pressures, a true hydride
forms in which H atoms become H ions (b-phase). On the right side, H atoms
can also be adsorbed from solution if the metal is used as an electrode in an
electrochemical device.
pressure. Just as importantly, 78.3% of the hydrogen can be released under ambient pressure at room temperature, and the
remainder can be released with heating.
There are several chemical methods of reversible hydrogen
storage as well. For example, heating NaAlH4, doped with titanium dioxide, releases hydrogen and the NaAlH4 can be rejuvenated by adding hydrogen under pressure.
2 NaAlH4(s) ¡ 2 NaH 2 Al(s) 3 H2(g)
4 kg
4 kg
Mg2NiH4
4 kg
LaNi5H6
Metal hydrides
No matter how hydrogen is used, it has to be delivered to vehicles and homes in a safe and practical manner. Work has also
been done in this area (Figure 11), but many problems remain to
be solved. European researchers have found that a tanker truck
that can deliver 2400 kg of compressed natural gas (mostly
methane) can deliver only 288 kg of H2 at the same pressure. Although hydrogen oxidation delivers about 2.4 times more energy
per gram (119.95 kJ/g) than methane,
4 kg
CH4(g) 2 O2(g) ¡ CO2(g) 2 H2O(g)
¢ H°rxn 802.30 kJ/mol or 50.14 kJ/g
Liquefied hydrogen
(below 250 °C)
Pressurized
hydrogen gas
(at 200 bar)
Figure 9 Comparison of the volumes required to store 4 kg of hydrogen relative to the size of a typical car. (L. Schlapbach and A. Züttel:
Nature, Vol. 414, pp. 353–358, 2001.)
the tanker can carry about 8 times more methane than H2. That
is, it will take more tanker trucks to deliver the hydrogen needed
to power the same number of cars or homes running on hydrogen than those running on methane.
How close are we to the realization of a hydrogen economy?
Not very near, and it is not clear whether it will ever come to pass.
Energy in the Future: Choices and Alternatives
291
Martin Bond/Science Photo Library/Photo Researchers, Inc.
Biosources of Energy
Figure 11 A prototype hydrogen-powered BMW. The car is being
refueled with hydrogen at a distribution center in Germany. Note the solar
panels in the background.
C2H5OH(g) 2 H2O(g) 12 O2(g) ¡ 2 CO2(g) 5 H2(g)
The heat of this reaction is approximately 70 kJ per mole of
ethanol (or about 1.5 kJ/g).
© 2002 Corbis
G.A. DeLuga, J.R. Salge, L.D. Schmidt, and X.E. Verykios, Science,
vol. 303, 2/13/2004, pp. 942 and 993
There is one interesting example in which the hydrogen economy has gained a real toehold. In 2001, Iceland announced that
the country would become a “carbon-free economy.” Icelanders
plan to rely on hydrogen-powered electric fuel cells to run vehicles and fishing boats. Iceland is fortunate in that two thirds of its
energy already comes from renewable sources—hydroelectric
and geothermal energy (Figure 12). The country has decided to
use the electricity produced by geothermal heat or hydroelectric
power to separate water into hydrogen and oxygen. The hydrogen will then be used in fuel cells or combined with CO2 to make
methanol, CH3OH, a liquid fuel that can either be burned or be
used in different types of fuel cells.
Gasoline sold today often contains ethanol, C2H5OH. In addition
to being a fuel, ethanol serves to improve the burning characteristics of gasoline.
Ethanol is readily made by fermentation of glucose from renewable resources such as corn or sugar cane. While it may not
emerge as the sole fuel of the future, this material is likely to contribute to the phasing-out process of fossil fuels and may be one
of multiple fuel sources in the future.
There are several interesting points to make about ethanol
as a fuel. Green plants use the sun’s energy to create biomass
from CO2 and H2O by photosynthesis. The sun is a renewable resource, as, in principle, is the ethanol derived from biomass. In
addition, the process recycles CO2. Plants use CO2 to create biomass, which is in turn used to make ethanol. In the final step in
this cycle, oxidation of ethanol returns CO2 to the atmosphere.
Recent research on ethanol has taken this topic in a new direction. Namely, ethanol can be used as a source of hydrogen. It
is possible to create hydrogen gas from ethanol by using a steam
re-forming process like the methane-related process. The recently developed method involves the partial oxidation of
ethanol mixed with water in a small fuel injector, like those used
in cars to deliver gasoline, along with rhodium and cerium catalysts to create hydrogen gas exothermically (Figure 13). The net
reaction is
Figure 12 Iceland, a “carbon-free,” hydrogen-based economy.
A geothermal field in Iceland. The country plans to use such renewable
resources to produce hydrogen from water and then to use the hydrogen
to produce electricity in fuel cells.
Figure 13 Hydrogen from ethanol. Ethanol can be obtained by fermentation from corn. In a prototype reactor (right), ethanol, water, and oxygen are
converted by a catalyst (glowing white solid) to hydrogen (and CO2).
292
The Chemistry of Fuels and Energy Sources
2 CO2 2 C2H5OH 4 H2O
20 kJ/mol
C6H12O6 4 H2O(6 O2)
140 kJ/mol
O2
Energy input from sun
for photosynthesis
6 CO2 10 H2
2540 kJ/mol
2420 kJ/mol
5 O2
6 CO2 10 H2O
Figure 14 An energy-level diagram for the reactions leading from the
production of biomass (glucose) to CO2 and H2. (Based on a Figure in
G. A. DeLuga, J. R. Salge, L. D. Schmidt, and X. E. Verykios: Science, Vol. 303,
pp. 942 and 993, 2004).
To examine the efficiency of this process, we must analyze the
overall energy cycle, starting with the photosynthesis of CO2 and
water to generate glucose (Figure 14). The sun provides the initial
2540 kJ input of energy for this cycle to produce 1 mol of glucose
(C6H12O6). The sugar is then converted 2 mol of ethanol per 1
mol of sugar. This conversion process requires a small energy input, 20 kJ. At this point, hydrogen can be generated exothermically using the catalytic fuel-injector method described earlier.
Once the hydrogen is generated, it can be used in a hydrogen fuel
cell to produce energy and water.
Solar Energy
Every year the earth’s surface receives about 10 times as much energy from sunlight as is contained in all the known reserves of
coal, oil, natural gas, and uranium combined! The amount of solar energy incident on the earth’s surface is equivalent to about
15,000 times the world’s annual consumption of energy. Although
solar energy is a renewable resource, today we are making very inefficient use of the sun’s energy. Less than 2% of the electricity
produced in the United States is generated using solar energy.
How might the sun’s energy be exploited more efficiently?
One strategy is to produce electricity using solar radiation. We already know how to do this. The direct conversion of solar energy
to electricity can be carried out using photovoltaic cells (see “The
Chemistry of Modern Materials,” page 648). These devices are
made from specific metal and metalloid combinations (often gallium and arsenic) that absorb light from the sun and produce an
electric current. They are now used in applications as diverse as
spacecraft and pocket calculators, and they have also been tested
for large-scale commercial use.
Before solar energy can be a viable alternative, a number of
issues need to be addressed, including the collection, storage,
and transmission of energy. Furthermore, electricity generated
from solar power stations is intermittent. (The output fluctuation
results from the normal cycles of daylight and changing weather
conditions.) Our current power grid cannot handle intermittent
energy, so solar energy would need to be stored in some way and
then doled out at a steady rate.
Likewise, we need to find ways to make solar cells cost-effective. Research has produced photovoltaic cells that can convert
20–30% of the energy that falls on them. However, even higher
efficiency is necessary to offset the high cost of making the devices. Currently, 1 kW-h of energy generated from solar cells costs
about 35 cents, compared to about 2 cents per kW-h generated
from fossil fuels.
What Does the Future Hold for Energy?
Our society is at an energy crossroads. The modern world is increasingly reliant on energy, but we have built an energy infrastructure that depends primarily on a type of fuel that is limited.
While fossil fuels provide an inexpensive and simple approach
for providing power, they have several drawbacks, among them
atmospheric contamination and diminishing supplies.
Alternative fuels, especially from renewable sources, and
new ways of generating energy do exist. A great deal more research and resources must be put into them to make them affordable and reliable, however. This is where the study of chemistry fits squarely into the picture. Chemists will have a great deal
of work to do in coming years to develop new means of generating and delivering energy. Meanwhile, numerous ways exist to
conserve the resources we have. Ultimately, it will be necessary to
bear in mind the various benefits and drawbacks of each technology so that they can be combined in the most rational ways,
rather than remaining in a system that is dependent on a single
form of energy.
Suggested Readings
1. R. A. Hinrichs and M. Kleinbach: Energy—Its Use and the
Environment, 3rd ed. Orlando, Harcourt, 2002.
2. M. L. Wald: “Questions About a Hydrogen Economy,”
Scientific American, pp. 67–73, May 2004.
3. U.S. Department of Energy: Energy Efficiency and Renewable Energy, www.eere.energy.gov/hydrogenandfuelcells.
Accessed May 2004.
4. G. T. Miller: Living in the Environment, 12th ed. Philadelphia,
Brooks/Cole, 2001.
5. L. D. Burns, J. B. McCormick, and C. E. Borroni-Bird:
“Vehicle of Change,” Scientific American, pp. 64–73, October
2002.
6. M. S. Dresselhaus and I. L. Thomas: “Alternative Energy
Technologies,” Nature, Vol. 414, pp. 332–337, November 15,
2001.
Study Questions
Study Questions
Blue numbered questions have answers in Appendix P and fully
worked solutions in the Student Solutions Manual.
1. Hydrogen can be produced using the reaction of steam
(H2O) with various hydrocarbons. Compare the mass of H2
expected from the reaction of steam with 100. g each of
methane, petroleum, and coal. (Assume complete reaction
in each case. Use CH2 and C as the representative formulas
for petroleum and coal, respectively.)
2. Use the value for “energy released” in kilojoules per gram
from gasoline in Table 2. Estimate the percentage of carbon, by weight, by comparing this value to the ¢ H° values
for burning pure C and H2.
3. Per capita energy consumption in the United States was
equated to the energy obtained by burning 70. lb of coal
per day. Use enthalpy of formation data to calculate the
energy evolved, in kilojoules, when 70 lb of coal is burned.
(Assume the heat of combustion of coal is 33 kJ/g.)
4. Some gasoline contains 10% (by volume) ethanol. Using
enthalpy of formation data in Appendix L, calculate the
heat evolved from the combustion of 1.00 g of ethanol to
CO2(g) and H2O(g). Compare this value to the heat
evolved from the combustion of ethane to the same products. Why should you expect that the energy evolved in the
combustion of ethanol is less than the energy evolved in
the combustion of ethane?
5. Energy consumption in the United States amounts to the
equivalent of the energy obtained by burning 7.0 gal of oil
or 70. lb of coal per day per person. Carry out calculations
to show that these energy quantities are approximately
equivalent using data in Table 2. The density of fuel oil is
approximately 0.8 g/mL.
293
would have to be burned to provide this quantity of energy,
assuming that the heat of combustion of coal is 33. kJ/g?
[Electrical energy for home use is measured in kilowatthours (kW-h). One watt is defined as 1 J/s, so 1 kW-h is the
quantity of energy involved when 1000 W is dispensed over
a 1.0-h period.]
9. Major home appliances purchased in the United States are
now labeled (with bright yellow “Energy Guide” tags) showing anticipated energy consumption. The tag on a recently
purchased washing machine indicated the anticipated
energy use would be 940 kW-h per year. Calculate the anticipated annual energy use in kilojoules. (See Question 8 for
a definition of kilowatt-hour.)
10. Define the terms renewable and nonrenewable as applied to
energy resources. Which of the following energy resources
are renewable: solar energy, coal, natural gas, geothermal
energy, wind power?
11. Confirm the statement in the text that oxidation of 1.0 L
of methanol to form CO2(g) and H2O(/) in a fuel cell will
provide at least 5.0 kW-h of energy. (The density of
methanol is 0.787 g/mL.)
12. List the following substances in order of energy content per
gram: C8H18, H2, C(s), CH4. (See Question 7 for the heat of
combustion of C8H18.)
13. A parking lot in Los Angeles, California, receives an average of 2.6 107 J/m2 of solar energy per day in the summer. If the parking lot is 325 m long and 50.0 m wide, what
is the total quantity of energy striking the area per day?
6. The energy required to recycle aluminum is one third of
the energy required to prepare aluminum from Al2O3
(bauxite). Calculate the energy required to recycle 1.0 lb
(= 454 g) of aluminum.
14. Your home loses heat in the winter through doors, windows, and any poorly insulated walls. A sliding glass door
(6 ft 6.5 ft with 0.5 in. of insulating glass) allows 1.0 106 J/h to pass through the glass if the inside temperature
is 22 ° C (72 ° F) and the outside temperature is 0 ° C
(32 ° F). What quantity of heat, expressed in kilojoules, is
lost per day? Assume that your house is heated by electricity. How many kilowatt-hours of energy are lost per day
through the door? (See Question 8.)
7. The heat of combustion of isooctane (C8H18) is 5.45 103
kJ/mol. Calculate the heat evolved per gram of isooctane
and per liter of isooctane (d 0.688 g/mL). (Isooctane is
one of the many hydrocarbons in gasoline, and its heat of
combustion will approximate the energy obtained when
gasoline burns.)
15. Palladium metal can absorb up to 935 times its volume in
hydrogen, H2. Assuming that 1.0 cm3 of Pd metal can absorb 0.084 g of the gas, what is the approximate formula
of the substance? (The a-form of hydrogen-saturated palladium has about the same density as palladium metal,
12.0 g/cm3.)
16. Microwave ovens are highly efficient, compared to other
means of cooking. A 1100 watt microwave oven, running at
full power for 90 sec will raise the temperature of 1 cup of
water (225 mL) from 20 ° C to 67 ° C. As a rough measure
of the efficiency of the microwave oven, compare its energy
consumption with the heat required to raise the water
temperature.
Isooctane
C8H18
8. Calculate the energy used, in kilojoules, to power a 100-W
lightbulb continuously over a 24-h period. How much coal
17. New fuel-efficient hybrid cars are rated at 55.0 miles per
gallon of gasoline. Calculate the energy consumed to drive
1.00 mile if gasoline produces 48.0 kJ/g and the density of
gasoline is 0.737 g/cm3.