Electric Cars Powered by Batteries
An alternative to vehicles that use fuel cells are those powered by batteries. Some electric cars have already been produced, and most use a number of the same sort of lead-acid batteries that gasoline-powered vehicles have traditionally employed singly to operate the starter motor. In the future, electric cars will probably use nickel-cadmium, nickel-metal hydride, and lithium-based batteries. The practical difficulties that discourage widespread adoption of such vehicles are their high cost, the low driving range between battery charges, the length of the battery recharge period, and the weight of the batteries. Like fuel-cell systems, they have the attraction of zero emissions during operation, little operating noise, and low maintenance costs. Of course, pollution is emitted into the environment when the electricity required for these cars is generated in the first place. Some researchers have predicted that lead pollution stemming from the manufacture, handling, disposal, and recycling of lead-acid batteries would raise lead emissions into the environment by an amount that would exceed the levels that were associated with leaded gasoline. Critics of this analysis have pointed out that the data used are flawed and that not all the lead lost in the processing steps will be emitted into the environment rather than disposed of properly.
Even electric vehicles are not really pollution-free if a fossil fuel is burned to generate the electricity to charge the battery, since the fossil fuel's combustion in any power plant yields NOx that is released into the atmosphere.
The turn of the century saw the introduction of hybrid combustion/ electric powered vehicles, such as the Toyota Prius, into the market. Such vehicles overcome the requirement of long and frequent recharging of all-battery systems, since the battery is recharged continuously when the (small) gasoline engine is in operation and is producing excess power. During braking, recovered kinetic energy is channeled back to the battery. Motive power for the vehicle is supplied by both the gasoline and electric motors, the proportion depending on the driving situation. Hybrid vehicles are highly fuel efficient, so they emit much less carbon dioxide and much less nitrogen oxides, carbon monoxide, and VOCs than conventional vehicles. The batteries involved are nickel-metal hydride, which are lighter and more compact than lead batteries of equivalent potential.
Other Uses for Fuel Cells
The first wave of consumer products powered by fuel cells will likely be on the market in the next year or two. Laptop computers will be powered by fuel cells rather than rechargeable batteries. These fuel cells will use methanol rather than hydrogen directly as fuel and will contain removable cartridges containing the alcohol. The advantage for the user of fuel cells over battery power is a much longer working time before the power runs out. Using methanol or natural gas rather than H2 directly as the fuel in fuel cells does avoid the problem of the generation and storage of hydrogen. One problem with methanol is the generation of by-product carbon monoxide, which poisons the catalyst, as discussed previously. Diluting the methanol with water lessens this problem but cuts the power output of the cell. In addition, neither methanol nor any other liquid fuel that could in principle be used directly instead of H2 in a fuel cell reacts fast enough to produce the required electrical current in a vehicle, although recent research on the use of dilute solutions of methanol indicates that these problems may be overcome eventually.
In rocketry applications, hydrogen is stored as a liquid, as is oxygen. Since hydrogen's boiling point of only 20 K ( — 253 °C) at 1 atm pressure is so low, a large amount of energy must be expended in keeping it very cold, in addition to the energy used to liquefy it. This drawback effectively limits the applications of liquid hydrogen to a few specialized situations in which its lightness (low density) is the most important factor.
Hydrogen could be stored as a compressed gas, in much the same way as is done for methane in the form of natural gas. However, compared to CH4, hydrogen has a drawback: A much greater amount of H2 gas needs to be stored in order to release the same amount of energy. Compared with methane, the combustion of one mole of hydrogen consumes only one-quarter of the oxygen and consequently generates about one-quarter of the energy, even though both occupy equal volumes under the same pressure
(ideal gas law). Thus the "bulky" nature of hydrogen gas limits its applications (see Problem 8-13).
It is instructive to compare the volumes of hydrogen under different conditions required to fuel a hydrogen-fuel-cell car (assuming 50% efficiency) to travel 400 km (240 miles), approximately the distance one can obtain in an efficient gasoline-powered car with a tank capacity of 40-50 L. The amount of hydrogen required is 4 kg, which occupies
• 45,000 L, or 45 m3—e.g., a balloon having a 5-m diameter or a cube 3.6 m on each side, if it exists as a gas at normal atmospheric pressure; or
• 225 L (about 60 gal, equal to about five normal-sized gasoline tanks) as a gas compressed to about 200 arm (routinely achievable); or
• 56 L as a liquid (or solid) maintained at 252 C (at 1 atm pressure); or
• 35-75 L if stored as a metal hydride, if effective systems can be developed, as discussed below.
A practical and safe way to store hydrogen for use in small vehicles may be in the form of a metal hydride. Many metals, including alloys, absorb large amounts of hydrogen gas reversibly—as a sponge absorbs water. The molecular form of hydrogen becomes dissociated into atoms at the surface of the metal as it is absorbed and forms metal hydrides by incorporating the small atoms of hydrogen in "holes" in the crystalline structure of the metal. Thus the hydrogen exists as atoms, not molecules, within the lattice, which expands slightly to incorporate them. For example, titanium metal absorbs hydrogen to form the hydride of formula Til 12, a compound in which the density of hydrogen is twice that of liquid H2! Heating the solid gradually releases the hydrogen as a molecular gas, which then can be burned in air or oxygen to power the vehicle.
Research continues to find a light metal alloy that can efficiently store hydrogen without making the vehicle excessively heavy. Even existing metal hydride systems are lighter than the pressurized tanks needed to store liquid hydrogen. Most industrial research now centers on metal systems. Practical considerations require that an alloy to store hydrogen
• be capable of quickly and reversibly absorbing hydrogen,
• not become brittle after many repeated cycles of absorption and desorption,
• operate in the pressure and temperature ranges of 1 -10 atm and 0- 100°C,
• not be so dense that it weighs down the vehicle excessively (a concentration of hydrogen of at least 6.5 mass % is the U.S. Department of Energy target), and
• not require a huge volume (at least 62 kg H/m3, equivalent to 4 kg in 65 L, is the target).
Lanthanum-nickel alloys derived from LaNi5 have all the above characteristics except one: They are too heavy (mass % < 2), a deficiency shared by all known metal hydrides that operate near ambient temperature. Many lighter hydrides and alloys, such as MgH2 and Mg2NiH4, are known, but they do not operate reversibly under moderate conditions. Research on systems formed by the lighter metals continues but has not yet been successful in producing alloys that fulfil all five conditions listed.
One of the practical difficulties in using hydrogen as a fuel is its tendency to react over time with the metal in pipelines or storage containers in which it is used. This reaction embrittles the metal, eventually deteriorating it to form a powder. Recent progress has been made in overcoming this difficulty by using composite materials rather than simple metals as the structural materials for storage and transport facilities.
Some research in the past reported that tiny fibers made of graphite, a light material, can store up to three times their weight in hydrogen between the graphite layers and would be a safe, lightweight storage mechanism for hydrogen. However, research in the ;late 1990s involving extensive hydrogen storage in carbon nanotubes has not proven to be reproducible. One of the difficulties is the very tiny (milligram) samples of carbon nanotubes that are available for experimentation. Some researchers believe that, if the nanotubes are broken so that they have an open end, hydrogen can enter the tube. Other experiments indicate that only a single layer of H2 gas conventionally adsorbed to the outside of the tubes is actually stored, a concentration too small (<2% by mass) to be useful.
Overall, the problem of devising a practical, economical, and safe way of storing hydrogen has not yet been achieved, and in the eyes of some analysts, "no breakthrough is yet in sight," despite much interest and research activity. It may be that the weight requirements associated with all practical methods of storing hydrogen will limit its use to large vehicles such as buses and airplanes.
As mentioned in the discussion of fuel cells, in some applications it may be more feasible to transport and store hydrogen in the form of an energy-dense liquid such as methanol and to use it as required for power generation. Toluene, C7H8, has also been proposed as a long-distance hydrogen carrier; it could then be dehydrogenated when the hydrogen is required.
Another way to temporarily store hydrogen is by means of the alkali salt (lithium or sodium) of the borohydride ion, BH4~. For their prototype minivan that runs on fuel cells, Chrysler uses a 20% solution of sodium borohydride in water to store hydrogen, which is released when the solution is pumped over a ruthenium catalyst, prompting the redox reaction of the 11 in borohydride with the H in water to produce H2:
- Low-temperature metal hydrides
Chemical storage O (NaBH4)
DOE target O
High-temperature metal hydrides
Gravimetric density (% weight H2)
FIGURE 8-15 The performance of various hydrogen storage techniques. Note that both horizontal and vertical scales are logarithmic. [Source: R. F. Service, "The Hydrogen Backlash/' Science 305 (2004): 958-961.)
The density of hydrogen in the borohydride solution is comparable to that in liquid hydrogen. The analogous alanate ion, AIH4~, in the form of its sodium salt is also a candidate for hydrogen storage in vehicular fuel cells:
2 NaAlH4-> 2 NaH + 2 Al + 3 H2
Several molecular boron compounds, including BH3NH3 and an organoboron-phosphorus system, have recently been proposed as hydrogen carriers.
The performance of metal systems for storing hydrogen is compared to that of the compressed and liquefied element and that of gasoline and diesel fuel in Figure 8-15. No practical system discovered to date has reached the target of the U.S. Department of Energy (Figure 8-15) in terms of combining high density with a high percentage of hydrogen in its mass ("gravimetric density" of at least 6%).
The possibility of storing hydrogen in clathrates in water—much like methane in water clathrates (Chapter 6)—may be feasible. Although hydrogen molecules are too small to be efficiently trapped at low pressures, it has been discovered recently that two or four H2 molecules can be stored in each ice clathrate at room temperature at enormous pressures, about 2000 atm. Once formed, though, the clathrate can be stored at liquid nitrogen temperatures at reduced pressure.
Calculate the mass of titanium metal required to absorb each kilogram of hydrogen and form TiH2 in a "tankful" of hydrogen. Repeat the calculation for magnesium if the hydride has the formula MgH?. Which metal is superior for storage of hydrogen from a weight standpoint?
Assuming that the energy released by combustion of H2 is proportional to the amount of oxygen it consumes, estimate the ratio of heat released by one mole of methane compared to one mole of hydrogen gas.
Using the thermochemical information in the H2 combustion equation, calculate the enthalpy (heat) of combustion of hydrogen per gram, and by comparing it to that of methane (see Problem 7-4), decide which fuel is superior on a weight basis. By comparing the actual energy released by combustion per mole of gas—and hence per molar volume—decide which fuel is superior on a volume basis.
It is important to realize that hydrogen is not an energy source, since it does not occur as the free element in the Earth's crust. Hydrogen gas is an energy-vector, or carrier, only; it must be produced, usually from water and/or methane, with the consumption of large amounts of energy and/or other fuels. The industrial infrastructure that would be required to produce enough hydrogen to fuel all the vehicles in the United States is enormous, since it would require about as much energy as the current electric power capacity.
The most expensive commercial way to produce hydrogen is by electrolysis of water, using electricity generated by some energy source:
2 Hp(l) ekct"cltY > 2 H,(g) + 02(g)
Unfortunately, about half the electrical energy is inadvertently converted to heat and therefore wasted in this process.
A hope for the future is that wind power or solar energy from photovoltaic collectors will become economically efficient in providing electricity to generate hydrogen. Currently, there are prototype plants in Saudi Arabia and Germany that use electricity from solar energy to produce hydrogen, a process about 7% efficient. The stored energy is later recovered by reacting the hydrogen with oxygen. Excess electricity from hydroelectric or nuclear power or wind-power installations—i.e., power generated but not required immediately for use—could be used to produce hydrogen by electrolysis of water.
Even better than the use of solar electricity to electrolyze water would be the direct decomposition of water into hydrogen and oxygen by absorbed sunlight, but no practical, efficient method has yet been devised to effect this transformation. One of the difficulties in using sunlight to decompose water is that H20 does not absorb light in the visible or UV-A regions; thus some substance must be found that can absorb sunlight, transfer the energy to the decomposition process, and finally be regenerated. The substances proposed to date for this purpose are very inefficient in converting sunlight into energy. In addition, since the light-absorbing substances and others required are not 100% recoverable at the end of the cycle, they must be continuously resupplied—thus the hydrogen that is produced is not really a renewable fuel.
One catalyst that has been found to convert sunlight into hydrogen by electrolyzing water is titanium dioxide, Ti02. A small potential is applied to the electrode in the cell's operation. Titanium dioxide is stable to sunlight (unlike many other potential light-absorbing materials) and cheap, but pure Ti02 absorbs only ultraviolet light. By blending carbon into TiOz so that C replaces some of the oxide ions, the efficiency in producing hydrogen gas is increased eightfold, to more than 8% of the Sun's energy, because the addition of carbon extends absorption into the visible region (to 535 nm).
Determine the longest wavelength of light that has photons capable of decomposing liquid water into H2 and 02 gases, given that for this process AH = +285.8 kj/mol of water. In which region of the spectrum does this wavelength lie? [Hint; Recall from Chapter I the relationship between reaction enthalpy and light wavelength J
In principle, the thermal conversion of sunlight into heat can produce temperatures hot enough to decompose water into hydrogen and oxygen. Research in Israel, using a solar tower of mirrors to concentrate sunlight by a factor of 10,000 and thereby produce temperatures of about 2200°C in a reactor, has succeeded in splitting about one-quarter of water vapor at low pressures into H2 and 02.
Various thermochemical cycles by which water can indirectly be decomposed by heat into hydrogen and oxygen have been proposed. Ideally, such cycles should operate at moderate temperatures, be efficient in conversion of heat into hydrogen, and not degrade the reactants so they can be recycled. Perhaps the most practical is the sulfur-iodine cycle, in which elemental iodine is first reduced by sulfur dioxide to hydrogen iodide and sulfuric acid:
I2 + S02 + 2 HzO-► 2 HI + H2S04 (at 120°C)
The hydrogen iodide is then thermally decomposed into hydrogen gas, recovering the elemental iodine, and the sulfuric acid is thermally decomposed into oxygen gas, recovering the sulfur dioxide:
2 HI + heat (320°C)-> H2 + I2
H2S04 + heat (830°C)-*• S02 + HzO + 1/2 02
Since the reactants HI and S02 are recovered in high yield, the cycle can be repeated over and over. Heat produced by nuclear reactors could drive this cycle, which has a conversion efficiency of about 50%.
Hydrogen gas can be produced by reacting a fossil fuel such as coal or petroleum or natural gas with steam to form hydrogen and carbon dioxide. The energy value of the fuel is transferred from carbon to the hydrogen atoms of water; chemically speaking, the reduced status of the carbon is transferred to the hydrogen. The net reactions, assuming coal to be mainly graphite, are:
C + 2 H20-► 2 H2 + C02
CH4 + 2 H20-»4 H2 + C02
Notice that as much carbon dioxide is produced in this way as would be obtained by combustion of the fossil fuels in oxygen. As discussed previously, the actual conversions occur in two steps: First the fossil fuel reacts with steam to yield carbon monoxide and some hydrogen (Figure 8-10). Then the CO/H2 synthesis gas mixture and additional steam are passed over a suitable catalyst to obtain additional hydrogen and complete the oxidation of the carbon by the water-gas shift reaction driven in the direction shown here:
co + h2o catalyst > c02 + h2
It is interesting to note that at the turn of the twentieth century and for several decades thereafter, the synthesis gas produced when coal reacts with steam was itself used as the fuel in many municipal street-lighting systems around the world.
Hydrogen gas could be produced in a renewable way from biomass grown for this purpose. Some research indicates that aqueous solutions of both glucose and glycerol can be decomposed at moderate temperatures (225-265°C) and pressures (27-54 atm) with a platinum-based catalyst to produce hydrogen and carbon dioxide (Figure 8-10).
Associated with every conversion of one fuel to another are energy losses, mainly to waste heat, some of which are dictated by the second law of thermodynamics and therefore cannot be avoided. The energy of natural gas can be transferred to hydrogen with an efficiency of about 72%; transfer from coal is 55-60% efficient. Thus, if the resulting fuel is used only to generate heat, significantly less COz is emitted if the original fossil fuel is burned rather than being first converted to hydrogen.
Finally, it should be mentioned that hydrogen is considered to be a dangerous fuel due to its high flammability and explosiveness; it ignites more easily than do most conventional fuels. On the positive side, however, spills of liquid hydrogen rapidly evaporate and rise high into the air. (Some of the fears surrounding hydrogen stem from the 1930s incident in which the airship Hindenburg was destroyed in a catastrophic fire. However, it was the thin layer of aluminum encasing the hydrogen gas that initially was ignited, not the H2 itself.)
1. Define the term renewable energy, and list several forms of it. Which form is growing the fastest?
2. Name four environmental/social problems associated with the expansion of hydroelectric power.
3. What is the mathematical relationship between the energy generated by a windmill and (a) the wind's speed and (b) the length of the windmill blades?
4. Explain the origin of coastal winds.
5. List four pros and cons of wind power.
6. Define energy payback, and state which form of renewable energy has the lowest payback period and the lowest cost at present.
7. What is meant by geothermal energy7. Give some examples of how and where it is tapped.
8. Describe the difference between the two methods of absorbing energy from sunlight. What is the difference between active and passive systems?
9. What is meant by solar thermal electricity, and how is it generated? What is meant by the term cogenerationl
10. State the second law of thermodynamics. According to this law, what formula gives the maximum fraction of heat that can be transformed into electricity?
11. Define the photovoltaic effect. What is the chief difficulty preventing the widespread use of solar cells?
12. List four advantages and four disadvantages of solar energy.
13. What are the advantages and disadvantages of using alcohol fuels in regard to air pollution? What is meant by E|0 fuel?
14. Describe the method used in producing ethanol in high volume for use as a fuel. What are the potential feedstocks for this process?
15. What are the highly energy-intensive steps involved in production of fuel ethanol? Why isn't ethanol a fully "renewable" fuel? What is meant by the term cellulostic ethanol?
16. What is the water-gas shift reaction? Describe the methods by which methanol can be produced in volume for use as a fuel. What does M85 mean?
17. Chemically speaking, what is biodiesel and how is it produced?
18. Describe the three ways in which hydrogen can be stored in vehicles for use as a fuel, and discuss briefly the disadvantages of each method.
19. Does the burning of hydrogen really produce no pollutants? Under what conditions do no pollutants form?
20. What is the difference between an energy source and an energy carrier (vector)? Into which category does H2 fall?
21. Describe how a hydrogen fuel cell works, and write balanced half-reactions for its operation in acidic media. What other types of fuel cells exist?
22. Describe the production of hydrogen gas by electrolysis. Can solar energy be used for this purpose? Why isn't water decomposed directly by absorption of sunlight?
Green Chemistry Questions
See the discussion of focus areas and the principles of green chemistry in the Introduction before attempting these questions.
1. The first reaction below is the Suppes synthesis of propylene glycol from glycerin (which won a Presidential Green Chemistry Challenge Award); the second reaction is the commercial synthesis of propylene glycol from propene. Glycerin (as was discussed in this chapter) can be obtained from biomass while propene is a petrochemical. Another aspect of these two preparations of propylene glycol to consider when assessing their environmental impact is their atom economy. Calculate the atom economy of these syntheses. To aid you, for each synthesis, the atoms of the reactants that are incorporated into propylene glycol are given in green while those that are wasted are in black.
| | | copper chromite_^ j j h2c-ch—ch2 h h, 200°c, 200 psi h2c-ch—ch3
glycerin propylene glycol
h3c- ch=ch2 h3c-ch-ch2 ^oh, h3c—-ho—ch2 h3c-hc-ch2
2. The development of the preparation of propylene glycol and acetol from glycerin by Suppes won a Presidential Green Chemistry Challenge Award.
(a) Into which of the three focus areas for these awards does this award best fit?
(b) List at least two of the twelve principles of green chemistry that are addressed by the green chemistry developed by Suppes.
3. If you have had a course in organic chemistry, try to deduce a reaction mechanism for the con version of fats and oils to biodiesel and glycerin as shown in Figure 8-11.
4. Where does diesel fuel come from? How is it produced? What compounds make up diesel fuel?
5. Ethanol and methanol are being used as automobile fuels. What do you think about the use of the alcohols propylene glycol and glycerin as automobile fuels? [Hint: It many be useful to look up the boiling points of ethanol, methanol, ethylene glycol, and glycerin.]
1. Given that an average of 342 W of sunlight energy falls on each square meter of Earth, that the surface area of a sphere is 4irr", and that the Earth's radius r is about 6400 km, calculate in joules the total amount of sunlight received annually by the Earth, What percentage of this quantity needs to be captured in order to provide our current commercial energy needs?
2. Consider the use of methanol, CHjOH, as an oxygenated liquid fuel for suitably modified cars.
(a) By writing the balanced chemical equation for its combustion in air, determine whether it is more similar to coal, oil, or natural gas in terms of the joules of energy released per mole of C02 produced*
(b) Determine the balanced equation by which methanol can be produced by reacting elemental carbon (coal) with water vapor, given that C02 is-the only other product in the reaction, (c) Does the combined scheme of parts (a) and (b) represent a way of using coal but producing less carbon dioxide per joule than by its direct combustion? Explain your answer.
3. Deduce the fraction of the CO or H2 produced by the reaction of coal with steam that must be converted to H2 or CO, respectively, by the watergas shift reaction in order to obtain the 2:1 ratio of hydrogen to carbon monoxide that is required to synthesize methanol. Deduce also the net reactions of conversion of coal to methanol.
4. Deduce the balanced reaction in which synthesis gas is formed by combining equal volumes of methane and carbon dioxide. From enthalpy of formation data given in Problems 7-4 and 8-7, deduce the enthalpy change for this reaction. By applying Le Chatelier's principle, deduce whether the conversion of the gases to carbon monoxide and hydrogen will be favored by low or by high pressures, and by low or by high temperatures. Combining these results with those obtained in Additional Problem 3, determine the fraction of the total carbon dioxide resulting from the production and combustion of methanol synthesized from this synthesis gas that would be renewable, i.e., recycled from the consumption of carbon dioxide in the process.
5. Contact several new car dealerships in your area to discover which vehicles currently on sale can use one or more of the alternative fuels CNG, LPG (propane; see Chapter 7), M85, E10q or Eg5, hydrogen, or electricity. For each fuel and vehicle, inquire about the average kilometers or miles per liter or gallon of fuel; from this information and using fuel prices obtained from local service stations, estimate the driving cost per kilometer or mile for each vehicle and fuel combination. Are any of the combinations competitive in cost with gasoline?
6. Contact a garage in your area that converts existing gasoline-fueled vehicles into those that accept CNG or LPG (propane; see Chapter 7). Determine the common conversion price and what the likely kilometers-per-liter or miles-per-gallon performance for the new fuel in a common vehicle will be. From the cost of the fuels in your area, estimate the distance that the vehicle must be driven before the cost of the conversion has been met by savings on the fuel.
7. The Bay of Fundy, located between the Canadian provinces of Nova Scotia and New Brunswick, has the highest tides in the world. The difference between high and low tides can be as much as 16 m. A total of 14 billion tonnes of sea-water flow into and out of Minas Basin, a part of the Bay of Fundy, during each tide. The energy tapped as tidal power comes from the change in potential energy of this water as it falls in the Earth's gravitation field. Given that the potential energy of a mass m at height h in a gravitational field is given by m X g X h, where g is the gravitational constant, 9,807 m/s2, calculate the amount of energy that corresponds to a tidal drop of 16 m in the Minas Basin.
8. One way of considering the direct environmental impact of the combustion of various fuels is to look at the amount of heat produced per unit of C02 generated. Determine and compare the values of kilojoules of heat per mole of C02 produced in the case of the combustion of methanol, ethanol, and n-octane. Use the following molar enthalpies of combustion (AHC, in kj/mol); methanol, — 726; ethanol, —1367; and n-octane, —5450. Comment on the values obtained.
9. The use of biomass fuels, including scavenged wood and wood-chip waste, has been proposed as a way of reducing C02 emissions, even though such fuels would produce a large amount of C02 per unit of heat produced. Explain the rationale behind this idea.
10, Assuming that MTBE reacts in the atmosphere at the methyl group attached to the oxygen, use the principles of atmospheric reactivity developed in Chapter 5 to show that the first stable product in its decomposition sequence in air is an ester.
1. S. Pacala and R. Socolow, "Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies," Science 305 (2004): 968-972.
2. M. Hoogwijk et al., "Assessment of the Global and Regional Geographical, Technical, and Economic Potential of Onshore Wind Energy," Energy Economics 26 (2004): 889-919.
3. N. Fell, "Deep Heat," New Scientist (22 February 2003): 40-42.
4. R. Gomez and J. L. Segura, "Plastic Solar Cells," Journal of Chemical Education 84 (2007): 253-258.
5. P. Hoffmann, Tomorrow's Energy: Hydrogen, Fuel Cells, and the Prospects for a Cleaner Planet (Cambridge, MA: MIT Press, 2001).
6. R. F. Service, "The Hydrogen Backlash," Science 305 (2004): 958-961 (the August 13, 2004, issue of Science contains many articles on the hydrogen economy).
Websites of Interest
Log on to www.whfi-eeman.com/envchem4/ and click on Chapter 8.
RADIOACTIVITY, RADON, AND NUCLEAR ENERGY
The San Onofre nuclear power generating station at San Diego, California. All current nuclear energy is generated by fission, though fusion plants may be viable in the future. (Corbis Images)
In this chapter, the following introductory chemistry topics are used:
■ Mass and atomic numbers; isotope symbolism; elementary particles
■ Half-lives; exponential decay and first-order processes
Background from previous chapters used in this chapter:
■ Concepts of free radicals (Chapter 1) and synergism (Chapter 4)
■ Half-life equation (Chapter 6)
■ Second law of thermodynamics (Chapter 8)
In all other chapters of this book, we are concerned with chemicals and chemical processes. In this chapter, we consider nuclear processes and how they affect the environment, our health, and our energy supply. These concerns all center on the effects of radioactivity, and it is with this topic that we begin. This allows us to discuss radon, the most important radioactive indoor air pollutant, and depleted uranium. We then switch to nuclear energy and explore the ways in which electricity can be produced from it and the environmental consequences of the radioactive waste that these processes generate.
The San Onofre nuclear power generating station at San Diego, California. All current nuclear energy is generated by fission, though fusion plants may be viable in the future. (Corbis Images)
Radioactivity and Radon Gas
The Nature of Radioactivity
Although most atomic nuclei are stable indefinitely, some are not. The unstable, or radioactive, nuclei spontaneously decompose by emitting a small particle that is very fast moving and therefore carries with it a great deal of energy. In some types of nuclear decomposition processes, atoms are converted from those of one element to those of another as a consequence of this emission. Very heavy elements are particularly prone to this type of decomposition, which occurs by the emission of a small particle. The nuclei produced by emission of the particle may or may not themselves be radioactive; if they are, they will undergo another decomposition at a later time.
Recall from introductory chemistry that the mass number is the number of heavy particles—protons and neutrons—and not the actual mass of the nucleus. An alpha (a) particle is a radioactively emitted particle that has a charge of +2 and a mass number of 4—it has two neutrons and two protons—and it is identical to a common helium nucleus. Thus an a particle is written as 2He, where 4 is its mass number and 2 refers to its nuclear charge (i.e., number of protons). The nucleus that remains behind after an atom has lost an a particle has a nuclear charge that is 2 units less than the original, and it is 4 units lighter. For example, when a 2ggRa (radium-226) nucleus emits an a particle, the resulting nucleus has a mass number of 226 — 4 — 222 units and a nuclear charge of 88 — 2 = 86; this is a wholly new element that is an isotope of the element radon. The process can be written as a nuclear reaction:
Notice that both the total mass number and the total nuclear charge individually balance in such equations.
A beta (/3) particle is an electron. It is formed when a neutron splits into a proton and an electron in the nucleus. Since the proton remains behind in the nucleus when the electron leaves it, the nuclear charge (or atomic number) increases by 1 unit (you may imagine this effect as "subtracting a negative particle"). There is no change in mass number of the nucleus, since the total number of neutrons plus protons remains the same. For example, when an atom of the lead isotope 2||Pb (lead-214) decays radioactively by the emission of a /3 particle, the nuclear charge of the product is 82 + 1 = 83, corresponding to the element bismuth; the mass number remains 214:
Notice that the symbol Jje used here for the electron shows its mass number (zero) and its charge; in the equation the total mass numbers and nuclear charge numbers each balance.
Particle Symbol Chemical and Name Symbol Comment
Effect on Nucleus of Particle Emission
?He Nucleus of a Atomic number reduced helium atom by 2
_°e Fast-moving Atomic number increased electron by 1
None High-energy photon
One other important type of radioactivity is the emission of a gamma (y ) particle (also called a ray) by a nucleus. This is a huge amount of energy concentrated in one photon and possesses no particle mass. Neither the nuclear mass number nor the nuclear charge changes when a y particle is emitted. The emission of a y ray often accompanies the emission of an a or /3 particle from a radioactive nucleus. The properties of all three types of nuclear radiation are summarized in Table 9-1.
Deduce the nature of the species that belongs in the blank for each of the following nuclear reactions:
The Health Effects of Ionizing Radiation
The a and /3 particles that are produced in the radioactive decay of a nucleus are not in themselves harmful chemicals, since they are simply the nucleus of a helium atom and an electron. However, they are ejected from the nucleus with an incredible amount of energy of motion. When this energy is absorbed by the matter encountered by the particle, it often ionizes atoms or molecules; for that reason, it is called ionizing radiation, or just radiation. This radiation is potentially dangerous if we absorb it, since the molecular components of our bodies can be ionized or otherwise damaged.
Although a and ¡3 particles are energetic, they cannot travel far within the human body, since they lose more and more of their energy—and consequently slow down—as they collide with more and more atoms.
(a) 28262Rn->42He +_
(c) 2I4Po->2I4Pb + _
Alpha particles can travel only a few thousandths of a centimeter within the body, so they are not penetrating. This is true because they are relatively massive, and when they interact with matter they slow down, capture electrons from it, and are converted into harmless atoms of helium gas. If an a particle is emitted outside the body, it will usually be absorbed in the air or by the layer of dead skin, so it will do you no harm. However, inhaled or ingested radioactive atoms can cause serious internal damage when they emit a particles. The damage is particularly severe with a particles since their energy is concentrated in a small area of absorption located within about 0.05 mm of the point of emission. In their interaction with matter, a particles are highly damaging—the most highly damaging of all particles—since they can knock atoms out of molecules or ions out of crystal sites. If the molecules affected are DNA or its associated enzymes, cell death can result. A more serious consequence for the individual can be the creation of mutations that could lead to cancer.
Beta particles move much faster than a particles since they are much lighter and can travel about 1 m in air or about 3 cm in water or biological tissue before losing their excess energy. Like a particles, they can cause considerable damage to cells if they are emitted from particles that have been inhaled or ingested and the radioactive nucleus is consequently close to the cell when it decays.
Gamma rays easily pass through concrete walls—and our skin. A few centimeters of lead are required to shield us from y rays. Gamma particles are the most penetrating and therefore the most damaging of the three, traveling a few dozen centimeters into our bodies or even right through them. They are generally the most dangerous type of radioactivity, since they can penetrate matter efficiently and do not have to be inhaled or ingested. Although they can pass through our bodies, y rays lose some of their energy in the process, and cells can be damaged by this transferred energy, since it can ionize molecules. Ionized DNA and protein molecules cannot carry out their normal functions, potentially resulting in radiation sickness and cancer.
The ions produced by radiation when its energy is transferred to molecules are free radicals; hence they are highly reactive (Chapters 1, 3, and 5). For example, a water molecule can be ionized by an a, (3, or y ray or by an X-ray. The resulting HzO+ free-radical ion subsequently dissociates into a hydrogen ion and the hydroxyl free radical, OH:
H20+-> H 1 + OH
If the affected water molecule is contained in a cell, the hydroxyl radical can engage in harmful reactions with biological molecules in the cell, such as DNA and proteins. In some cases, radiation damage is sufficient to kill cells of living organisms. This is the basis of food irradiation, where the death of microorganisms helps prevent subsequent spoilage of the food.
If human beings are exposed to substantial, though sublethal, amounts of ionizing radiation, they can develop radiation sickness. The earliest effects of this malady to be observed occur in tissues containing cells that divide rapidly, because damage to the cell's DNA or protein can affect cell division. Such rapidly dividing cells are found in bone marrow, where white blood cells are produced, and in the lining of the stomach. It is not surprising, then, to find that early symptoms of radiation sickness include nausea and a drop in white blood cell count. Children are more susceptible to radiation than adults because their tissues involve more cell division. On the other hand, radiation can he effectively used to kill cancer cells since they are dividing rapidly. Unfortunately, radiation therapy cannot be completely selective in terms of the cells it affects, so it has side effects such as nausea.
Long-term effects from radiation may show up in genetic damage, because chromosomes may have undergone damage or their DNA may have mutated. Such damage may lead to cancer in the person exposed or to effects in her or his offspring if the changes occurred in the ovaries or testes.
Quantifying the Amount of Radiation Energy Absorbed
The amount of radiation absorbed by the human body is measured in rad (radiation absorbed dose) units, where 1 rad is the quantity of radiation that deposits 0.01 joule of energy to 1 kilogram of body tissue. The rad is not a particularly useful quantity, however, since the damage inflicted by 1 rad of a particles is 10 to 20 times greater than that inflicted by 1 rad of (3 particles or y rays. The scale of absorbed radiation that incorporates this biological effectiveness factor is the rem (roentgen equivalent man). A more modern unit than the rem is the sievert, Sv, which equals 100 rem.
On average, we each receive about 0.3 rem, i.e., 300 mrem or 3000 ¿tSv, of radiation annually. The origin on average is about
• 55% from radon in indoor and outdoor air;
• 8% from cosmic rays from outer space;
• 11% from natural radioactive isotopes (e.g., 40K, 14C ) of elements that are present in our own bodies; and
• 18% from anthropogenic sources, chiefly medical X-rays.
The average contribution from nuclear power production is negligible at present.
An acute exposure of more than 25 rem results in a measurable decrease in a person's white blood cell count; over 100 rem produces nausea and hair loss; and an exposure of over 500 rem results in a 50% chance of death within a few weeks.
Of particular interest is the portion of the 14-step sequence of 238U radioactive decay that involves radon, since this element is the only one, other than the helium produced from the a particles, that is gaseous and therefore is mobile. Details concerning this portion of the radioactive decay series are shown in Figure 9-lb. The immediate precursor of the radon is radium-226, which has a half-life of 1600 years and decays by emission of an a particle:
The 222Rn isotope has a half-life of 3.8 days, which can be long enough for it to diffuse through the solid rock or soil in which it is initially formed. Most radon escapes directly into outdoor air when the surface of the Earth where it appears is not covered, e.g., by a building. The very small background concentration of radon in air that this produces nevertheless yields about half of our exposure to radioactivity, as listed above. Although the radon decays in a few days, it is constantly replaced by the decay of more radium.
Some scientists have pointed out that radon gas accumulates to unhealthy levels in caves, including some that are often used for recreational purposes. However, it is in certain homes that radon becomes an important indoor air pollutant. Most radon that seeps into homes comes from the top meter of soil below and around the foundation; radon produced much deeper than this will probably decay to a nongaseous and therefore immobile element before it reaches the surface. Loose, sandy soil allows the maximum diffusion of radon gas, whereas frozen, compacted, or clay soil inhibits its flow. Radon enters the basements of homes through holes and cracks in their concrete foundations. The intake is increased significantly if the air pressure in the basement is low. The material used to construct the homes and water from artesian wells are other potential sources of radon in homes. Groundwater systems serving up to a few hundred people often have radon levels almost 10 times those of surface waters. When well water is heated and exposed to air, as occurs when it exits from a showerhead, radon is released to the air. However, radon from water usually represents only a small fraction of that arising from soil, although it represents a greater health hazard than that contributed from water disinfection by-products and other dissolved chemicals.
Measuring the Rate of Disintegration and Health Threat from Environmental Radiation
The rate of radioactive disintegrations in a sample of matter is usually measured in bequerels, Bq, where 1 Bq corresponds to the disintegration of one atomic nucleus per second. The other unit used is the curie, Ci, which equals 3.7 X 1010 Bq and is the radioactivity produced by one gram of 226Ra. Environmental regulations are usually expressed in the number of bequerels per unit volume or, in the United States, in terms of the number of picocuries, where 1 pCi = 10~12 Ci. For example, the U.S. EFA uses 4 pCi per liter of air as the upper limit for a safe radon level in houses.
We can calculate the amount of energy from radiation that is absorbed by a person's lungs in a year if he or she breathes air containing radon at the 4 pCi/L level, since the energy of each a particle emitted by a radon atom is known by measurement to be 9.0 X 10 J. Since 4 pCi/L is equivalent to 4 X 10~° X 3.7 X 1010 = 0.15 disintegrations per liter per second, and since there are 60 X 60 X 24 X 365 sec in a year, the total annual number of disintegrations in 1 L of air is 4.7 X 106. Thus the total amount of energy liberated annually in the process is 4.7 X 106 X 9.0 X 10~13 J = 4.2 X 10^6 J. If we assume that all this energy is absorbed by a person's lung tissues (rather than by the air in the lungs), that the lung volume is about 1 L, and that the lung mass is about 3 kg, then since 1 rad = 0.01 J/kg, the energy absorbed is 1.4 X 10^4 rad or 0.14 mrad. Using a factor of 10 to convert rads to rems for a-particle radiation, we find that the annual radiation dose is about 1.4 mrem, i.e., about 0.5% of background exposure.
The Daughters of Radon
Radon, the heaviest member of the noble gas group, is chemically inert under ambient conditions and remains a monatomic gas. As such, it becomes part of the air that we breathe when it enters our homes. Because of its inertness, physical state, and low solubility in body fluids, radon itself does not pose much of a danger; the chance that it will disintegrate during the short time it is present in our lungs is small and, as discussed above, the range of a particles in air before they lose most of their energy is less than 10 cm.
The dang er arises instead from the radioactivity of the next three elements in the disintegration sequence of radon—namely, polonium, lead, and bismuth (see Figure 9-lb). These descendants are termed daughters of radon, which in turn is called the parent element. In macroscopic amounts, these particular daughter elements are solids, and when formed in the air from radon they all quickly adhere to dust particles. Some dust particles adhere to lung surfaces when inhaled, and it is under these conditions that the elements pose a health threat. In particular, both the 21'"Po, which is formed directly from 222Rn, and the 214Po, which is formed later in the sequence (Figure 9-lb), emit energetic a particles that can cause radiation damage to the bronchial cells near which the dust particles reside. This damage can eventually lead to lung cancer. Indeed, as will be discussed, radon (or rather its daughters) is the second leading cause of such cancers, although it follows smoking by a wide margin.
Although some radon daughters in the sequence disintegrate by j3-particle emission, the deleterious health effects of these particles are considered negligible because the a particles carry much more energy and, as discussed, it is the disruption of cell molecules by the burst of high energy that initiates cancer.
Notice that the sequence (see Figure 9-lb) of radon decay to 2I0Pb formation takes less than a week on average. In contrast, disintegration of "10Pb to Bi has a half-life of 22 years, and, in. fact, most of the lead will have been cleared from the body before this process occurs.
Measuring the Health Danger from Radon and Its Daughters
The greatest exposure to a particles from radon disintegration is experienced by miners who work in poorly ventilated underground uranium mines. Their rate of lung cancer is indeed higher than that of the general public, even after corrections to the data have been made for the effects of smoking. From statistical data relating their excess incidence of lung cancer to their cumulative level of exposure to radiation, a mathematical relationship between cancer incidence and radon exposure has been developed. Scientists have extrapolated this relationship to determine the risk to the general population from the generally lower levels of radon to which the public is exposed.
Based on linear extrapolation from the miners' data and other sources, the U.S. EPA estimates that radon currently causes about 21,000 excess lung cancer deaths annually; the estimate for the United Kingdom is 2000 cases per year. Most of the excess deaths are associated with smokers, since radon and cigarette smoke are synergistic (see Chapter 4) in causing lung cancer. In particular, the risk of lung cancer by age 75 is 4 in 1000 for a nonsmoker living in a house with zero radon and is only increased to 7 in 1000 if he or she has constant exposure by inhalation to 400 Bq/rn3 of radioactivity from the gas, for a net increase of 3 in 1000. However, a smoker's chances of lung cancer rise from 100 to 160 in 1000, an increase of 60, by exposure to the same level of radioactivity. It is estimated that radon causes about 10% of all lung cancers, which is about half the mortality rate from automobile accidents, for example.
The level of radioactivity in air is stated in units of becquerels (Bq) per cubic meter. The average indoor radon concentration globally is about 39 Bq/m3, compared to the usual outdoor level of about 10 Bq/m3. The radioactivity "action level" for indoor air, beyond which mitigation measures should be taken, is 150 Bq/m (4 pCi/L) in the United States; that in Great Britain, Norway, and Sweden—and that proposed for Canada—is 200 Bq/m3. Because radon dissolved in drinking water can escape into the air when it comes out of the tap, a maximum contaminant level of 150 Bq/L has been established by the U.S. EPA; the World Health Organization (WHO) guideline is 100 Bq/L. Radon concentrations at these levels are associated with groundwater that has passed through rock formations that contain natural uranium and radium.
Those skeptical of using uranium miners' data point out that the calculated estimates of radon-caused lung cancer may be too high, since miners work in much dustier conditions than are found in homes and their breathing during hard labor is much deeper than normal. Consequently, there is a much greater chance that radon daughters will find their way deep into the lungs of miners in comparison with the general population. The miners' exposure to arsenic and diesel exhaust may also contribute to an increase in the lung cancer rate that would have been counted as due to radon.
In order to establish whether or not radon gas buildup in homes causes lung cancer, several epidemiological studies were undertaken in the 1990s. These analyses, one from Sweden, one from Canada, and one from the United States, reached contradictory conclusions about the risk of radon to householders. In the Swedish report the rate of lung cancer in nonsmokers and especially in smokers was found to increase with increasing levels of radon in their homes. The Canadian study focused on residents of Winnipeg, Manitoba, which has the highest average radon levels in Canada; no linkage between radon levels and lung cancer incidence was found. The U.S. study, conducted among nonsmoking women in Missouri, found little evidence for a trend of increasing lung cancer with increasing indoor radon concentration.
The environmental radon problem has received the greatest attention in the United States, where there are currently programs to test the air in the basements of a large number of homes for significantly elevated levels of the gas. Once radon is identified, the owners can' then alter the air circulation patterns to reduce radon levels in living areas, thereby reducing the additional risk of contracting lung cancer. Gaps and cracks in basement walls are sealed, and venting pipes through basement floor slabs are installed. About 800,000 U.S. homes have undergone mitigation to reduce high radon levels, at an average cost of $1200 per house, since the 1980s. It has been pointed out, however, that the normally high mobility of the U.S. population means that, on average, a given individual who happens to live in a house with a high level of radon will be there for only a few years and likely spend most of his or her life in a house with a lower level (since only about 7% of houses have high levels). Consequently, the estimate of increased lung cancer death mentioned at the beginning of this section is likely much too high.
A minority of scientists do not believe that radioactivity at very low levels causes harm to humans. They are skeptical about the linear-no-threshold (LNT) assumption—that the observed effects of high doses of radioactivity can be extrapolated linearly to very low doses and that there is no threshold below which radioactivity causes no harmful effects such as cancer. Some point to evidence of a threshold near about 1000 Bq/m3 for the effects of residential radon in causing lung cancer, and others point to the lack of cancer incidence in many areas having high natural radon levels. However, reviews published in 2005 and 2006 of epidemiological data from North America and Europe find no threshold for increase in lung cancer with residential radon.
Some scientists believe in a theory called hormesis, which states that exposure to radioactivity (and some chemicals) at very low doses for short periods of time can be positive to human health. Although there are some animal and cell studies that support this idea, it is not widely accepted among scientists. Indeed, the LNT theory is supported by recent evidence concerning cancer incidence among Russian citizens who were exposed inadvertently to very low doses of radioactivity from nuclear weapons production.
Depleted uranium is what remains of natural uranium once most ofthe 235U isotope, used in power reactors and for nuclear weapons, and the 234U have been extracted from it. Indeed, about 200 kg of depleted uranium is produced for every kilogram of the highly enriched element, since uranium is only 0.7% 238U. Because uranium is so dense (70% denser than lead), it is useful for making armor-piercing weapons, especially projectiles used against tanks. When the shells hit hard targets, the uranium ignites and the combustion creates clouds of dust containing uranium oxide. This dust settles and contaminates the soil in the area, but before that, it can be inhaled by people in the vicinity. Since most of the 235U has been extracted from the uranium (for use in bombs and power production), and since i38U has such a very long half-life, depleted uranium is less radioactive in terms of a-particle emissions (by almost half) than the naturally occurring element. However, the ¡3 emission from daughters of 238U is still present. Concerns have been expressed about the effect of the residual radioactivity from depleted uranium on troops and civilians exposed to it during wartime.
A dirty bomb is a conventional, chemical-based explosive mixed with radioactive material that would be distributed over a wide area as a result of the bomb's explosion. Dirty bombs could be made by terrorists, using radioactive material stolen from either hospitals or research institutes, or purchased on the black market from supplies originating in the former Soviet Union or other countries that have undergone disruption with a consequent loss of security at their nuclear energy facilities. Although the actual danger to human health from a dispersal of dirty bomb materials into the environment may well be quite small, the fear it would generate in the public and the expense of decontamination of wide areas would be substantial.
A similar terrorist threat would be the deliberate crashing of hijacked aircraft into nuclear power plants or into containers of nuclear fuel. However, most nuclear energy industry executives deny that the power plants or containers could be breached, and radioactivity released, by such an event.
Although most of the energy we use originates as heat generated by the combustion of carbon-containing fuels, heat in commercial quantities can also be produced indirectly when certain processes involving atomic nuclei occur; this power source is called nuclear energy, used mainly to produce electricity. Since nuclear forces are much stronger than chemical bond forces, the energy released per atom in nuclear reactions is immense compared to that obtained in combustion reactions. One of the attractions of nuclear power is that it does not generate carbon dioxide or other greenhouse gases during its operation. Some policymakers have promoted expansion of nuclear power as a way to combat global warming in the future.
There are two processes by which energy is obtained from atomic nuclei: fission and fusion.
• In fission, the collision of certain types of heavy nuclei (all of which have many neutrons and protons) with a neutron results in the splitting of the nucleus into two similarly sized fragments. Since the separated fragments are more stable energetically than was the original heavy nucleus, energy is released by the process.
• The combination of two very light nuclei to form one combined nucleus is called fusion. It also results in the release of huge amounts of energy, since the combined nucleus is more stable than the original, lighter ones.
Currently, there are 440 fission-based nuclear power plants in operation in more than 30 countries in the world. Collectively, they generate 17% of global electricity demand, including 23% of that in developed countries, 16% of that in the former Soviet Union, but only 2% of that in developing countries. Global production more than tripled between 1980 and 2000. The fraction of electricity produced by nuclear energy in various countries is listed in Table 9-2.
Nuclear Power Around the World (2005)
Number of Power Reactors
Proportion of Electricity Generated by Nuclear Power
The most economically useful example of fission, and the one mainly used by power plants, is induced by the collision of a 235U nucleus with a neutron. The combination of these two particles is unstable. When it decomposes, the products vary but are typically a nucleus of barium, 142Ba, one of krypton, 91 Kr, and three neutrons:
Not all the uranium nuclei that absorb a neutron form exactly the same products, but the process always produces two nuclei of about the sizes of Ba and Kr, together with several neutrons.
The two new nuclei produced in fission reactions are very fast-moving, as are the neutrons. It is heat energy from this excess kinetic energy that is used to produce electrical power. Indeed, the generation of electricity by nuclear energy and by the burning of fossil fuels both involve using the energy source to produce steam, which is then used to turn large turbines that produce the electricity.
An average of about three neutrons are produced per 23)U nucleus that reacts; one of these neutrons can produce the fission of another 235U nucleus, and so on, yielding a chain reaction. In atomic bombs, the extra neutrons are used to induce a very rapid fission of al
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