ANSWERS TO END-OF-CHAPTER QUESTIONS
ANSWERS TO END-OF-CHAPTER QUESTIONS
ANSWERS TO END-OF-CHAPTER QUESTIONS
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
<strong>ANSWERS</strong> <strong>TO</strong><br />
<strong>END</strong>-<strong>OF</strong>-<strong>CHAPTER</strong> <strong>QUESTIONS</strong><br />
<strong>CHAPTER</strong> 8: ENERGY FROM ELECTRON TRANSFER<br />
Emphasizing Essentials<br />
1. a. Define the terms oxidation and reduction.<br />
b. Why must these processes take place together?<br />
Answer:<br />
a. Oxidation is a process in which an atom, ion, or molecule loses one or more electrons.<br />
Reduction is a process in which an atom, ion, or molecule gains one or more electrons.<br />
b. Electrons must be transferred from the species losing electrons to the species gaining<br />
electrons.<br />
2. Which of these half-reactions represent oxidation and which reduction? Explain your<br />
reasoning.<br />
a. Fe(s) ⎯→ Fe 2+ (aq) + 2 e –<br />
b. Ni 4+ (aq) + 2 e – ⎯→ Ni 2+ (aq)<br />
c. 2 H2O(l) + 2 e – ⎯→ H2(g) + 2 OH – (aq)<br />
Answer:<br />
a. Oxidation. Iron loses two electrons to form the iron(II) ion.<br />
b. Reduction. The nickel(IV) ion gains two electrons to form the nickel(II) ion.<br />
c. Reduction. Water gains two electrons to the hydrogen ion and the hydroxide ion.<br />
3. You have seen several examples of oxidation–reduction reactions in this chapter. Now<br />
examine these equations and decide which are oxidation–reduction reactions and which are<br />
not. Explain your decisions.<br />
a. Zn(s) + 2 MnO2(s) + H2O(l) ⎯→ Zn(OH)2(s) + Mn2O3(s)<br />
b. HCl(aq) + NaOH(aq) ⎯→ NaCl(aq) + H2O(l)<br />
c. CH4(g) + 2 O2(g) ⎯→ CO2(g) + 2 H2O(g)<br />
Answer:<br />
Parts a and c are redox reactions. Electrons must be transferred when an element (Zn in<br />
part a, O2 in part c) reacts to form a compound. Part b is a neutralization reaction in which<br />
ions combine to form a soluble salt and water. Electron transfer does not occur in this case.<br />
4. Comment on the statement: Every combustion reaction is an oxidation–reduction reaction.<br />
Answer:<br />
This is a true statement. Combustion is the rapid combination of a fuel and oxygen to form<br />
products. In the case of burning a hydrocarbon, these products are CO2 and H2O. The oxygen<br />
is reduced, and the hydrogen and the carbon are oxidized to form H2O and CO2, respectively.<br />
PAGE 8-1
Even when substances that do not contain carbon are burned, the reactions can still be<br />
described in terms of oxidation and reduction.<br />
5. Two common units associated with electricity are the volt and the amp. What does each unit<br />
measure?<br />
Answer:<br />
Electric current (an amount of charge per second) is measured in amps. In contrast, the volt is<br />
a measure of electric potential, that is, the force or “pressure” behind this current.<br />
6. Consider this galvanic cell. A coating of impure silver metal begins to appear on the surface<br />
of the silver electrode as the cell discharges.<br />
a. Identify the anode and write the oxidation half-reaction.<br />
b. Identify the cathode and write the reduction half-reaction.<br />
Answer:<br />
a. The anode is Zn(s) and the oxidation half-reaction is:<br />
Zn(s) ⎯→ Zn 2+ (aq) + 2 e –<br />
b. The cathode is Ag(s) and the reduction half-reaction is:<br />
2 Ag + (aq) + 2 e – ⎯→ 2 Ag(s)<br />
7. In the lithium–iodine cell, Li is oxidized to Li + ; I2 is reduced to 2 I – .<br />
a. Write equations for the two half-reactions that take place in this cell, labeling one as<br />
oxidation and the other as reduction.<br />
b. Write an equation for the overall reaction in this cell.<br />
c. Identify the half-reaction that occurs at the anode and the half-reaction that occurs at the<br />
cathode.<br />
Answer:<br />
a. oxidation half-reaction: Li(s) ⎯→ Li + + e –<br />
reduction half-reaction: I2(s) + 2 e – ⎯→ 2 I –<br />
b. overall reaction: 2 Li(s) + I2(s) ⎯→ 2 LiI(s)<br />
c. The oxidation reaction, Li(s) ⎯→ Li + (aq) + e – , occurs at the anode.<br />
The reduction reaction, I2(s) + 2 e – ⎯→ 2 I – (aq), occurs at the cathode.<br />
PAGE 8-2
8. a. Is the voltage from a tiny AAA-size alkaline cell the same as that from a large D alkaline<br />
cell? Explain.<br />
b. Will both batteries sustain the flow of electrons for the same time? Why or why not?<br />
Answer:<br />
a. The voltage from both kinds of cells is the same (1.54 V) because voltage depends on the<br />
chemical reaction that is producing the electrical energy and not the size of the electrodes.<br />
The biggest difference in these two kinds of cells is the amount of current each produces. The<br />
amount of current produced by a cell does depend on the size of the cell: Larger cells<br />
generate more current than smaller cells do. The larger D cell will generate a larger current,<br />
but the same voltage as the smaller AAA cells.<br />
b. Larger cells contain more materials and can sustain the transfer of electrons over a longer<br />
period.<br />
9. Identify the type of battery commonly used in each of these consumer electronic products.<br />
Assume none uses solar cells.<br />
a. battery-powered watch c. digital camera<br />
b. MP3 player d. handheld calculator<br />
Answer:<br />
a. battery-powered watch: silver oxide battery<br />
b. MP3 player: Lithium ion battery<br />
c. digital camera: Lithium ion battery<br />
d. calculator: AA or AAA alkaline batteries<br />
10. The mercury battery has been used extensively in medicine and industry. Its overall reaction<br />
can be represented by this equation.<br />
HgO(l) + Zn(s) ⎯→ ZnO(s) + Hg(l)<br />
a. Write the oxidation half-reaction.<br />
b. Write the reduction half-reaction.<br />
c. Why is the mercury battery no longer in common use?<br />
Answer:<br />
a. Oxidation: Zn(s) + 2 OH – (aq) ⎯→ ZnO(s) + H2O(l) + 2 e –<br />
b. Reduction: HgO(s) + H2O(l) + 2 e – ⎯→ Hg(l) + 2 OH – (aq)<br />
c. In both 1970 and 1980, a major use for mercury was in batteries. By 1990, awareness of<br />
the dangers of mercury in urban trash had grown. Mercury is a toxic metal and (in some<br />
forms) can accumulate in the biosphere. Safer batteries and the need to recycle batteries led<br />
to the passage of the Mercury-Containing and Rechargeable Battery Management Act (The<br />
Battery Act) in 1996.<br />
11. a. What is the function of the electrolyte in a galvanic cell?<br />
b. What is the electrolyte in an alkaline cell?<br />
c. What is the electrolyte in a lead–acid storage battery?<br />
PAGE 8-3
Answer:<br />
a. The electrolyte completes the electrical circuit. It provides a medium for transport of ions,<br />
thus allowing charge to be transferred.<br />
b. KOH paste<br />
c. H2SO4(aq)<br />
12. These are the incomplete equations for the half-reactions in a lead storage battery. They do<br />
not show the electrons either lost or gained.<br />
Pb(s) + SO4 2– (aq) ⎯→ PbSO4(s)<br />
PbO2(s) + 4 H + (aq) + SO4 2– (aq) ⎯→ PbSO4(s) + 2 H2O(l)<br />
a. Balance both equations with respect to charge by adding electrons on either side of the<br />
equations, as needed.<br />
b. Which half-reaction represents oxidation and which represents reduction?<br />
c. One of the electrodes is made of lead, the other of lead dioxide. Which is the anode and<br />
which is the cathode?<br />
Answer:<br />
a. Pb(s) + SO4 2– (aq) ⎯→ PbSO4(s) + 2 e –<br />
PbO2(s) + 4 H + (aq) + SO4 2– (aq) + 2 e – ⎯→ PbSO4(s) + 2 H2O(l)<br />
b. The first half-reaction shows electrons being lost, so it is the half-reaction of oxidation.<br />
The second half-reaction shows electrons being gained, so it is the half-reaction of reduction.<br />
c. Lead is being oxidized, so lead is the anode. Lead dioxide is being reduced, so it is the<br />
cathode.<br />
13. What is meant by the term hybrid car?<br />
Answer:<br />
In current usage, the term hybrid car refers to the combination of a gasoline engine with a<br />
nickel-metal hydride battery, an electric motor, and an electric generator. Other hybrids using<br />
fuel cells are either available or under development.<br />
14. a. What is the role of the electrolyte in a fuel cell?<br />
b. List two advantages fuel cells have over internal combustion engines.<br />
Answer:<br />
a. The electrolyte allows ions to move between the electrodes, thus completing the circuit.<br />
b. Fuel cells using hydrogen gas (H2) as the fuel produce only water as a product, whereas<br />
internal combustion engines produce many products including the greenhouse gas carbon<br />
dioxide and pollutants such as carbon monoxide, soot, VOCs, and nitrogen monoxide. When<br />
a fuel is oxidized in a fuel cell, 45–55% of the energy released is transferred into electricity.<br />
Combustion engines are only 20–30% efficient because combustion releases energy in the<br />
form of heat, which then must be transferred to electrical and/or mechanical energy.<br />
15. Is the conversion of O2(g) to H2O(l) in a fuel cell an example of oxidation or reduction? Use<br />
electron loss or gain to support your answer.<br />
PAGE 8-4
Answer:<br />
The conversion of O2 to H2O requires a supply of electrons (see equation 8.13) so it is a<br />
reduction reaction. This is the reduction half-reaction:<br />
1/2 O2(g) + 2 H + (aq) + 2 e – ⎯→ H2O(l)<br />
16. Consider this diagram of a hydrogen–oxygen fuel cell used in earlier space missions.<br />
a. How does the reaction between hydrogen and oxygen in a fuel cell differ from the<br />
combustion of hydrogen with oxygen?<br />
b. Write the half-reaction that takes place at the anode in this fuel cell.<br />
c. Write the half-reaction that takes place at the cathode in this fuel cell.<br />
Answer:<br />
a. In the combustion of hydrogen with oxygen, the chemical energy is released in the form of<br />
heat and light, often explosively. In a fuel cell utilizing the same chemical reaction, the<br />
chemical energy is released in a controlled manner in the form of electricity.<br />
b. Anode reaction: H2(g) ⎯→ 2 H + (aq) + 2 e –<br />
c. Cathode reaction: 1/2 O2(g) + 2 H + (aq) + 2 e – ⎯→ H2O(l)<br />
17. a. What is a PEM fuel cell? How does it differ from the fuel cell represented in question #16?<br />
b. What is an S<strong>OF</strong>C? How does it differ from the fuel cell represented in question #16?<br />
Answer:<br />
a. PEM stands for proton exchange membrane. In a PEM fuel cell, H2 is oxidized to form H + .<br />
These H + move through the membrane to react with O2 (which is reduced) to form water. In<br />
the cell described in question #16, the same oxidation-reduction reactions occur, but the<br />
electrodes and electrolytes are different. In addition, the PEM fuel cell operates at room<br />
temperature and the one used for the space mission does not.<br />
b. S<strong>OF</strong>C stands for Solid Oxide Fuel Cell. An S<strong>OF</strong>C uses solid zirconium oxide and yttrium<br />
oxide as the electrolyte instead of the hot KOH solution shown in question #16. Also, oxide<br />
ions (O 2– ) instead of H + ions are transported from one electrode to the other through the<br />
electrolyte.<br />
18. In addition to hydrogen, methane also has been studied for use in PEM fuel cells. Balance the<br />
given oxidation and reduction half-reactions and write the overall equation for a methanebased<br />
fuel cell.<br />
PAGE 8-5
Oxidation half-reaction:<br />
__CH4(g) + __OH – (aq) ⎯→ __CO2(g) + __H2O(l) + __e –<br />
Reduction half-reaction:<br />
__O2(g) + __H2O(l) + __e – ⎯→ __OH – (aq)<br />
Answer:<br />
Oxidation half-reaction:<br />
CH4(g) + 8 OH – (aq) ⎯→ CO2(g) + 6 H2O(l) + 8 e –<br />
Reduction half-reaction:<br />
2 O2(g) + 4 H2O(l) + 8 e – ⎯→ 8 OH – (aq)<br />
Overall reaction:<br />
CH4(g) + 2 O2(g) ⎯→ CO2(g) + 2 H2O(l)<br />
19. The reactions in a hydrogen-fueled solid oxide fuel cell (S<strong>OF</strong>C) are shown in equations<br />
8.19–8.21. This is the skeleton equation for the oxidation half-reaction if CO, rather than H2,<br />
is the fuel.<br />
a. Balance by adding electrons as needed.<br />
b. Combine the balanced equation for oxidation with that for reduction.<br />
c. Write the overall equation for a carbon monoxide-based S<strong>OF</strong>C.<br />
Answer:<br />
The reactions in a hydrogen-fueled solid oxide fuel cell (S<strong>OF</strong>C) are shown in equations<br />
8.19–8.21.<br />
a. CO(g) + O 2– ⎯→ CO2(g) + 2 e –<br />
b. The reduction half reaction is: ½ O2(g) + 2 e – ⎯→ O 2–<br />
Combined, this gives: CO(g) + O 2– + ½ O2(g) + 2 e – ⎯→ CO2(g) + 2 e – + O 2–<br />
c. CO(g) + ½ O2(g) ⎯→ CO2(g)<br />
20. a. Potassium, a Group 1A metal, reacts with H2 to form potassium hydride, KH. Write the<br />
chemical equation for the reaction.<br />
b. Potassium hydride reacts with water to release H2 and form potassium hydroxide. Write<br />
the chemical equation.<br />
c. Offer a possible reason that potassium is not used to store H2 for use in fuel cells.<br />
Answer:<br />
a. K(s) + ½ H2(g) ⎯→ 2 KH(s)<br />
b. KH(s) + H2O(l) ⎯→ H2(g) + KOH(s)<br />
c. Potassium is a highly reactive metal that can react explosively with water. The risks would<br />
outweigh the benefits.<br />
PAGE 8-6
!<br />
21. a. What is meant by “the hydrogen economy”?<br />
b. Even if methods for producing hydrogen cheaply and in large quantities were to become<br />
available, what problems would still remain for the hydrogen economy?<br />
Answer:<br />
a. The hydrogen economy refers to producing, storing, and using hydrogen gas to produce<br />
energy.<br />
b. Before achieving all the benefits of the hydrogen economy, the problems of the safe<br />
production, transportation, and storage of large quantities of hydrogen will need to be solved.<br />
22. a. How are equations 8.24 and 8.25 the same and how are they different?<br />
b. How will the energy released in the reaction shown in equation 8.24 compare with the<br />
energy released in the reaction represented by equation 8.25? Explain your reasoning.<br />
Answer:<br />
a. Equations 8.24 and 8.25 are alike in that they both represent the same chemical process,<br />
the oxidation of O2 and reduction of H2 to form H2O. The coefficients differ by a factor of 2.<br />
Equation 8.24 represents the reaction of 2 moles of H2 with 1 mole of O2 to form 2 moles of<br />
H2O, while equation 8.25 represents the reaction of 1 mole of H2 with ½ mole of O2 to<br />
produce 1 mole of O2.<br />
b. The energy released in the reaction shown in equation 8.24 is double the amount released<br />
by the reaction shown in equation 8.25 because the first equation represents twice as many<br />
moles of reactants.<br />
23. Given that 286 kJ of energy is released per mole of H2 burned, what is the maximum amount<br />
of energy that can be released when 370 kg of H2 is burned?<br />
Answer:<br />
370 kg H 2 "<br />
1000 g<br />
1 kg " 1 mol H 2<br />
2.0 g H 2<br />
" 286 kJ<br />
1 mol H 2<br />
PAGE 8-7<br />
= 5.3 × 10 7 kJ<br />
24. a. Use bond energies from Table 4.2 to calculate the energy released when 1 mol of H2 burns.<br />
b. Compare your result with the stated value of 286 kJ. Account for any difference.<br />
Answer:<br />
a. First write the chemical equation for the reaction.<br />
Then consider the Lewis structures for the reaction.<br />
Energy needed to break bonds:<br />
436 kJ + 1/2 (498 kJ) = 685 kJ<br />
Energy released as new bonds form:<br />
2(467 kJ) = 2934 kJ<br />
The net energy change is (685 kJ) + (2934 kJ) = 2249 kJ.
. Average bond energies are based on bonds within molecules in the gaseous state. In the<br />
given chemical equation, the H2O formed is present as a liquid rather than as a gas.<br />
Additional energy is released when gaseous water condenses to the liquid state, so the stated<br />
value of 286 kJ is greater than the 249 kJ calculated in part a.<br />
25. Every year, 5.6 × 10 21 kJ of energy comes to Earth from the Sun. Why can’t this energy be<br />
used to meet all of our energy needs?<br />
Answer:<br />
Solar energy is distributed over Earth’s surface. Therefore, it is not very easy to capture,<br />
store, and transform solar energy to meet our energy needs.<br />
26. This unbalanced equation represents the last step in the production of pure silicon for use in<br />
solar cells.<br />
__Mg(s) + __SiCl4(l) ⎯→ __MgCl2(l) + __Si(s)<br />
a. How many electrons are transferred per atom of pure silicon formed?<br />
b. Is the production of pure silicon an oxidation or a reduction reaction? Why do you think<br />
so?<br />
Answer:<br />
a. This is the balanced equation: 2 Mg(s) + SiCl4(l) ⎯→ 2 MgCl2(l) + Si(s)<br />
Each magnesium atom loses 2 electrons, and there are 2 magnesium atoms providing<br />
electrons for one silicon atom. Silicon in SiCl4 picks up the four electrons.<br />
b. The silicon atoms in SiCl4 gain four electrons to form elemental silicon, a reduction<br />
process. The two magnesium atoms each lose two electrons, an oxidation process. Overall,<br />
the production of pure silicon is an oxidation-reduction reaction.<br />
27. The symbol represents an electron and the symbol represents a silicon atom. Does this<br />
diagram represent a gallium-doped p-type silicon semiconductor, or does it represent an<br />
arsenic-doped n-type silicon semiconductor? Explain your answer.<br />
Answer:<br />
Each Si atom is surrounded by 8 electrons, but the atom in the center (the one that is doping<br />
the semiconductor) is surrounded by 9 electrons. Silicon is in Group 4A and has 4 outer<br />
electrons. Thus central atom in the figure must have 5 outer electrons. This is consistent with<br />
an element in Group 5A such as arsenic, so this is an n-type silicon semiconductor.<br />
28. Describe the main reasons why solar cells have solar energy conversion efficiencies<br />
significantly less than the theoretical value of 31%.<br />
PAGE 8-8
Answer:<br />
The efficiency is reduced because some of the radiant energy is reflected by the cell or<br />
absorbed to produce heat instead of an electric current.<br />
Concentrating on Concepts<br />
29. Explain the significance of the title of this chapter, “Energy from Electron Transfer.”<br />
Answer:<br />
In every electrochemical process described in this chapter, energy is produced through<br />
electron transfer. Chemical reaction, such as those that take place in galvanic cells, batteries<br />
and fuel cells, produce electrons that can do work because the anode and cathode are<br />
physically separated in space. The transfer of electrons also may be initiated when light<br />
strikes a photovoltaic cell.<br />
30. Consider these three sources of light: a candle, a battery-powered flashlight, and an electric<br />
light bulb. For each source, provide:<br />
a. The origin of the light<br />
b. The immediate source of the energy that appears as light<br />
c. The original source of the energy that appears as light. Hint: Trace this back stepwise as<br />
far as possible.<br />
d. The end-products and by-products of using each<br />
e. The environmental costs associated with each<br />
f. The advantages and disadvantages of each light source<br />
Answer:<br />
Candlelight<br />
Origin The hot gases that burn and emit light.<br />
Immediate E The hydrocarbon wax, made either by bees or from petroleum.<br />
source<br />
Original E source Sunlight that drove photosynthesis, which in turn produced the plants<br />
from which bees gathered their food (or years ago died and formed<br />
fossil fuels).<br />
Products Products: CO2, H2O and small amounts of soot and CO.<br />
Environmental<br />
costs<br />
Advantages<br />
Disadvantages<br />
Mainly from the pollutants (and CO2, not classified as a pollutant as of<br />
2007) produced while burning the candles.<br />
Convenient, pretty to look at, produce dirty soot and sometimes start<br />
fires.<br />
PAGE 8-9
Light in a battery-powered flashlight<br />
Origin A wire that glows when it is heated to a high temperature.<br />
Immediate E Energy to heats the wire comes from a chemical reaction in the battery.<br />
source<br />
Original E source Several possibilities, depending on what energy source was used to<br />
produce the battery. Could have been fossil fuel consumption<br />
(ultimately solar energy) or nuclear power plant (nuclear fission).<br />
Products The end products are different chemicals in the battery while the byproducts<br />
are those that are produced during the manufacture of the<br />
battery, bulb, and flashlight.<br />
Environmental<br />
costs<br />
Advantages<br />
Disadvantages<br />
All those associated with the production and disposal of the battery<br />
materials, as well as the side products during the combustion of fossil<br />
fuels (or nuclear fission).<br />
Portable, convenient, clean for the user.<br />
Somewhat expensive.<br />
Light from an electric light bulb<br />
Origin A wire that glows when it is heated to a high temperature.<br />
Immediate E<br />
source<br />
Several possibilities, depending on what energy source was used to<br />
produce the electricity. Could have been fossil fuel consumption or<br />
nuclear power plant.<br />
Original E source The Sun or the ancient stellar synthesis that produced the uranium and<br />
other metals on our planet.<br />
Products The light bulb is very clean at the site where it is used, but produces<br />
pollutants such as NOx, SO2 and particulate matter at the power plant<br />
(if coal combustion) or spent nuclear fuel (if nuclear).<br />
Environmental<br />
costs<br />
Advantages<br />
Disadvantages<br />
See above.<br />
Convenient, safe, inexpensive.<br />
Few to the user, except that the energy costs are relatively high in<br />
comparison to using a fluorescent bulb.<br />
31. Explain the difference between a rechargeable battery and one that must be discarded. Use a<br />
NiCad battery and an alkaline battery as examples.<br />
PAGE 8-10
Answer:<br />
These batteries derive their voltage from different sets of chemical reactions. A rechargeable<br />
battery (such as a NiCad battery) is one in which the oxidation-reduction reaction can be<br />
reversed with the input of energy. This recharges the battery. The oxidation-reduction<br />
reaction in a non-rechargeable battery, such as an alkaline battery, cannot easily be reversed.<br />
Since no way exists to recharge alkaline batteries, they are discarded once they stop<br />
producing electrical energy.<br />
32. Is there a difference between a galvanic cell and an electrochemical cell? Explain, giving<br />
examples to support your answer.<br />
Answer:<br />
The chapter defines only one type of cell: galvanic. A galvanic cell (sometimes called a<br />
voltaic cell) is a device that converts the energy released in a spontaneous chemical reaction<br />
into electrical energy. The term electrochemical cell is broader than this. It also includes cells<br />
that require the input of energy to make them run. Batteries are galvanic cells. So are fuel<br />
cells. Electrolysis, a process described in Chapter 5 that splits water into oxygen and<br />
hydrogen, is an electrolytic cell.<br />
33. What is the difference between a storage battery and a fuel cell?<br />
Answer:<br />
A storage battery converts chemical energy into electrical energy by means of a reversible<br />
reaction. No reactants or products leave the “storage” battery and the reactants can be<br />
reformed during the recharging cycle. A fuel cell also converts chemical energy into<br />
electrical energy but the reaction is not reversible. A fuel cell continues to operate only if fuel<br />
and oxidant are continuously added, which is why it is classed as a “flow” battery.<br />
34. Why are electric cars powered by lead–acid storage batteries alone only a short-term solution<br />
to the problem of air pollution emissions from automobiles? Outline your reasoning.<br />
Answer:<br />
The emissions in question are the oxides of sulfur and nitrogen (SO2 and NOx) as well as<br />
particulate matter. Although cars powered by lead storage batteries do not emit these<br />
chemicals, nonetheless all of them are associated with the manufacture of these batteries.<br />
Thus the emissions still exist; they are just one step removed from the automobile. In<br />
addition, lead is a toxic metal with environmental consequences. Because of its high density,<br />
it is heavy (and expensive) to transport.<br />
35. AgZn batteries are replacing lead–acid batteries in small airplanes, such as<br />
Cessna172s.<br />
a. Why are these batteries, although more expensive, preferable to the lead–acid batteries<br />
used previously?<br />
b. Write the half-reaction of oxidation and of reduction.<br />
PAGE 8-11
Answer:<br />
a. These batteries are lighter and less toxic than the lead-acid batteries.<br />
b. The overall reaction is:<br />
Zn(s) + Ag2O(s) ⎯→ ZnO(s) + 2 Ag(s)<br />
The oxidation half-reaction is: Zn(s) + O 2– (aq) ⎯→ ZnO(s) + 2 e –<br />
The reduction half-reaction is: Ag2O(s) + 2 e – ⎯→ 2 Ag(s) + O 2– (aq)<br />
36. The battery of a cell phone discharges when the phone is in use. A manufacturer, while<br />
testing a new “power boost” system, reported these data.<br />
Time, min.sec Voltage, V<br />
0.00 6.56<br />
1.00 6.31<br />
2.00 6.24<br />
3.00 6.18<br />
4.00 6.12<br />
5.00 6.07<br />
6.35 6.03<br />
8.35 6.00<br />
11.05 5.90<br />
13.50 5.80<br />
16.00 5.70<br />
16.50 5.60<br />
a. Prepare a graph of these data.<br />
b. The manufacturer’s goal was to retain 90% of its initial voltage after 15 minutes of<br />
continuous use. Has that goal been achieved? Justify your answer using your graph.<br />
Answer:<br />
a.<br />
PAGE 8-12
. The manufacturer’s goal of retaining 90% of the initial battery voltage after 15 minutes of<br />
continuous use has not been achieved. 90% of the initial 6.56 V is 5.90 V, a level that was<br />
reached when the cell phone had been used for only 11 minutes.<br />
37. Assuming that hybrid cars are available in your area, what questions would you ask the car<br />
dealer before deciding to buy or lease a hybrid? Which of these questions do you consider<br />
most important? Offer reasons for your choices.<br />
Answer:<br />
The primary question is likely to be that of fuel economy. How do the new hybrids compare<br />
to other vehicles on the market? Follow-up questions include purchase price, safety record,<br />
maintenance costs, battery lifetime, environmental impacts, and available tax credits. All of<br />
these factors will influence the costs of purchasing and operating the car. Each factor’s<br />
relative importance will depend on personal choices and on the region of the country where<br />
the car is being purchased and driven.<br />
38. You never need to plug in Toyota’s gasoline–battery hybrid car to recharge the batteries.<br />
Explain.<br />
Answer:<br />
The Toyota hybrid car (the Prius) has a gasoline engine sitting side-by-side with nickel-metal<br />
hydride batteries, an electric motor, and a generator. The battery is recharged by regenerative<br />
braking, in which the kinetic energy of the car is transferred to stored electrical energy.<br />
39. Prepare a list of the environmental costs and benefits associated with hybrid vehicles.<br />
Compare that list with the environmental costs and benefits of vehicles powered by gasoline.<br />
On balance, which energy source do you favor, and why?<br />
Answer:<br />
Many hybrid vehicles have greater average fuel economy than their gasoline counterparts.<br />
(Note that many smaller gasoline-powered cars currently achieve better fuel efficiency than<br />
PAGE 8-13
large hybrid vehicles such as SUVs.) Hybrid vehicles generally have fewer emissions (CO,<br />
CO2, NOx, and particulate matter) than gasoline-powered cars. When hybrid vehicles are<br />
running on the electricity generated by braking, they are not depleting fossil fuel supplies.<br />
However, the greater consumer familiarity and lower price of gasoline-powered vehicles<br />
makes them attractive to many people.<br />
40. William C. Ford, Jr., chairman of the board of Ford Motor Company, is quoted as saying that<br />
going “totally green” with zero-emissions vehicles will be a real challenge. Regular drivers<br />
won’t buy high-tech clean cars, Ford admits, until the industry has a “no-trade-off” vehicle<br />
widely available. What do you think he means by a no-trade-off vehicle? Do you think he is<br />
justified in this opinion?<br />
Answer:<br />
A no-trade-off vehicle will perform just as well as a gasoline vehicle at the same price (and<br />
still have zero emissions). Although Mr. Ford’s words may have made sense when he spoke<br />
them, they are less true today (2007 as the 6 th edition goes to press). Those working in the<br />
industry are seeing the need to “go green” and are finding that “green” practices give them a<br />
competitive edge and that people are buying them.<br />
41. Fuel cells were invented in 1839, but never developed into practical devices for producing<br />
electrical energy until the U.S. space program in the 1960s. What advantages did fuel cells<br />
have over previous power sources?<br />
Answer:<br />
The space program required reliable, relatively lightweight power sources. Fuels cells, when<br />
compared to other types of batteries that were available at that time, met those specifications<br />
and did not “run down” or require recharging. A fuel cell will continue to operate as long as<br />
fuel is available.<br />
42. Hydrogen, H2, and methane, CH4, can each be used with oxygen in a fuel cell. Hydrogen and<br />
methane also can be burned directly. Which has greater heat content when burned, 1.00 g of<br />
H2 or 1.00 g of CH4? Hint: Write the balanced chemical equation for each reaction and use<br />
the bond energies in Table 4.2 to help answer this question.<br />
Answer:<br />
The bond energies listed in Table 4.2 for bonds breaking and forming in gases can be used to<br />
calculate the following heats of combustion. These differ somewhat from the values given in<br />
the beginning of this chapter, where the product water is given in the liquid state.<br />
H2(g) + ½ O2(g) ⎯→ H2O(g) heat of combustion = 249 kJ/mol<br />
CH4(g) + 2 O2(g) ⎯→ CO2(g)+ 2 H2O(g) heat of combustion = 814 kJ/mol<br />
In each case, the units of the calculated heat of combustion can be changed to kJ/gram by<br />
dividing by the molar mass of the fuel.<br />
249 kJ<br />
For hydrogen: " 1 mol H2 = 124 kJ<br />
For methane:<br />
!<br />
!<br />
mol H2 2.01g H2 814 kJ<br />
mol CH4 " 1 mol CH4 16.0 g CH4 g H 2<br />
= 50.9 kJ<br />
PAGE 8-14<br />
g CH 4
43. Engineers have developed a prototype fuel cell that converts gasoline to hydrogen and carbon<br />
monoxide. The carbon monoxide, in contact with a catalyst, then reacts with steam to<br />
produce carbon dioxide and more hydrogen.<br />
a. Write a set of reactions that describes this prototype fuel cell, using octane (C8H18) to<br />
represent the hydrocarbons in gasoline.<br />
b. Speculate as to the future economic success of this prototype fuel cell.<br />
Answer:<br />
a. Conversion of fuel: C8H18(l) + 4 O2(g) ⎯→ 9 H2(g) + 8 CO(g)<br />
catalyst<br />
Elimination of CO: CO(g) + H2O(g) ⎯→ CO2(g) + H2(g)<br />
b. This type of fuel cell is convenient because it runs on a liquid fuel, gasoline, rather than<br />
using gaseous hydrogen. However, the liquid fuel is still petroleum-based and therefore nonrenewable.<br />
It also burns to produce CO2, a greenhouse gas. Therefore, although such fuel<br />
cells may find specialty applications in the near future, their long-term prospects are not<br />
good.<br />
44. At this time, the U.S. Department of Transportation (DOT) prohibits passengers from<br />
carrying flammable fluids aboard aircraft. Explain how this might affect the development of<br />
microfuel cells for use in consumer electronics such as portable computers.<br />
Answer:<br />
While the microcells described in section 8.6 use methanol as a fuel, the small amounts<br />
required would be unlikely to cause a flammability concern. However, the volume of liquid<br />
fuel required to power a laptop or iPod may exceed the three ounces of liquid per container<br />
currently allowed by U.S. regulations.<br />
45. Consider this representation of two water molecules in the liquid state.<br />
a. What bonds are broken when water boils? Are these intermolecular or intramolecular<br />
bonds? Hint: See Chapter 5 for definitions.<br />
b. What bonds are broken when water is electrolyzed? Are these intermolecular or<br />
intramolecular bonds?<br />
Answer:<br />
a. When water boils, the hydrogen bonds between water molecules are broken. These are<br />
intermolecular bonds.<br />
PAGE 8-15
!<br />
!<br />
b. When water is electrolyzed, the covalent bonds within water molecules are broken. These<br />
are intramolecular bonds.<br />
46. Although hydrogen gas can be produced by the electrolysis of water, this reaction is usually<br />
not carried out on a large scale. Suggest a reason for this fact.<br />
Answer:<br />
Electrolysis of water requires an input of 286 kJ of energy per mole of water electrolyzed.<br />
Most of this energy comes from the burning of fossil fuels in conventional power plants. The<br />
inherent inefficiency associated with transforming heat into work limits the usefulness of<br />
large-scale electrolysis and makes the process energy intensive.<br />
47. Small quantities of hydrogen gas can be prepared in the lab by reacting metallic sodium with<br />
water, as shown in this equation.<br />
a. Calculate the grams of sodium needed to produce 1.0 mol of hydrogen gas.<br />
b. Calculate the grams of sodium needed to produce sufficient hydrogen to meet an<br />
American’s daily energy requirement of 1.1 × 10 6 kJ.<br />
c. If the price of sodium were $94/kg, what would be the cost of producing 1.0 mol of<br />
hydrogen? Assume the cost of water is negligible.<br />
Answer:<br />
a. 1.0 mol H2 "<br />
b.<br />
2 mol Na<br />
1 mol H 2<br />
1.1"10 6 kJ " 1 mol H 2<br />
286 kJ<br />
c. Using the result from part a:<br />
" 23.0 g Na<br />
1 mol Na<br />
" 2 mol Na<br />
1 mol H 2<br />
PAGE 8-16<br />
= 46.0 g Na<br />
" 23.0 g Na<br />
46.0 g Na "<br />
1 mol Na<br />
= 1.8 "10 5 g Na<br />
1 kg Na<br />
10 3 g Na "<br />
$94<br />
1 kg Na<br />
= $4.32<br />
48. a. As a fuel, hydrogen has both advantages and disadvantages. Set up parallel lists for the<br />
advantages and the disadvantages of using hydrogen as the fuel for transportation and for<br />
producing electricity. !<br />
b. Do you advocate the use of hydrogen as a fuel for transportation or for the production of<br />
electricity? Explain your position in a short article for your student newspaper.
Answer:<br />
a.<br />
Advantages of H 2<br />
Disadvantages of H 2<br />
Transportation Electricity<br />
• lightweight fuel<br />
• saves fossil fuels if the<br />
hydrogen is derived from<br />
renewable sources<br />
• can be used in fuel cells<br />
• potentially explosive<br />
• difficult to store, handle<br />
PAGE 8-17<br />
• large supply potentially<br />
available from water through<br />
electrolysis<br />
• fuel cells using H 2 are<br />
practical for some<br />
applications<br />
• expensive to extract H 2 from<br />
water<br />
• difficult to transport, store,<br />
and handle<br />
b. Personal decisions about the use of hydrogen as a fuel for transportation and/or the<br />
production of electricity should be based largely on the real advantages and disadvantages of<br />
this fuel in the two different applications.<br />
49. Fossil fuels have been called “. . . Sun’s ancient investment on Earth.” Explain this statement<br />
to a friend who is not enrolled in your course.<br />
Answer:<br />
Fossil fuels were formed as the result of photosynthetic processes that took place hundreds of<br />
millions of years ago. Solar energy was a necessary ingredient in the formation, so the energy<br />
that is stored in chemical bonds in fossil fuels originally was invested from the Sun.<br />
50. The cost of electricity generated by solar thermal power plants currently is greater than that<br />
of electricity produced by burning fossil fuels. Given this economic fact, suggest some<br />
strategies that might be used to promote the use of environmentally cleaner electricity from<br />
photovoltaics.<br />
Answer:<br />
Environmentally cleaner electricity may be required to meet the mandates of the Kyoto<br />
Accord. One strategy to promote the use of environmentally cleaner electricity could be to<br />
provide a tax break or a subsidy for companies that produce electricity without consuming<br />
fossil fuels. Changes in the tax system could be imposed that reflect the true dollar and<br />
environmental costs of burning fossil fuels.<br />
51. Name some of the current applications of photovoltaic cells other than the production of<br />
electricity in remote areas.<br />
Answer:<br />
PV devices have demonstrated their practical utility for satellites, highway signs, security and<br />
safety lighting, navigational buoys, and automobile recharging stations.
Exploring Extensions<br />
52. The aluminum–air battery is being explored for use in automobiles. In this battery, aluminum<br />
metal undergoes oxidation to Al 3+ and forms Al(OH)3. Oxygen from the air undergoes<br />
reduction to OH – ions.<br />
a. Write equations for the oxidation and reduction half-reactions. Use H2O as needed to<br />
balance the number of hydrogen atoms present, and add electrons as needed to balance the<br />
charge.<br />
b. Add the half-reactions to obtain the equation for the overall reaction in this cell.<br />
c. Specify which half-reaction occurs at the anode and which occurs at the cathode in the<br />
battery.<br />
d. What are the benefits of the widespread use of the aluminum–air battery? What are<br />
some of the limitations? Write a brief summary of your findings.<br />
e. What is the current state of development of this battery? Is it in use in any vehicles at<br />
the present time? What is its projected future use?<br />
Answer:<br />
a. Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – half-reaction of oxidation<br />
O2 + 2 H2O + 4 e – ⎯→ 4 OH – half-reaction of reduction<br />
b. To make the number of electrons the same, the first half-reaction must be multiplied by 4,<br />
and the second half-reaction must be multiplied by 3. In this way, 12 moles of electrons are<br />
exchanged in the overall reaction.<br />
4 (Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – )<br />
3 (O2 + 2 H2O + 4 e – ⎯→ 4 OH – )<br />
The net equation is 4 Al + 3 O2 + 6 H2O ⎯→ 4 Al(OH)3.<br />
c. The half-reaction of oxidation, Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – , occurs at the<br />
anode.<br />
The half-reaction of reduction, O2 + 2 H2O + 4 e – ⎯→ 4 OH – , occurs at the cathode.<br />
d. Potential benefits are that aluminum is recyclable, non-combustible, and it is capable of<br />
storing a lot of energy. An aluminum–air battery is quite light and uses air as a reactant. It is a<br />
non-polluting source of power. Some of the limitations are that obtaining aluminum from its<br />
ore is a very energy-intensive process. Even recycling aluminum in a large volume requires<br />
considerable energy, potentially limiting the cost and availability of aluminum. In early<br />
versions of the battery, the electrolyte degraded the aluminum even when the battery was not<br />
in use. Recently, companies have overcome this problem.<br />
e. Despite twenty-five years of research to develop the aluminum–air battery, its use has been<br />
mostly in military applications and no commercial products use the technology. A French<br />
company, Métalectrique (http://www.metalectrique.com), is developing aluminum–air<br />
batteries for use in mobile refrigerators, electric cars, and electric wheelchairs.<br />
53. An iron-based “superbattery” is a promising alternative for delivering more power with<br />
fewer environmental effects than alkaline batteries. Find out how the superbattery is designed<br />
and its state of commercial acceptance.<br />
PAGE 8-18
Answer:<br />
Developed by Stuart Licht of the Technion-Israel Institute of Technology in Haifa, Israel,<br />
this rechargeable battery is designed using less toxic materials than other batteries and has a<br />
50% higher energy advantage. In these batteries the manganese dioxide cathode found in a<br />
traditional battery is replaced with compounds containing iron in a +6 oxidation state. (Thus<br />
the batteries are also called “super-iron” batteries.) The batteries can be used anywhere where<br />
AAA batteries are currently used, but the superbatteries are not yet on the market. Additional<br />
information about Licht’s battery is available at:<br />
http://www.weizmann.ac.il/ICS/booklet/3/pdf3/3.pdf.<br />
54. Although Alessandro Volta is credited with the invention of the first electric battery in<br />
1800, some feel this is a reinvention. Research the “Baghdad battery” to evaluate the merit of<br />
this claim.<br />
Answer:<br />
Clay jars found in the National Museum of Iraq have been described as the oldest known<br />
electric batteries in existence. They were initially attributed to the Parthian Empire, an<br />
ancient Asian culture that ruled most of the Middle East from 247 B.C. to A.D. 228.<br />
However, modern archaeologists think the jars may have been created in the later Sassanid<br />
era (A. D. 224 to 640).<br />
The jars were first described in 1938 by German archaeologist Wilhelm König, and to this<br />
day, it is uncertain whether Konig dug them up himself or found them archived in the<br />
museum.<br />
Those who have examined the jars closely say that there is little else that they can be but a<br />
battery. A typical nondescript earthen jar is only 5½ inches high by 3 inches across. The<br />
opening was sealed with an asphalt plug, which held in place a copper sheet, rolled into a<br />
tube. This tube was capped at the bottom with a copper disc held in place by more asphalt. A<br />
narrow iron rod was stuck through the upper asphalt plug and hung down into the center of<br />
the copper tube — not touching any part of it.<br />
Fill the jar with an acidic liquid, such as vinegar or fermented grape juice, and the jar<br />
becomes a battery capable of generating a small current. The acidic liquid permits a flow of<br />
electrons from the copper tube to the iron rod – an electric flow – when the two metal<br />
terminals are connected on the outside of the jar.<br />
Some archeologists remain skeptical that the people making these batteries understood the<br />
batteries’ electrical properties. It is simple to prove that the batteries can generate an<br />
electrical current today, but it is much more difficult to predict what the batteries were used<br />
for in ancient times.<br />
PAGE 8-19
55. If all of today’s technology presently based on fossil fuel combustion were replaced by<br />
H2–O2 fuel cells, significantly more H2O would be released into the environment. Is this<br />
effect a concern? Find out what other effects might be anticipated from switching to a<br />
hydrogen economy.<br />
Answer:<br />
While water is not a criteria air pollutant, it is a greenhouse gas. Excess water in the<br />
atmosphere could therefore contribute to global warming. Other effects include the release of<br />
excess CO2 from the hydrogen production, and those listed in 56b below.<br />
56. a. Hydrogen is generally considered as an environmentally friendly fuel, only<br />
producing water after reacting with oxygen. What effect could the widespread use of<br />
hydrogen have on urban air quality?<br />
b. Some scientists are reporting concerns that leakage of hydrogen gas from cars, hydrogen<br />
production plants, and fuel transportation could cause problems in Earth’s ozone layer. How<br />
significant are these concerns? What is the mechanism by which hydrogen could destroy<br />
ozone?<br />
Answer:<br />
a. The widespread use of hydrogen fuel could dramatically improve urban air quality by<br />
reducing levels of particulate matter and SOx produced by internal combustion engines.<br />
b. In June of 2003, geochemists at Caltech created a stir in the scientific community when<br />
they published theoretical models in the journal Science that predicted hydrogen could leak<br />
from production plants, transport systems, and hydrogen-powered cars in the future. They<br />
predicted a large amount of hydrogen would reach the stratosphere assuming all current<br />
fossil fuel combustion was replaced by hydrogen fuel cells and 10-20% of the hydrogen was<br />
lost to leakage. The additional hydrogen in the stratosphere would react and significantly<br />
increase the amount of water molecules found as ice in the stratosphere. Because ozone is<br />
destroyed by reactions with chlorine radicals that take place on stratospheric ice, the<br />
researchers proposed that the rate of ozone destruction would be enhanced. However, other<br />
scientists in academic and industrial research claim the Caltech group’s hydrogen leakage<br />
estimate is too high. One automobile executive argues that the industry would not allow such<br />
a large amount of a valuable commodity (hydrogen) to go to waste. Another scientist points<br />
out that the leaking hydrogen can be recovered. Others use the Caltech group’s research to<br />
support the idea of on-site hydrogen production.<br />
57. At the cutting edge of technology the line between science and science fiction often<br />
blurs. Investigate the “futuristic” idea of putting mirrors in orbit around Earth to focus and<br />
concentrate solar energy for use in generating electricity.<br />
Answer:<br />
Mirrors or lenses could be used to concentrate solar energy on a collection array in space and<br />
send the electricity back to Earth. This technology would counteract one of the major<br />
PAGE 8-20
limitations of current solar technology – the light from the Sun spreads out over Earth and is<br />
relatively weak. Unlike Earth-based solar collectors, orbiting collecting satellites would also<br />
have the advantage of being in the light all the time.<br />
http://www.thespacereview.com/article/214/1<br />
58. Although silicon, used to make solar cells, is one of the most abundant elements in<br />
Earth’s crust, extracting it from minerals is costly. The increased demand for solar cells has<br />
some companies worried about a “silicon shortage.” Use the resources of the Web to find out<br />
how silicon is purified and how the PV industry is coping with the rising prices.<br />
Answer:<br />
Crystalline silicon is used for the production of photovoltaic cells. Two of the common<br />
procedures for synthesizing crystalline silicon are Czochralski Crystal Growth and Float<br />
Zone Crystal Growth. Information can be found at:<br />
http://www.siliconfareast.com/crystal.htm.<br />
To cope with the shortage of silicon, the photovoltaic industry is undergoing a series of<br />
changes. Companies are pursuing business models to increase productivity and secure supply<br />
lines. Researchers are developing and testing a number of synthetic molecules for the next<br />
generation of photovoltaic collectors.<br />
http://www.renewableenergyaccess.com/rea/news/story?id=43983<br />
59. Figure 8.27 shows an array of photovoltaic cells installed at the Bavaria Solar Park in<br />
Germany. Where in the United States is the largest photovoltaic power plant? Use the Web to<br />
learn of other large-scale photovoltaic cell installations. What factors help to influence this<br />
approach, one that uses a centralized array rather than individual rooftop solar units?<br />
Answer:<br />
Photovoltaics (PV) are being used in parts of California and in Arizona in large-scale<br />
demonstration projects. For example, Pacific Gas and Electric in Kerman, CA is close to<br />
rapidly growing Fresno, CA. The PV substation experiment is designed to measure the value<br />
to the utility of a 500-kW generating plant that can be quickly (within 6 months) placed<br />
where extra power is needed. The results of this experiment will be widely distributed.<br />
In April 2007, construction began on an array of photovoltaics at an Air Force base in<br />
Nevada. When completed, the array will be the largest in the US and will provide the base<br />
with 15-18 megawatts of power, about 25-30% of its need.<br />
PV generation plants have several characteristics that have slowed their use by utilities.<br />
Under current utility accounting, PV-generated electricity still costs considerably more than<br />
electricity generated by conventional plants, and regulatory agencies require most utilities to<br />
supply electricity for the lowest cash cost. Furthermore, photovoltaic systems produce power<br />
PAGE 8-21
only during daylight hours and their output varies with the weather. Utility planners must<br />
therefore treat a PV power plant differently than they would treat a conventional plant.<br />
Despite the costs, utilities are becoming more involved with PV. An alliance has been formed<br />
between DOE, the Electric Power Research Institute (EPRI), and several utilities. The<br />
program is called Photovoltaics for Utility-Scale Applications (PVUSA) and there are<br />
presently three pilot projects, including the Kerman PV plant. Photovoltaics have found some<br />
use in other parts of the world, including Australia, Japan, and as the text discusses,<br />
Switzerland. Developing countries have been slower to use photovoltaics on a large scale<br />
because of the high cost.<br />
60. The Solar America Initiative program receives so little publicity that most people in the<br />
United States are unaware of its existence. Design a poster to explain and promote some part<br />
of this program to the general public.<br />
Answer:<br />
The Solar America Initiative is coordinated by the U.S. Department of Energy. The program<br />
aims to improve the cost-effectiveness of solar energy to the levels of conventional forms of<br />
energy (coal, nuclear power, hydroelectric power, etc) by 2015. Funding is available to<br />
universities and companies for research and development of solar technologies. The program<br />
also gives grants to local governments to encourage changes in energy regulations and<br />
promotion of solar technology among residents. Thirteen U.S. cities were chosen to be Solar<br />
America Cities in 2007, and a second round of grants will be announced in 2008. A third<br />
grant program supports Solar America Showcases, which are large projects that draw public<br />
attention to the benefits of solar energy. The first showcases funded are powering homes on a<br />
military base in Oahu, Hawaii, large buildings in San Jose, CA, and the Orange County<br />
convention center in Florida. Student posters could focus on any of these aspects of the Solar<br />
America Initiative.<br />
PAGE 8-22