ANSWERS TO END-OF-CHAPTER QUESTIONS CHAPTER 8: ENERGY FROM ELECTRON TRANSFER Emphasizing Essentials 1. a. Define the terms oxidation and reduction. b. Why must these processes take place together? 2. Which of these half-reactions represent oxidation and which reduction? Explain your reasoning. a. Fe(s) Fe 2+ (aq) + 2 e b. Ni 4+ (aq) + 2 e Ni 2+ (aq) c. 2 H 2 O(l) + 2 e H 2 (g) + 2 OH (aq) 3. You have seen several examples of oxidation reduction reactions in this chapter. Now examine these equations and decide which are oxidation reduction reactions and which are not. Explain your decisions. a. Zn(s) + 2 MnO 2 (s) + H 2 O(l) Zn(OH) 2 (s) + Mn 2 O 3 (s) b. HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) c. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(g) 4. Comment on the statement: Every combustion reaction is an oxidation reduction reaction. Even when substances that do not contain carbon are burned, the reactions can still be described in terms of oxidation and reduction. 5. Two common units associated with electricity are the volt and the amp. What does each unit measure? 6. Consider this galvanic cell. A coating of impure silver metal begins to appear on the surface of the silver electrode as the cell discharges. PAGE 8-1
a. Identify the anode and write the oxidation half-reaction. b. Identify the cathode and write the reduction half-reaction. 7. In the lithium iodine cell, Li is oxidized to Li + ; I 2 is reduced to 2 I. a. Write equations for the two half-reactions that take place in this cell, labeling one as oxidation and the other as reduction. b. Write an equation for the overall reaction in this cell. c. Identify the half-reaction that occurs at the anode and the half-reaction that occurs at the cathode. 8. a. Is the voltage from a tiny AAA-size alkaline cell the same as that from a large D alkaline cell? Explain. b. Will both batteries sustain the flow of electrons for the same time? Why or why not? 9. Identify the type of battery commonly used in each of these consumer electronic products. Assume none uses solar cells. a. battery-powered watch c. digital camera b. MP3 player d. handheld calculator 10. The mercury battery has been used extensively in medicine and industry. Its overall reaction can be represented by this equation. HgO(l) + Zn(s) ZnO(s) + Hg(l) a. Write the oxidation half-reaction. b. Write the reduction half-reaction. c. Why is the mercury battery no longer in common use? 11. a. What is the function of the electrolyte in a galvanic cell? b. What is the electrolyte in an alkaline cell? c. What is the electrolyte in a lead acid storage battery? PAGE 8-2
12. These are the incomplete equations for the half-reactions in a lead storage battery. They do not show the electrons either lost or gained. Pb(s) + SO 4 2 (aq) PbSO 4 (s) PbO 2 (s) + 4 H + (aq) + SO 4 2 (aq) PbSO 4 (s) + 2 H 2 O(l) a. Balance both equations with respect to charge by adding electrons on either side of the equations, as needed. b. Which half-reaction represents oxidation and which represents reduction? c. One of the electrodes is made of lead, the other of lead dioxide. Which is the anode and which is the cathode? 13. What is meant by the term hybrid car? 14. a. What is the role of the electrolyte in a fuel cell? b. List two advantages fuel cells have over internal combustion engines. 15. Is the conversion of O 2 (g) to H 2 O(l) in a fuel cell an example of oxidation or reduction? Use electron loss or gain to support your answer. 16. Consider this diagram of a hydrogen oxygen fuel cell used in earlier space missions. a. How does the reaction between hydrogen and oxygen in a fuel cell differ from the combustion of hydrogen with oxygen? b. Write the half-reaction that takes place at the anode in this fuel cell. c. Write the half-reaction that takes place at the cathode in this fuel cell. 17. a. What is a PEM fuel cell? How does it differ from the fuel cell represented in question #16? b. What is an SOFC? How does it differ from the fuel cell represented in question #16? PAGE 8-3
18. In addition to hydrogen, methane also has been studied for use in PEM fuel cells. Balance the given oxidation and reduction half-reactions and write the overall equation for a methanebased fuel cell. 19. The reactions in a hydrogen-fueled solid oxide fuel cell (SOFC) are shown in equations 8.19 8.21. This is the skeleton equation for the oxidation half-reaction if CO, rather than H 2, is the fuel. a. Balance by adding electrons as needed. b. Combine the balanced equation for oxidation with that for reduction. c. Write the overall equation for a carbon monoxide-based SOFC. 20. a. Potassium, a Group 1A metal, reacts with H 2 to form potassium hydride, KH. Write the chemical equation for the reaction. b. Potassium hydride reacts with water to release H 2 and form potassium hydroxide. Write the chemical equation. c. Offer a possible reason that potassium is not used to store H 2 for use in fuel cells. 21. a. What is meant by the hydrogen economy? b. Even if methods for producing hydrogen cheaply and in large quantities were to become available, what problems would still remain for the hydrogen economy? 22. a. How are equations 8.24 and 8.25 the same and how are they different? b. How will the energy released in the reaction shown in equation 8.24 compare with the energy released in the reaction represented by equation 8.25? Explain your reasoning. 23. Given that 286 kj of energy is released per mole of H 2 burned, what is the maximum amount of energy that can be released when 370 kg of H 2 is burned? 24. a. Use bond energies from Table 4.2 to calculate the energy released when 1 mol of H 2 burns. b. Compare your result with the stated value of 286 kj. Account for any difference. 25. Every year, 5.6 10 21 kj of energy comes to Earth from the Sun. Why can t this energy be used to meet all of our energy needs? 26. This unbalanced equation represents the last step in the production of pure silicon for use in solar cells. Mg(s) + SiCl 4 (l) MgCl 2 (l) + Si(s) PAGE 8-4
a. How many electrons are transferred per atom of pure silicon formed? b. Is the production of pure silicon an oxidation or a reduction reaction? Why do you think so? 27. The symbol represents an electron and the symbol represents a silicon atom. Does this diagram represent a gallium-doped p-type silicon semiconductor, or does it represent an arsenic-doped n-type silicon semiconductor? Explain your answer. 28. Describe the main reasons why solar cells have solar energy conversion efficiencies significantly less than the theoretical value of 31%. Concentrating on Concepts 29. Explain the significance of the title of this chapter, Energy from Electron Transfer. 30. Consider these three sources of light: a candle, a battery-powered flashlight, and an electric light bulb. For each source, provide: a. The origin of the light b. The immediate source of the energy that appears as light c. The original source of the energy that appears as light. Hint: Trace this back stepwise as far as possible. d. The end-products and by-products of using each e. The environmental costs associated with each f. The advantages and disadvantages of each light source Answer: Candlelight Origin Immediate E source Original E source Products The hot gases that burn and emit light. The hydrocarbon wax, made either by bees or from petroleum. Sunlight that drove photosynthesis, which in turn produced the plants from which bees gathered their food (or years ago died and formed fossil fuels). Products: CO 2, H 2 O and small amounts of soot and CO. PAGE 8-5
Environmental costs Advantages Disadvantages Mainly from the pollutants (and CO 2, not classified as a pollutant as of 2007) produced while burning the candles. Convenient, pretty to look at, produce dirty soot and sometimes start fires. Light in a battery-powered flashlight Origin A wire that glows when it is heated to a high temperature. Immediate E Energy to heats the wire comes from a chemical reaction in the battery. source Original E source Several possibilities, depending on what energy source was used to produce the battery. Could have been fossil fuel consumption (ultimately solar energy) or nuclear power plant (nuclear fission). Products Environmental costs Advantages Disadvantages The end products are different chemicals in the battery while the byproducts are those that are produced during the manufacture of the battery, bulb, and flashlight. All those associated with the production and disposal of the battery materials, as well as the side products during the combustion of fossil fuels (or nuclear fission). Portable, convenient, clean for the user. Somewhat expensive. Light from an electric light bulb Origin A wire that glows when it is heated to a high temperature. Immediate E source Original E source Products Environmental Several possibilities, depending on what energy source was used to produce the electricity. Could have been fossil fuel consumption or nuclear power plant. The Sun or the ancient stellar synthesis that produced the uranium and other metals on our planet. The light bulb is very clean at the site where it is used, but produces pollutants such as NO x, SO 2 and particulate matter at the power plant (if coal combustion) or spent nuclear fuel (if nuclear). See above. PAGE 8-6
costs Advantages Disadvantages Convenient, safe, inexpensive. Few to the user, except that the energy costs are relatively high in comparison to using a fluorescent bulb. 31. Explain the difference between a rechargeable battery and one that must be discarded. Use a NiCad battery and an alkaline battery as examples. 32. Is there a difference between a galvanic cell and an electrochemical cell? Explain, giving examples to support your answer. 33. What is the difference between a storage battery and a fuel cell? 34. Why are electric cars powered by lead acid storage batteries alone only a short-term solution to the problem of air pollution emissions from automobiles? Outline your reasoning. 35. AgZn batteries are replacing lead acid batteries in small airplanes, such as Cessna172s. a. Why are these batteries, although more expensive, preferable to the lead acid batteries used previously? b. Write the half-reaction of oxidation and of reduction. 36. The battery of a cell phone discharges when the phone is in use. A manufacturer, while testing a new power boost system, reported these data. Time, min.sec Voltage, V 0.00 6.56 1.00 6.31 2.00 6.24 3.00 6.18 4.00 6.12 5.00 6.07 6.35 6.03 8.35 6.00 11.05 5.90 13.50 5.80 16.00 5.70 PAGE 8-7
16.50 5.60 a. Prepare a graph of these data. b. The manufacturer s goal was to retain 90% of its initial voltage after 15 minutes of continuous use. Has that goal been achieved? Justify your answer using your graph. Answer: a. b. The manufacturer s goal of retaining 90% of the initial battery voltage after 15 minutes of continuous use has not been achieved. 90% of the initial 6.56 V is 5.90 V, a level that was reached when the cell phone had been used for only 11 minutes. 37. Assuming that hybrid cars are available in your area, what questions would you ask the car dealer before deciding to buy or lease a hybrid? Which of these questions do you consider most important? Offer reasons for your choices. PAGE 8-8
38. You never need to plug in Toyota s gasoline battery hybrid car to recharge the batteries. Explain. 39. Prepare a list of the environmental costs and benefits associated with hybrid vehicles. Compare that list with the environmental costs and benefits of vehicles powered by gasoline. On balance, which energy source do you favor, and why? 40. William C. Ford, Jr., chairman of the board of Ford Motor Company, is quoted as saying that going totally green with zero-emissions vehicles will be a real challenge. Regular drivers won t buy high-tech clean cars, Ford admits, until the industry has a no-trade-off vehicle widely available. What do you think he means by a no-trade-off vehicle? Do you think he is justified in this opinion? 41. Fuel cells were invented in 1839, but never developed into practical devices for producing electrical energy until the U.S. space program in the 1960s. What advantages did fuel cells have over previous power sources? 42. Hydrogen, H 2, and methane, CH 4, can each be used with oxygen in a fuel cell. Hydrogen and methane also can be burned directly. Which has greater heat content when burned, 1.00 g of H 2 or 1.00 g of CH 4? Hint: Write the balanced chemical equation for each reaction and use the bond energies in Table 4.2 to help answer this question. 43. Engineers have developed a prototype fuel cell that converts gasoline to hydrogen and carbon monoxide. The carbon monoxide, in contact with a catalyst, then reacts with steam to produce carbon dioxide and more hydrogen. a. Write a set of reactions that describes this prototype fuel cell, using octane (C 8 H 18 ) to represent the hydrocarbons in gasoline. b. Speculate as to the future economic success of this prototype fuel cell. 44. At this time, the U.S. Department of Transportation (DOT) prohibits passengers from carrying flammable fluids aboard aircraft. Explain how this might affect the development of microfuel cells for use in consumer electronics such as portable computers. 45. Consider this representation of two water molecules in the liquid state. PAGE 8-9
a. What bonds are broken when water boils? Are these intermolecular or intramolecular bonds? Hint: See Chapter 5 for definitions. b. What bonds are broken when water is electrolyzed? Are these intermolecular or intramolecular bonds? 46. Although hydrogen gas can be produced by the electrolysis of water, this reaction is usually not carried out on a large scale. Suggest a reason for this fact. 47. Small quantities of hydrogen gas can be prepared in the lab by reacting metallic sodium with water, as shown in this equation. a. Calculate the grams of sodium needed to produce 1.0 mol of hydrogen gas. b. Calculate the grams of sodium needed to produce sufficient hydrogen to meet an American s daily energy requirement of 1.1 10 6 kj. c. If the price of sodium were $94/kg, what would be the cost of producing 1.0 mol of hydrogen? Assume the cost of water is negligible. 48. a. As a fuel, hydrogen has both advantages and disadvantages. Set up parallel lists for the advantages and the disadvantages of using hydrogen as the fuel for transportation and for producing electricity. b. Do you advocate the use of hydrogen as a fuel for transportation or for the production of electricity? Explain your position in a short article for your student newspaper. Answer: a. Advantages of H 2 Transportation lightweight fuel saves fossil fuels if the Electricity large supply potentially available from water through PAGE 8-10
Disadvantages of H 2 hydrogen is derived from renewable sources can be used in fuel cells potentially explosive difficult to store, handle electrolysis fuel cells using H 2 are practical for some applications expensive to extract H 2 from water difficult to transport, store, and handle b. Personal decisions about the use of hydrogen as a fuel for transportation and/or the production of electricity should be based largely on the real advantages and disadvantages of this fuel in the two different applications. 49. Fossil fuels have been called... Sun s ancient investment on Earth. Explain this statement to a friend who is not enrolled in your course. 50. The cost of electricity generated by solar thermal power plants currently is greater than that of electricity produced by burning fossil fuels. Given this economic fact, suggest some strategies that might be used to promote the use of environmentally cleaner electricity from photovoltaics. 51. Name some of the current applications of photovoltaic cells other than the production of electricity in remote areas. Exploring Extensions 52. The aluminum air battery is being explored for use in automobiles. In this battery, aluminum metal undergoes oxidation to Al 3 and forms Al(OH) 3. Oxygen from the air undergoes reduction to OH ions. a. Write equations for the oxidation and reduction half-reactions. Use H 2 O as needed to balance the number of hydrogen atoms present, and add electrons as needed to balance the charge. b. Add the half-reactions to obtain the equation for the overall reaction in this cell. c. Specify which half-reaction occurs at the anode and which occurs at the cathode in the battery. d. What are the benefits of the widespread use of the aluminum air battery? What are some of the limitations? Write a brief summary of your findings. e. What is the current state of development of this battery? Is it in use in any vehicles at the present time? What is its projected future use? PAGE 8-11
53. An iron-based superbattery is a promising alternative for delivering more power with fewer environmental effects than alkaline batteries. Find out how the superbattery is designed and its state of commercial acceptance. 54. Although Alessandro Volta is credited with the invention of the first electric battery in 1800, some feel this is a reinvention. Research the Baghdad battery to evaluate the merit of this claim. 55. If all of today s technology presently based on fossil fuel combustion were replaced by H 2 O 2 fuel cells, significantly more H 2 O would be released into the environment. Is this effect a concern? Find out what other effects might be anticipated from switching to a hydrogen economy. 56. a. Hydrogen is generally considered as an environmentally friendly fuel, only producing water after reacting with oxygen. What effect could the widespread use of hydrogen have on urban air quality? b. Some scientists are reporting concerns that leakage of hydrogen gas from cars, hydrogen production plants, and fuel transportation could cause problems in Earth s ozone layer. How significant are these concerns? What is the mechanism by which hydrogen could destroy ozone? 57. At the cutting edge of technology the line between science and science fiction often blurs. Investigate the futuristic idea of putting mirrors in orbit around Earth to focus and concentrate solar energy for use in generating electricity. 58. Although silicon, used to make solar cells, is one of the most abundant elements in Earth s crust, extracting it from minerals is costly. The increased demand for solar cells has some companies worried about a silicon shortage. Use the resources of the Web to find out how silicon is purified and how the PV industry is coping with the rising prices. 59. Figure 8.27 shows an array of photovoltaic cells installed at the Bavaria Solar Park in Germany. Where in the United States is the largest photovoltaic power plant? Use the Web to PAGE 8-12
learn of other large-scale photovoltaic cell installations. What factors help to influence this approach, one that uses a centralized array rather than individual rooftop solar units? 60. The Solar America Initiative program receives so little publicity that most people in the United States are unaware of its existence. Design a poster to explain and promote some part of this program to the general public. PAGE 8-13