ANSWERS TO END-OF-CHAPTER QUESTIONS

large hybrid vehicles such as SUVs.) Hybrid vehicles generally have fewer emissions (CO,
CO2, NOx, and particulate matter) than gasoline-powered cars. When hybrid vehicles are
running on the electricity generated by braking, they are not depleting fossil fuel supplies.
However, the greater consumer familiarity and lower price of gasoline-powered vehicles
makes them attractive to many people.
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?
Answer:
A no-trade-off vehicle will perform just as well as a gasoline vehicle at the same price (and
still have zero emissions). Although Mr. Ford’s words may have made sense when he spoke
them, they are less true today (2007 as the 6 th edition goes to press). Those working in the
industry are seeing the need to “go green” and are finding that “green” practices give them a
competitive edge and that people are buying them.
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?
Answer:
The space program required reliable, relatively lightweight power sources. Fuels cells, when
compared to other types of batteries that were available at that time, met those specifications
and did not “run down” or require recharging. A fuel cell will continue to operate as long as
fuel is available.
42. Hydrogen, H2, and methane, CH4, 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
H2 or 1.00 g of CH4? Hint: Write the balanced chemical equation for each reaction and use
the bond energies in Table 4.2 to help answer this question.
Answer:
The bond energies listed in Table 4.2 for bonds breaking and forming in gases can be used to
calculate the following heats of combustion. These differ somewhat from the values given in
the beginning of this chapter, where the product water is given in the liquid state.
H2(g) + ½ O2(g) ⎯→ H2O(g) heat of combustion = 249 kJ/mol
CH4(g) + 2 O2(g) ⎯→ CO2(g)+ 2 H2O(g) heat of combustion = 814 kJ/mol
In each case, the units of the calculated heat of combustion can be changed to kJ/gram by
dividing by the molar mass of the fuel.
249 kJ
For hydrogen: " 1 mol H2 = 124 kJ
For methane:
!
!
mol H2 2.01g H2 814 kJ
mol CH4 " 1 mol CH4 16.0 g CH4 g H 2
= 50.9 kJ
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g CH 4