ANSWERS TO END-OF-CHAPTER QUESTIONS

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b. When water is electrolyzed, the covalent bonds within water molecules are broken. These
are 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.
Answer:
Electrolysis of water requires an input of 286 kJ of energy per mole of water electrolyzed.
Most of this energy comes from the burning of fossil fuels in conventional power plants. The
inherent inefficiency associated with transforming heat into work limits the usefulness of
large-scale electrolysis and makes the process energy intensive.
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.
Answer:
a. 1.0 mol H2 "
b.
2 mol Na
1 mol H 2
1.1"10 6 kJ " 1 mol H 2
286 kJ
c. Using the result from part a:
" 23.0 g Na
1 mol Na
" 2 mol Na
1 mol H 2
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= 46.0 g Na
" 23.0 g Na
46.0 g Na "
1 mol Na
= 1.8 "10 5 g Na
1 kg Na
10 3 g Na "
$94
1 kg Na
= $4.32
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.