Chapter 12
Batteries, Fuel Cells, and the Hydrogen Economy: Oxidation and Reduction
Review Questions
1. a) Oxidation is defined as the loss of electrons by a substance.
b) Reduction is defined as the gain of electrons by a substance.
c) Half-reactions are processes that describe the transfer of electrons between atoms. Half-reactions, one representing an oxidation reaction and the other a reduction reaction, are not viable alone. Rather, one species represented in one half-reaction must give up electrons and another species represented in the other half-reaction must receive them.
d) Redox reactions are oxidation–reduction reactions. Oxidation and reduction reactions must occur together.
2. a) Fe (s) ® Fe+2 + 2 e-
b) Na (s) ® Na+1 + 1 e-
c) O2 (g) + 4 e- ® 2 O-2
d) Cl2 (g) + 2 e- ® 2 Cl-1
3. An oxidizing agent is a substance that promotes oxidation. When an oxidizing agent pulls electrons from another substance, the substance is oxidized and the oxidizing agent is reduced. H+, Cl2, O2, and O3 are common oxidizing agents.
4. A reducing agent is a substance that promotes reduction. When a reducing agent gives electrons to another substance, the substance is reduced and the reducing agent is oxidized. Fuels like natural gas and many metals are reducing agents.
5. a) Oxygen goes from 0 to –2, is reduced, and is the oxidizing agent. Sulfur goes from –2 to +4, is oxidized, and is the reducing agent. A total of 12 electrons are transferred in this process.
b) Chromium goes from +3 to 0, is reduced, and is the oxidizing agent. Aluminum goes from 0 to +3, is oxidized, and is the reducing agent. A total of six electrons are transferred in this process.
c) Tungsten goes from +6 to 0, is reduced, and is the oxidizing agent. Hydrogen goes from 0 to +1, is oxidized, and is the reducing agent. A total of six electrons are transferred in this process.
6. Electrochemical cells are units that produce electricity from chemical oxidation-reduction reactions.
7. a) Electrodes are metal surfaces at which oxidation and reduction reactions occur.
b) Electrolytes are ion-containing solutions into which the electrodes in an electrochemical cell are placed.
c) A salt bridge is a device, composed of either an aqueous solution or a water-based gel, that maintains the anion balance in an electrochemical cell.
8. a) Electrodes in an electrochemical cell are metal surfaces at which oxidation and reduction reactions occur. One electrode dissolves over time from its oxidation and the other electrode gains in mass over time as the soluble metal cations in the solution are reduced and plated onto the electrode.
b) Electrolytes are ion-containing solutions into which the electrodes in an electrochemical cell are placed. The cation in the electrolyte has to be the oxidized form of the metal composing the electrode that is placed in the solution.
c) A salt bridge is a device, composed of either an aqueous solution or a water-based gel, that maintains the anion balance in an electrochemical cell.
9. During the discharge of an electrochemical cell, the metal surface of one electrode is oxidized to the corresponding ion, which enters the aqueous solution. The electrons from this oxidation travel to the other electrode where they are transferred to another ion in solution resulting in the plating of the metal on the electrode. The salt bridge serves to redistribute the anions in order to maintain positive and negative ion balance.
10. A battery is a commercially available electrochemical cell.
11. A fuel cell is an electrochemical cell that can be continuously operated and does not become discharged. In fuel cells, the reactants are continuously replenished as the unit produces electrical power.
12. Electrolysis is a process of passing an electric current through a material to drive a chemical reaction. Electrolysis reactions do not occur spontaneously and often require substantial expenditures of energy. Electrolysis is used to prepare and purify materials, such as metals.
13. Electroplating is a technique using electrolysis in which a thin film of metal is deposited on an object.
14. Since hydrogen gas is likely to have a key role as a fuel for both vehicles and power plants, many people believe that hydrogen will become a valuable commodity and, as such, will be an integral part of the economic infrastructure of most nations.
15. 2 H2 (g) + O2 (g) ® 2 H2O (g) + heat energy.
16. Hydrogen gas is superior to electricity as an energy carrier because its transmission over long distances consumes less energy than electricity, and it does not require batteries for storage.
17. Hydrogen can be stored as a liquid, a compressed gas, a gas between graphite layers or reversibly absorbed by metals or metal alloys, and as an elemental component of liquid fuels.
Understanding Concepts
18. a) Zn (s) + 2 H+ (aq) ® Zn+2 (aq) + H2 (g)
b) 4 Li (s) + O2 (g) ® 2 Li2O (s)
19. In oxidation-reduction reactions, solid metals are oxidized to ions, aqueous acids are reduced to hydrogen gas, oxygen gas is reduced to the oxide anion and typically forms insoluble ionic salts, and hydroxide is often formed in reduction reactions and forms insoluble ionic salts.
20. In the reaction 2 H+ (aq) + Fe (s) ® H2 (g) + Fe+2 (aq), H+ is the oxidizing agent, as it is reduced from +1 to 0, and Fe is the reducing agent, as it is oxidized from 0 to +2.
21. Electron transfer occurs between two metal strips in solution even though they are not touching as long as there is an electrical connection between the two metals and the two metals spontaneously undergo an oxidation – reduction reaction. All that is required for electron transfer is a means to allow the electrons given up by one metal to be transferred to the ionic form of another metal without much resistance.
22. A salt bridge is a device, composed of either an aqueous solution or a water-based gel that maintains the anion balance in an electrochemical cell.
23. It is inconvenient to use a liquid electrolyte in a battery because of the need to prevent evaporation and leakage.
24. Batteries contain a small, finite amount of reactants and eventually become fully discharged and useless. Secondary or rechargeable batteries overcome the problem of a finite amount of reactants, but are limited by the considerable recharge times required. In a fuel cell, the reactants are continuously replenished while the unit delivers electric power. Thus, fuel cells can be continuously operated and never become discharged.
25. Hydrogen gas is a desirable choice as a reducing agent in fuel cells because it can be produced from both fossil fuels and from renewable sources, and it has been the fuel used in most fuel cell technology development efforts.
26. A proton-exchange membrane is a hair-thin synthetic polymer, or plastic film, that acts as an electrolyte in a hydrogen-oxygen fuel cell. Hydrogen ions produced at the hydrogen electrode pass through this membrane and travel to the oxygen electrode where they combine with oxygen to form water vapor.
27. Fuel cells are attractive for use in small vehicles because they avoid the limitations associated with carrying and recharging heavy conventional battery systems.
28. Aluminum ore is produced by the electrolytic reduction of its +3 ion found in aluminum oxide (Al2O3) ore, or bauxite. The half-reaction for the reduction is given below.
Al+3 + 3 e- ® Al (s)
29. Hydrogen gas can be used as a fuel because it combines with oxygen to produce water and a substantial amount of energy. The reaction for the combustion of hydrogen gas is given below.
2 H2 (g) + O2 (g) ® 2 H2O (g) + heat energy
30. No, it is not true that the combustion of hydrogen gas is a pollution-free process. The combustion of hydrogen gas requires a high temperature flame. As a result, some of the nitrogen contained in the air is converted into various nitrogen oxides. Hydrogen peroxide, H2O2, is also released in this process.
31. Hydrogen combustion is accomplished at low temperatures through the use of catalytic heaters that reduce the production of nitrogen oxide pollutants.
32. An energy source consists of an element or compound that exists as a free substance on the Earth’s crust. Fuels that are not found on the Earth’s crust, but have to be produced are called energy vectors or carriers.
33. Solar energy could be used to drive the electrolysis of water to H2 and O2.
34. Problems associated with the use of sunlight to generate hydrogen gas directly from water include its lack of practicality and efficiency. Since H2O does not directly absorb sunlight, a secondary substance must be used to absorb the sunlight and transfer the energy. Currently, the materials used for this purpose are inefficient in absorbing and converting the Sun’s energy, and are not completely recoverable, thereby requiring constant replenishment. Due to the costs involved, this process is extremely uneconomical.
35. The advantage of storing hydrogen as a liquid is the relatively small holding vessels required. The condensation of hydrogen gas, however, requires substantial energy and cost. The advantage of storing hydrogen as a gas is the lack of cooling required to condense the gas to a liquid. The major disadvantage is the bulky nature of the holding vessels required.
36. There are two mechanisms by which hydrogen gas can be stored in a solid. First, hydrogen gas can be absorbed by graphite and become wedged between the its layers. Second, hydrogen can be reversibly absorbed by several metals and alloys. Hydrogen forms hydrides by incorporating itself in the “holes” in the crystalline structure of the metal.
37. Hydrogen gas can be formed from methanol by a reforming reaction. The stepwise and overall reactions are given below.
CH3OH ® 2 H2 + CO
CO + H2O ® CO2 + H2
Overall: CH3OH + H2O ® CO2 + 3 H2
38. It is impractical to use methanol as the fuel in a fuel cell because it cannot undergo reaction fast enough to produce the required electric current.
Synthesizing Ideas
39. a) This is an oxidation-reduction reaction in which Na is oxidized from 0 to +1 and Cl is reduced from 0 to -1.
b) This is an oxidation-reduction reaction in which carbon is oxidized and oxygen is reduced.
c) This is not an oxidation-reduction reaction. None of the species involved in the reaction change oxidation states.
40. The two half reactions are:
2 Al (s) ® 2 Al+3 + 6 e-
and
O2 (g) + 4 e- ® 2O-2
The overall reaction is:
4 Al (s) + 3 O2 (g) ® 2 Al2O3 (s)
Aluminum is oxidized from 0 to +3 and is the reducing agent. Oxygen is reduced from 0 to –2 and is the oxidizing agent.
41. An electrochemical reaction between two different metal strips in an electrolytic solution stops after a period of time when either the electrode where oxidation occurs is dissolved or the ions undergoing reduction are consumed.
42. The solution at the beginning of the redox reaction would contain Cu+2 ions. The solution during the reaction would contain both Cu+2 and Zn+2 ions. At the end of the reaction, the solution would contain only Zn+2 ions.
43. In general, it is extremely difficult to stop a spontaneous oxidation – reduction reaction. The only ways in which you could stop the spontaneous redox reaction between two metals are to either increase the resistance experienced by the electrons or deplete the concentration of the metal ion undergoing reduction.
44. The disadvantages of battery-driven electric cars are their high cost, their short driving range between charges, and their excessive battery weight. The advantages of these vehicles include their zero pollution emissions during operation, their low operating noise, and their low maintenance costs.
45. Unlike hydrogen gas, fuels containing carbon produce carbon dioxide when combusted. The reactions are given below.
2 CH3OH + 3 O2 ® 2 CO2 + 4 H2O
CH4 + 2 O2 ® CO2 + 2 H2O
2 C8H18 + 25 O2 ® 16 CO2 + 18 H2O
2 H2 + O2 ® 2 H2O
46. In the hydrogen-oxygen fuel cell, hydrogen gas is oxidized at one electrode and air or oxygen is reduced at the other electrode to produce water, which is emitted as a gas from the cell. The electrodes in a hydrogen-oxygen fuel cell consist of graphite carbon with finely divided platinum metal catalyst dispersed throughout.
47. The use of fuel cells in all-electric vehicles will eliminate the need to frequently recharge heavy batteries. Other advantages of using fuel cells to power vehicles and power plants include their high efficiency in extracting useful energy from fuels, the lack of pollutants generated and emitted (about 1% of the nitrogen oxides and sulfur oxides generated from burning fossil fuels), their silent and low maintenance operation, and their ability to use hydrogen from fossil fuels and renewable sources.
48. Reduction: O2 + 2 H2O + 4 e- ® 4 OH-
Oxidation: 2 OH- + H2 ® 2 H2O + 2 e-
Overall: (You need to multiply the oxidation reaction by 2 to cancel the electrons)
O2 + 2 H2O + 4 e- + 4 OH- + 2 H2 ® 4 OH- + 4 H2O + 4 e-
Canceling like terms and simplifying yields:
O2 + 2 H2 ® 2 H2O
49. The most practical storage system for hydrogen use in small vehicles would be hydrogen absorbed by a solid material. For example, hydrogen gas can occupy the space that exists between layers of graphite or between carbon nanotubes. Also, hydrogen can be reversibly absorbed by many metals, including alloys. These metals form hydrides by incorporating hydrogen atoms in spaces present in their crystalline structure.