1. Fuel cells differ from batteries in that the former do not store chemical energy. Reactants must be constantly resupplied and products must be constantly removed from a fuel cell. In this respect, a fuel cell resembles an engine more than it does a battery. 2041

2. Fuel cells differ from batteries in that the former do not store chemical energy. Reactants must be constantly resupplied and products must be constantly removed from a fuel cell. In this respect, a fuel cell resembles an engine more than it does a battery. Properly designed fuel cells may reach efficiencies as high as 70%. 2042

3. Fuel cells differ from batteries in that the former do not store chemical energy. Reactants must be constantly resupplied and products must be constantly removed from a fuel cell. In this respect, a fuel cell resembles an engine more than it does a battery. Properly designed fuel cells may reach efficiencies as high as 70%. Fuel cells generating electricity are free of noise, vibration, heat transfer thermal pollution and other problems normally associated with conventional power plants. 2043

4. Corrosion 2044

5. Corrosion Corrosion reactions are redox reactions, in which a metal is attacked by some substance in its environment and converted to an unwanted compound. 2045

6. Corrosion Corrosion reactions are redox reactions, in which a metal is attacked by some substance in its environment and converted to an unwanted compound. Rusting of Iron Rusting of Iron 2046

7. Corrosion Corrosion reactions are redox reactions, in which a metal is attacked by some substance in its environment and converted to an unwanted compound. Rusting of Iron Rusting of Iron The rusting of iron is known to involve dioxygen; iron does not rust in water unless O 2 is present. 2047

8. Corrosion Corrosion reactions are redox reactions, in which a metal is attacked by some substance in its environment and converted to an unwanted compound. Rusting of Iron Rusting of Iron The rusting of iron is known to involve dioxygen; iron does not rust in water unless O 2 is present. Rusting also involves water; iron does not rust in oil, even if it contains O 2, unless water is also present. 2048

9. Other factors such as the pH of the solution, the presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron can accelerate rusting. 2049

10. Other factors such as the pH of the solution, the presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron can accelerate rusting. The corrosion of iron is generally believed to be electrochemical in nature. A region on the surface of the iron serves as an anode: 2050

11. Other factors such as the pH of the solution, the presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron can accelerate rusting. The corrosion of iron is generally believed to be electrochemical in nature. A region on the surface of the iron serves as an anode: Anode: Fe (s) Fe 2+ (aq) + 2e- = 0. 44 V 2051

12. Other factors such as the pH of the solution, the presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron can accelerate rusting. The corrosion of iron is generally believed to be electrochemical in nature. A region on the surface of the iron serves as an anode: Anode: Fe (s) Fe 2+ (aq) + 2e- = 0. 44 V The electrons produced migrate through the metal to another portion of the surface that serves as the cathode. Here O 2 can be reduced: 2052

13. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V 2053

14. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: 2054

15. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 0.815 V 2055

16. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 0.815 V (1.0x10 -7 M) 2056

17. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 0.815 V (1.0x10 -7 M) O 2(g) + 2H 2 O (l) + 4e - 4OH - (aq) = 0.401 V 2057

18. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 0.815 V (1.0x10 -7 M) O 2(g) + 2H 2 O (l) + 4e - 4OH - (aq) = 0.401 V As the concentration of H + (aq) is lowered, the reduction of O 2(g) becomes less favorable. 2058

19. Cathode: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 1.23 V A couple of other less favorable possibilities: O 2(g) + 4H + (aq) + 4e - 2H 2 O (l) = 0.815 V (1.0x10 -7 M) O 2(g) + 2H 2 O (l) + 4e - 4OH - (aq) = 0.401 V As the concentration of H + (aq) is lowered, the reduction of O 2(g) becomes less favorable. It is observed that iron in contact with a solution whose pH is above 9-10 does not corrode. 2059

20. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. 2060

21. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. The Fe 3+ forms the hydrated iron(III) oxide known as rust. 2061

22. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. The Fe 3+ forms the hydrated iron(III) oxide known as rust. 4 Fe 2+ (aq) + O 2(g) + (4 +2 x )H 2 O (l) 2Fe 2 O 3. x H 2 O + 8H + (aq) 2062

23. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. The Fe 3+ forms the hydrated iron(III) oxide known as rust. 4 Fe 2+ (aq) + O 2(g) + (4 +2 x )H 2 O (l) 2Fe 2 O 3. x H 2 O rust + 8H + (aq) 2063

24. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. The Fe 3+ forms the hydrated iron(III) oxide known as rust. 4 Fe 2+ (aq) + O 2(g) + (4 +2 x )H 2 O (l) 2Fe 2 O 3. x H 2 O rust + 8H + (aq) The x indicates a variable amount of water of hydration. Rust is a non-stoichiometric compound. 2064

25. In the course of the corrosion, the Fe 2+ (aq) formed at the anode is further oxidized to Fe 3+. The Fe 3+ forms the hydrated iron(III) oxide known as rust. 4 Fe 2+ (aq) + O 2(g) + (4 +2 x )H 2 O (l) 2Fe 2 O 3. x H 2 O rust + 8H + (aq) The x indicates a variable amount of water of hydration. Rust is a non-stoichiometric compound. Because the cathode is generally the area having the largest supply of O 2, the rust often deposits there. 2065

26. 2066

27. The enhanced corrosion caused by the presence of salts is readily explained by a voltaic mechanism – the electric circuit is completed by the migration of ions – i.e. the salt acts as an electrolyte. 2067

28. The enhanced corrosion caused by the presence of salts is readily explained by a voltaic mechanism – the electric circuit is completed by the migration of ions – i.e. the salt acts as an electrolyte. The presence of anodic and cathodic sites on the iron requires two different chemical environments on the surface. 2068

29. The enhanced corrosion caused by the presence of salts is readily explained by a voltaic mechanism – the electric circuit is completed by the migration of ions – i.e. the salt acts as an electrolyte. The presence of anodic and cathodic sites on the iron requires two different chemical environments on the surface. At the sites of such impurities or defects the atomic-level environment around the iron atom may permit the metal to be either more or less easily oxidized than at normal lattice sites. 2069

30. These sites may serve as either anodes or cathodes. Ultrapure iron, prepared in such a way as to minimize lattice defects, is far less susceptible to corrosion than is ordinary iron. 2070

31. These sites may serve as either anodes or cathodes. Ultrapure iron, prepared in such a way as to minimize lattice defects, is far less susceptible to corrosion than is ordinary iron. Prevention of Corrosion 2071

32. These sites may serve as either anodes or cathodes. Ultrapure iron, prepared in such a way as to minimize lattice defects, is far less susceptible to corrosion than is ordinary iron. Prevention of Corrosion Iron is often covered with a coat of paint or another metal such as zinc, chromium, etc., to protect its surface against corrosion. 2072

33. The tin layer of a tin can protects the iron only as long as the protective layer remains intact. Once it is broken and the iron exposed to air and H 2 O, tin actually promotes the corrosion of iron. Tin serves as the cathode. 2073

34. The tin layer of a tin can protects the iron only as long as the protective layer remains intact. Once it is broken and the iron exposed to air and H 2 O, tin actually promotes the corrosion of iron. Tin serves as the cathode. Anode: Fe (s) Fe 2+ (aq) + 2e - = 0.44 V 2074

35. The tin layer of a tin can protects the iron only as long as the protective layer remains intact. Once it is broken and the iron exposed to air and H 2 O, tin actually promotes the corrosion of iron. Tin serves as the cathode. Anode: Fe (s) Fe 2+ (aq) + 2e - = 0.44 V Sn (s) Sn 2+ (aq) + 2e - = 0.14 V 2075

36. The tin layer of a tin can protects the iron only as long as the protective layer remains intact. Once it is broken and the iron exposed to air and H 2 O, tin actually promotes the corrosion of iron. Tin serves as the cathode. Anode: Fe (s) Fe 2+ (aq) + 2e - = 0.44 V Sn (s) Sn 2+ (aq) + 2e - = 0.14 V Iron is more easily oxidized than Sn. 2076

37. 2077

38. Galvanized iron is produced by coating iron with a thin layer of zinc. The zinc protects the iron against corrosion even after the surface coating is broken. In this case the iron serves as the cathode in the electrochemical corrosion because the zinc is oxidized more easily than iron: 2078

39. Galvanized iron is produced by coating iron with a thin layer of zinc. The zinc protects the iron against corrosion even after the surface coating is broken. In this case the iron serves as the cathode in the electrochemical corrosion because the zinc is oxidized more easily than iron: Zn (s) Zn 2+ (aq) + 2e - = 0.76 V 2079

40. Galvanized iron is produced by coating iron with a thin layer of zinc. The zinc protects the iron against corrosion even after the surface coating is broken. In this case the iron serves as the cathode in the electrochemical corrosion because the zinc is oxidized more easily than iron: Zn (s) Zn 2+ (aq) + 2e - = 0.76 V The zinc therefore serves as the anode and is corroded instead of the iron. 2080

41. 2081

42. Such protection of a metal by making it the cathode in an electrochemical cell is known as cathodic protection. 2082

43. Such protection of a metal by making it the cathode in an electrochemical cell is known as cathodic protection. Underground pipelines are often protected against corrosion by making the pipeline the cathode of a voltaic cell. 2083

44. Such protection of a metal by making it the cathode in an electrochemical cell is known as cathodic protection. Underground pipelines are often protected against corrosion by making the pipeline the cathode of a voltaic cell. Pieces of a reactive metal such as magnesium are buried along the pipeline and connected to it by a wire. In moist soil, the reactive metal serves as the anode and the pipe experiences cathodic protection. 2084

45. 2085

46. 2086

47. Other metals such as Al and Mg corrode slowly, due to the formation of a thin compact oxide coating that forms on the metal surface. This protects the underlying metal from further corrosion. 2087

48. Other metals such as Al and Mg corrode slowly, due to the formation of a thin compact oxide coating that forms on the metal surface. This protects the underlying metal from further corrosion. The oxide coat on iron is too porous to offer similar protection. However, when iron is alloyed with chromium, a protective oxide coating does form. Such alloys are called stainless steel. 2088

49. Key Summary Thermodynamics, Equilibria, and Electrochemistry 2089

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