CHAPTER 21 ELECTROCHEMISTRY

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1 Chapter 21: Electrochemistry Page 1 CHAPTER 21 ELECTROCHEMISTRY Consider an electrochemical cell formed from a Cu(s) electrode submerged in an aqueous Cu(NO 3 ) 2 solution and a Cd(s) electrode submerged in a Cd(NO 3 ) 2 (aq) solution. The two electrodes are connected by a wire and the two solutions are connected by a salt bridge containing NaNO 3 (aq). The following reaction takes place: Cu 2+ (aq) + Cd(s) Cu(s) + Cd 2+ (aq) Which statement describes how the electrons or nitrate ions will flow? (a) electrons will flow from Cu(s) to Cd(s) (b) nitrate ions will flow from Cu compartment to Cd compartment (c) nitrate ions will not flow between compartments (d) nitrate ions will flow from Cd compartment to Cd compartment Consider an electrochemical cell formed from a Cu(s) electrode submerged in an aqueous Cu(NO 3 ) 2 solution and a Cd(s) electrode submerged in a Cd(NO 3 ) 2 solution. The two electrodes are connected by a wire and the two solutions are connected by a salt bridge containing NaNO 3 (aq). The following reaction takes place: Cu 2+ (aq) + Cd(s) Cu(s) + Cd 2+ (aq) Which statement describes how the electrons or nitrate ions will flow? (a) electrons will flow from Cu(s) to Cd(s) (b) electrons will flow from Cd(s) to Cu(s) (c) electrons will not flow between electrodes (d) nitrate ions will flow from Cd compartment to Cd compartment Consider an electrochemical cell where the following reaction takes place: 2 Au 3+ (aq) + 3 Zn(s) 2 Au(s) + 3 Zn 2+ (aq) Which of the following is the correct cell notation for this cell? (a) Au Au 3+ Zn 2+ Zn (b) Au 3+ Au Zn Zn 2+ (c) Zn Zn 2+ Au 3+ Au (d) Zn Au 3+ Au Zn 2+ NOTE: for spring 2001 ignore questions with an underlined number.

2 Chapter 21: Electrochemistry Page Consider an electrochemical cell where the following reaction takes place: 3 Sn 2+ (aq) + 2 Al(s) 3 Sn(s) + 2 Al 3+ (aq) Which of the following is the correct cell notation for this cell? (a) Al Al 3+ Sn 2+ Sn (b) Al 3+ Al Sn Sn 2+ (c) Sn Sn 2+ Al 3+ Al (d) Sn Al 3+ Al Sn E o for the following redox reaction is V. 2 Ag + (aq) + Ni(s) 2 Ag(s) + Ni 2+ (aq) What is G o for this reaction? (a) +203 kj (b) -203 kj (c) +101 kj (d) -101 kj E o for the following redox reaction is V. Fe 3+ (aq) + 3 Ag(s) Fe 2+ (aq) + 3 Ag + (aq) What is G o for this reaction? (a) +2.8 kj (b) -2.8 kj (c) +8.4 kj (d) -8.4 kj An early method of producing aluminum metal was the reaction of aluminum salts with sodium metal: Al Na(s) Al(s) + 3 Na + E o = V What is G o for this reaction? (a) -304 kj (b) +101 kj (c) -101 kj (d) +304 kj The following redox reaction can be used to form iodine. Br 2 (aq) + 2 I - (aq) 2 Br - (aq) + I 2 (aq) G o = -105 kj What is E o for this reaction? (a) V (b) V (c) V (d) V

3 Chapter 21: Electrochemistry Page The following reaction can be used to form bromine. Cl 2 (aq) + 2 Br - (aq) 2 Cl - (aq) + Br 2 (aq) G o = -102 kj (a) V (b) V (c) V (d) V It would be useful if a battery could be made utilizing the following reaction: 2 Li(s) + F 2 (g) 2 LiF(s) G o = kj What is E o for this reaction? (a) V (b) V (c) +2.7 V (d) +5.9 V Given the two following half reactions, Ni 2+ (aq) + 2e - Ni(s) Hg 2+ (aq) + 2e - Hg(l) E o = V E o = V calculate E o for the following reaction: Hg 2+ (aq) + Ni(s) Ni 2+ (aq) + Hg(l) (a) V (b) V (c) V (d) V Given the two following half reactions, Cu 2+ (aq) + 2e - Cu(s) Hg 2+ (aq) + 2e - Hg(l) E o = V E o = V calculate E o for the following reaction: Hg 2+ (aq) + Cu(s) Cu 2+ (aq) + Hg(l) (a) V (b) V (c) V (d) V

4 Chapter 21: Electrochemistry Page Given the two following half reactions, Mg 2+ (aq) + 2e - Mg(s) I 2 (aq) + 2e - 2 I - (aq) E o = V E o = V calculate E o for the following reaction: Mg 2+ (aq) + 2 I - (aq) I 2 (aq) + Mg(s) (a) V (b) V (c) V (d) V Given the two following half reactions, Sn 2+ (aq) + 2e - Sn(s) E o = V Sn 4+ (aq) + 2e - Sn 2+ (aq) E o = V calculate E o for the following reaction: Sn 4+ (aq) + Sn(s) 2 Sn 2+ (aq) (a) V (b) V (c) V (d) V An electrochemical cell of notation Pd Pd 2+ Cu 2+ Cu has E o = V. If we know that the standard reduction potential of Cu 2+ /Cu is E o = 0.34 V, what is the standard reduction potential for Pd 2+ /Pd? (a) V (b) V (c) V (d) V An electrochemical cell of notation Pt Pt 2+ Ni 2+ Ni has E o = V. If we know that the standard reduction potential of Ni 2+ /Ni is E o = V, what is the standard reduction potential for Pt 2+ /Pt? (a) V (b) V (c) V (d) V Given the following two half reactions, Cd 2+ (aq) + 2e - Cd(s) E o = V Zr 4+ (aq) + 4e - Zr(s) E o = V determine E o and the spontaneity of the following reaction 2 Cd 2+ (aq) + Zr(s) 2 Cd(s) + Zr 4+ (aq) (a) V and not spontaneous (b) V and spontaneous (c) V and not spontaneous (d) V and spontaneous

5 Chapter 21: Electrochemistry Page Given the following two half reactions, Ca 2+ (aq) + 2e - Ca(s) E o = V V 2+ (aq) + 2e - V(s) E o = V determine E o and the spontaneity of the following reaction Ca 2+ (aq) + V(s) > Ca(s) + V 2+ (aq) (a) V and not spontaneous (b) V and spontaneous (c) V and not spontaneous (d) V and spontaneous Given the following two half reactions, Sr 2+ (aq) + 2e - Sr(s) E o = V Mn 2+ (aq) + 2e - Mn(s) E o = V determine E o and the spontaneity of the following reaction Sr 2+ (aq) + Mn(s) Sr(s) + Mn 2+ (aq) (a) V and spontaneous (b) V and spontaneous (c) V and not spontaneous (d) V and spontaneous Given the following two half reactions, Sn 2+ (aq) + 2e - Sn(s) I 2 + 2e - 2 I - (aq) E o = V E o = V determine E o and the spontaneity of the following reaction Sn(s) + 2 I - (aq) Sn 2+ (aq) + I 2 (aq) (a) V and not spontaneous (b) V and spontaneous (c) V and not spontaneous (d) V and not spontaneous Determine the cell potential for Mg(s) + 2 Ag + (aq) Mg 2+ (aq) + 2 Ag(s) where [Ag + ] = 0.30 M and [Mg 2+ ] = M using the following standard reduction potentials. Mg 2+ (aq) + 2e - Mg(s) E o = V Ag + (aq) + e - Ag(s) E o =+0.80 V (a) V (b) V (c) V (d) V

6 Chapter 21: Electrochemistry Page Determine the cell potential for Ni(s) + Fe 2+ (aq) Ni 2+ (aq) + Fe(s) where [Ni 2+ ] = 0.60 M and [Fe 2+ ] = M using the following standard reduction potentials. Ni 2+ (aq) + 2e - Ni(s) E o = V Fe 2+ (aq) + 2e - Fe(s) E o = V (a) V (b) V (c) V (d) V Determine the cell potential for Cu(s) + Pt 2+ (aq) Cu 2+ (aq) + Pt(s) where [Cu 2+ ] = 0.85 M and [Pt 2+ ] = M using the following standard reduction potentials. Pt 2+ (aq) + 2e - Pt(s) E o = V Cu 2+ (aq) + 2e - Cu(s) E o = V (a) V (b) V (c) V (d) V Determine the cell potential for Cu(s) + Ag + (aq) Cu + (aq) + Ag(s) where [Ag + ] = 0.25 M and [Cu + ] = M using the following standard reduction potentials. Ag + (aq) + e - Ag(s) E o = V Cu + (aq) + e - Cu(s) E o = V (a) V (b) V (c) V (d) V Determine the equilibrium constant for the following reaction at 25 o C. Cl 2 (aq) + 2I - (aq) 2 Cl - (aq) + I 2 (aq) E o = V (a) 1.08 x (b) (c) 9.25 x (d) 0.217

7 Chapter 21: Electrochemistry Page Determine the equilibrium constant for the following reaction at 25 o C. Sn 2+ (aq) + V(s) Sn(s) + V 2+ (aq) E o = V (a) 1.41 x (b) 7.10 x (c) (d) 1.19 x Given the following two half reactions, Hg 2+ (aq) + 2e - Hg(l) E o = V Ni 2+ (aq) + 2e - Ni(s) E o = V determine the equilibrium constant for Hg(l ) + Ni 2+ (aq) Hg 2+ (aq) + Ni(s) (a) 1.75 x (b) 7.56 x (c) 1.32 x 10 9 (d) 5.71 x Given the following two half reactions, Al 3+ (aq) + 3e - Al(s) Cl 2 (aq) + 2e - 2 Cl - (aq) E o = V E o = V determine the equilibrium constant for 2 Al 3+ (aq) + 6 Cl - (aq) R 2 Al(s) + 3 Cl 2 (aq) (a) (b) (c) (d) How many coulombs of charge are required to deposit 1.00 g Ag from a solution of Ag + (aq)? (a) 9.27 x 10-3 (b) 894 (c) 1790 (d) How many coulombs of charge are required to form 1.00 pound of Al(s) from an Al 3+ salt? (a) 4.87 x 10 6 (b) 50.5 (c) 1.62 x 10 6 (d) If we wish to convert 1.00 g of Au 3+ (aq) ion into Au(s) in a gold-plating" process, how long must we electrolyze a solution if the current passing through the circuit is 2.00 amps? (a) 245 sec (b) 0.50 sec (c) sec (d) 735 sec

8 Chapter 21: Electrochemistry Page If we electrolyze a solution of Ni 2+ (aq) to form Ni(s) and use a current of 0.15 amps for 10 minutes, how many grams of Ni(s) are produced? (a) g (b) g (c) 4.56 x 10-4 g (d) 5280 g We have 100. ml of a solution of Cu 2+ (aq) ion and wish to determine [Cu 2+ ]. We electrolyze the solution to produce Cu(s) and use 1.5 amps of current for 1120 seconds to complete the process. What was the concentration of Cu 2+ in the solution? (a) M (b) M (c) M (d) 1.74 M Assuming 100% efficiency, how many kilowatt hours (kwh) of electrical energy are required to produce 1.00 kg of chlorine gas by the following reaction: NaCl(aq) + H 2 O(l ) NaOH(aq) + 1/2 Cl 2 (g) + 1/2 H 2 (g) E o = V (a) 2.06 kwh (b) 22.5 kwh (c) 1.03 kwh (d) kwh In the production of Al metal by electrolysis, approximately 15 kwh of electrical energy are used to produce each kilogram of Al from an Al 3+. Assuming that all the charge passed is used in producing aluminum, at what voltage does the cell operate? (a) 2.51 V (b) 15.1 V (c) 1.68 V (d) 5.03 V How many kilowatt hours of electrical energy are required to plate 1.00 grams of gold from an Au 3+ (aq) solution onto a piece of jewelry? Assume an operating voltage of 3.00 V is necessary. (a) 4.08 x 10-4 kwh (b) 1.22 x 10-3 kwh (c) 1.36 x 10-4 kwh (d) 51.4 kwh In reducing Ag + ion for plating onto jewelry, an operating cell voltage of 4.00 V is required. If you run a business that does this kind of work, how much will the electrical energy cost to coat 1.0 x 10 4 necklaces with 0.10 g silver each? Assume that the cost of electricity is $0.10/kwh. (a) 26 cents (b) 10. cents (c) $122 (d) $14.56

9 Chapter 21: Electrochemistry Page How is aluminum currently produced in industry? (a) by reduction of Al 3+ in Al 2 O 3 with Na(s) (b) electrochemical reduction of pure Al 2 O 3 to give Al and O 2 (c) electrolysis of AlF 3 to give Al and F 2 (d) electrolysis of a mixture of Al 2 O 3 and Na 3 AlF 6 to give Al and O How is sodium currently produced in industry? (a) by reduction of Na + in NaCl with Al(s) (b) electrolysis of pure NaCl(l) (c) (d) electrolysis of a molten mixture of NaCl and other salts electrolysis of an aqueous solution of NaCl and other salts The products obtained from electrolysis of brine (aqueous NaCl) are: (a) HCl(g), Na(s) and O 2 (g) (b) H 2 (g), NaOH(aq), and Cl 2 (g) (c) Na(s) and Cl 2 (g) (d) NaOH(aq) and HCl(aq) A possible anodic reaction that takes place during corrosion of iron is: (a) O 2 (g) + 2 H 2 O(l) + 4e - 4 OH - (aq) (b) Fe(s) Fe e - (c) 2 H 2 O(l) + 2e - H 2 (g) + 2 OH - (aq) (d) SO 2 (g) + O 2 (g) + 2e - SO 4 2- (aq) Which of the following is NOT a possible cathodic reaction in the corrosion of iron? (a) O 2 (g) + 2 H 2 O(l) + 4e - 4 OH - (aq) (b) (c) 2 H 2 O(l) + 2e - H 2 (g) + 2 OH - (aq) SO 2 (g) + O 2 (g) + 2e - SO 4 2- (aq) (d) Fe(s) Fe 2+ (aq) + 2e -

10 Chapter 21: Electrochemistry Page Which of the following reactions does a common alkaline battery, such as those used in portable tape players, utilize? (a) Zn(s) + MgO(s) ZnO(s) + Mg(s) (b) ZnO(s) + Mg(s) Zn(s) + MgO(s) (c) 2 Zn(s) + 2 MnO 2 (s) 2 ZnO(s) + Mn 2 O 3 (s) (d) Zn(s) + HgO(s) Hg(l) + ZnO(s) Which of the following reactions does a lead-acid battery, such as that used in an automobile, utilize? (a) PbO 2 (s) + H 2 SO 4 (aq) Pb 2+ (aq) + H 2 (g) + SO 4 2- (aq) (b) (c) (d) 2 PbO 2 (s) Pb(s) + PbO 4 (s) HCl(aq) + Pb(s) H 2 (g) + PbCl 2 (s) Pb(s) + PbO 2 (s) + 2 H 2 SO 4 (aq) 2 PbSO H 2 O(l) Given the following half-reactions, Ag + (aq) + e - Ag(s) E o = V AgCl(s) + e - Ag(s) + Cl - (aq) E o = V calculate the molar solubility of AgCl. (a) 4.46 x 10-9 M (b) 1.34 x 10-5 M (c) 1.79 x M (d) M Given the following half reactions, Cd 2+ (aq) + 2e - Cd(s) E o = V CdS(s) + 2e - Cd(s) + S 2- (aq) E o = V estimate the molar solubility of CdS. (a) 2.33 x M (b) 5.43 x M (c) 1.84 x M (d) 4.29 x M

11 Chapter 21: Electrochemistry Page An electrochemical cell is constructed with two iron electrodes. The solution in the left hand compartment of the cell has 1.0 M Fe 2+ (aq) while the solution in the right hand compartment contains 0.10 M Fe 2+ (aq). Which of the following is true about the cell? (a) no current flows through the wire connecting the electrodes (b) both electrodes are oxidized (c) reduction occurs in the left hand compartment (d) reduction occurs in the right hand compartment An electrochemical cell is constructed with two inert electrodes that transfer electrons but are not oxidized or reduced. In the left hand compartment of the cell is a solution containing 0.10 M Cl 2 (aq) and 0.50 M Cl - (aq). In the right hand compartment of the cell is a solution containing 0.50 M Cl 2 (aq) and 0.50 M Cl - (aq). Which of the following is true about the cell? (a) no current flows through the wire connecting the electrodes (b) both electrodes are oxidized (c) reduction occurs in the left hand compartment (d) reduction occurs in the right hand compartment An electrochemical cell is constructed with two Ni(s) electrodes, each in a cell containing Ni 2+ (aq). Electrons flow through the wire connecting the electrodes from the left hand compartment to the right hand compartment. What can be said about the overall cell? (a) [Ni 2+ ] in the right compartment > [Ni 2+ ] left compartment (b) [Ni 2+ ] in the right compartment < [Ni 2+ ] left compartment (c) no magnetic fields penetrate the walls of the cell (d) Ni 2+ (aq) is more easily oxidized than Ni(s)

12 Chapter 21: Electrochemistry Page We have a sample of an unknown metal that we know can be oxidized by one electron. We wish to determine which element we have. Which of the following would be a possible means of determining which element we have? (a) construct an electrochemical cell and measure E o of the metal, and compare it to a table of potentials (b) determine if the metal oxidizes Fe 2+ Fe 3+ (c) construct an electrochemical cell and measure how quickly current passes from compartment to compartment (d) determine if the metal heats up when it is oxidized Consider the following half reactions: HALF REACTION E (V) F 2 (g) + 2 e - 2 F - (aq) I 2 (g) + 2 e - 2I - (aq) Cu 2+ (aq) + 2 e - Cu(s) Al 3+ (aq) + 3 e - Al(s) (i) Which is the strongest oxidizing agent? (ii) Which is the strongest reducing agent? (iii) Will Al(s) reduce Cu 2+ (aq) to Cu(s)? (iv) Will F - (aq) reduce I 2 (aq) to I - (g)? (v) Name the elements or ions that can be reduced by Cu(s). (vi) If you build a battery using the I 2 /I - and Al 3+ /Al half reactions, what is the voltage of the battery (assuming all reagents are at standard conditions)? (a) V (b) V (c) V (d) V

13 Chapter 21: Electrochemistry Page Consider the following half reactions: HALF REACTION E (V) Cl 2 (g) + 2 e - 2 Cl - (aq) I 2 (g) + 2 e - 2I - (aq) Pb 2+ (aq) + 2 e - Pb(s) V 2+ (aq) + 2 e - V(s) (i) Which is the weakest oxidizing agent in the list? (ii) Which is the strongest oxidizing agent? (iii) Which is the strongest reducing agent? (iv) Which is the weakest reducing agent? (v) Will I - (aq) reduce Cl 2 (aq) to Cl - (g)? (vi) Name the elements or ions that can be reduced by Pb(s) The basic reaction occurring in the cell in which Al 2 O 3 and aluminum salts are electrolyzed is Al e - Al(s). If the cell operates at 5.0 V and 1.0 x 10 5 amps, and you want to produce 600 pounds of aluminum metal (2.7 x 10 5 g), how many hours will you need to operate the cell? (a) 1.0 hour (b) 4.0 hours (c) 8.0 hours (d) 24.0 hours The space shuttle uses fuel cells for electricity, and one cell that has been proposed is based on hydrazine, N 2 H 4, and O 2. The reactions are (a) N 2 H 4 (aq) + 4 OH - (aq) N 2 (g) + 4 H 2 O(l) + 4 e - (b) O 2 (g) + 2 H 2 O(l ) + 4 e - 4 OH - (aq) (i) Which reaction occurs at the anode? (ii) If the cell is to produce 0.50 amps of current for 50.0 hours, how many grams of hydrazine (N 2 H 4 ) must be present? (1 Faraday = 96,500 coulombs/mol e - ) (a) 7.5 g (b) 29.9 g (c) 3.25 g (d) g

14 Chapter 21: Electrochemistry Page Imagine an electrochemical cell using the setup below. Voltmeter Ni salt bridge Cu M Ni 1 M Cu (i) (ii) (iii) (iv) (v) Ni 2+ (aq) + 2e- Ni(s) E = V Cu 2+ (aq) + 2e- Cu(s) E = V Write a balanced equation depicting the reaction occurring in the cell above and give its E value. (a) Ni 2+ (aq) + Cu(s) Cu 2+ (aq) + Ni(s) E = V (b) Cu 2+ (aq) + Ni(s) Ni 2+ (aq) + Cu(s) E = V (c) Cu 2+ (aq) + Ni(s) Ni 2+ (aq) + Cu(s) E = V (d) Ni 2+ (aq) + Cu(s) Cu 2+ (aq) + Ni(s) E = V What is oxidized? (a) Ni 2+ (aq) (b) Cu 2+ (aq) (c) Ni(s) (d) Cu(s) Which is the anode? (a) Ni (b) Cu What is the polarity of the Cu electrode? (a) positive (b) negative What is the direction of electron flow in the external wire? (a) from Cu to Ni (b) from Ni to Cu

15 Chapter 21: Electrochemistry Page Ni-cad batteries are rechargeable and are commonly used in cordless appliances. Although such batteries actually function under basic conditions, imagine an electrochemical cell using the setup below. Voltmeter Cd salt bridge Ni M Cd 1 M Ni (i) (ii) (iii) Cd2+(aq) + 2e- Cd(s) E = V Ni 2+ (aq) + 2e- Ni(s) E = V What is E for the cell? (a) V (b) V (c) V (d) V Which is the cathode? (a) Cd (b) Ni Which statement below is true? (a) Electrons flow from Cd to Ni in the external wire, and NO - 3 ions in the salt bridge move (b) (c) toward the Ni 2+ /Ni compartment. Electrons flow from Cd to Ni in the external wire, and NO - 3 ions in the salt bridge move toward the Cd 2+ /Cd compartment. Electrons flow from Ni to Cd in the external wire, and NO - 3 ions in the salt bridge move (iv) toward the Ni 2+ /Ni compartment. If amps are drawn from the battery, how long can it last if you begin with 100. ml of each of the solutions and each was initially 0.10 M in dissolved species? The electrodes each weigh 50.0 g in the beginning. (a) 5.36 hours (b) 10.7 hours (c) 107 hours (d) 477 hours

16 Chapter 21: Electrochemistry Page 16 ANSWERS CHAPTER b 11. c 21. d 2. b 12. b 22. c 3. c 13. d 23. a 4. a 14. c 24. a 5. b 15. d 25. c 6. c 16. c 26. a 7. a 17. b 27. d 8. d 18. b 28. a 9. c 19. c 29. b 10. d 20. a 30. a 31. d 41. b 32. a 42. d 33. c 43. c 34. c 44. d 35. d 45. b 36. b 46. a 37. b 47. c 38. d 48. d 39. c 49. a 40. b 50. a 51. F 2, Al, yes, no, F 2 and I 2, c 52. V 2+, Cl 2, V, Cl -, yes, I 2 and Cl c 54. a, a 55. b, c, a, a, b 56. b, b, b, b