Electrochemistry HW. PSI Chemistry

Transcription

1 Electrochemistry HW PSI Chemistry Name Multiple choice questions: I. Oxidation numbers and half-reactions 1) What is the oxidation number of potassium in KMnO4? A) 0 B) +1 C) +2 D) -1 E) +3 2) What is the oxidation number of bromine in the BrO3 - ion? A) -1 B) +1 C) +3 D) +5 E) +7 3) What is the oxidation number of nitrogen in the HNO3? A) -1 B) +1 C) +3 D) +5 E) +7 4) What is the oxidation number of chromium in Cr2O7 2- ion? A) +3 B) +6 C) +7 D) +12 E) +14 5) is reduced in the following reaction: Cr2O S2O H + 2Cr S4O H2O A) Cr 6+ B) S 2+ C) H + D) O 2- E) S4O6 2-6) is the oxidizing agent in the reaction below. Cr2O S2O H + 2Cr S4O H2O A) Cr2O7 2- B) S2O3 2- C) H +

2 D) Cr 3+ 2 E) SO6 4 7) Which substance is serving as the reducing agent in the following reaction? 14H + + Cr2O Ni 3Ni Cr H2O A) H + B) Cr2O7 2- C) Ni D) H2O E) Ni 2+ 8) Which substance is the reducing agent in the reaction below? Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O A) Pb B) H2SO4 C) PbO2 D) PbSO4 E) H2O 9) Which element is reduced in the reaction below? Fe 2+ + H + + Cr2O7 2- Fe 3+ + Cr 3+ + H2O A) Fe B) Cr C) O D) H 10) Which element is oxidized in the reaction below? I - + MnO4 - + H + I2 + MnO2 + H2O A) I B) Mn C) O D) H 11) Which element is reduced in the reaction below? I - + MnO4 - + H + I2 + MnO2 + H2O A) I B) Mn C) O D) H 12) The half-reaction occurring at the anode in the balanced reaction shown below is. MnO4 - (aq) + 24H + (aq) +5Fe(s) 3Mn 2+ (aq) + 5Fe 3+ (aq) + 12H2O(l) A) MnO4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq) + 4H2O(l) B) 2MnO4 - (aq) + 12H + (aq) + 6e - 2Mn 2+ (aq) + 3H2O(l) C) Fe(s) Fe 3+ (aq) + 3e -

3 D) Fe(s) Fe 2+ (aq) + 2e - E) Fe(s) Fe + (aq) + e - 13) Which one of the following reactions is a redox reaction? A) NaOH + HCl NaCl +H2O B) Pb 2+ + Cl - PbCl2 C) AgNO3 +HCI HNO3 + AgCI D) None of the above is a redox reaction. 14) Which element is oxidized in the reaction below? Fe(CO)5(l) + 2HI(g) Fe(CO)4I2(s) + CO(g) + H2(g) A) Fe B) C C) O D) H E) I 15) Which element is reduced in the reaction below? Fe(CO)5(l) + 2HI(g) Fe(CO)4I2(s) + CO(g) + H2(g) A) Fe B) C C) O D) H E) I 16) Which substance is the reducing agent in the following reaction? Fe2S3 + 12HNO3 2Fe(NO3)3 + 3S +6NO2 + 6H2O A) HNO3 B) S C) NO2 D) Fe2S3 E) H2O Table 20.1 Half Reaction E (V) F2(g) + 2e - 2F - (aq) CI2(g) + 2e - 2CI - (aq) Br2(g) + 2e - 2Br - (aq) O2(g) + 4H + (aq) + 4e - 2H2O(l) Ag + (aq) + e - Ag(s) Fe 3+ (aq) + e - Fe 2+ (aq) I2(g) + 2e - 2I - (aq) Cu 2+ (aq) + 2e - Cu(s) H + (aq) + 2e - H2(g) 0 Pb 2+ (aq) + 2e - Pb(s) Ni 2+ (aq) + 2e - Ni(s) -0.28

4 Li + (aq) + e - Li(s) ) Which of the halogens in Table 20.1 is the strongest oxidizing agent? A) CI2 B) Br2 C) F2 D) I2 E) All of the halogens have equal strength as oxidizing agents. 18) Which one of the following types of elements is most likely to be a good oxidizing agent? A) alkali metals B) lanthanides C) alkaline earth elements D) transition elements E) halogens 19) Which one of the following is the best oxidizing agent? A) H2 B) Na C) O2 D) Li E) Ca II. Voltaic Cells and Cell EMF 20) In a voltaic cell, electrons flow from the to the. A) salt bride, anode B) anode, salt bridge C) cathode, anode D) salt bridge, cathode E) anode, cathode 21) Which transformation could take place at the anode of an electrochemical cell? A) NO NO3 - B) CO2 C2O4 2- C) VO2 VO 2+ D) H2AsO4 H3AsO3 E) O2 H2O2 22) Which transformation could take place at the cathode of an electrochemical cell? A) MnO2 MnO4 - B) Br2 BrO3 - C) NO HNO2 D) HSO4 - H2SO3 E) Mn 2+ MnO4 -

5 23) Which transformation could take place at the anode of an electrochemical cell? A) Cr2O7 2- Cr 2+ B) F2 F - C) O2 H2O D) HAsO2 As E) None of the above could take place at the anode. 24) The standard cell potential (E cell) for the voltaic cell based on the reaction below is V. Sn 2+ (aq) + 2Fe 3+ (aq) 2Fe 2+ (aq) + Sn 4+ (aq) A) B) C) D) E) ) The standard cell potential (E cell) for the voltaic cell based on the reaction below is V. Cr(s) +3Fe 3+ (aq) 3Fe 2+ (aq) + Cr 3+ (aq) A) B) C) D) E) ) The standard cell potential (E cell) for the voltaic cell based on the reaction below is V. 2Cr(s) +3Fe 3+ (aq) 3Fe 2+ (aq) + 2Cr 3+ (aq) A) B) C) D) E) ) The standard cell potential (E cell) for the voltaic cell based on the reaction below is V. 3Sn 4+ (aq) + 2Cr(s) 2Cr 3+ (aq) + 3Sn 2+ (aq) A) B) C) D) E) III. Spontaneity of redox reactions /Gibbs Free Energy & EMF Table 20.2

6 Half Reaction E (V) Cr 3+ (aq) + 2e - Cr(s) Fe 2+ (aq) + 2e - Fe(s) Fe 3+ (aq) + e - Fe 2+ (s) Sn 4+ (aq) + 2e - Sn 2+ (aq) ) Which of the following reactions will occur spontaneously as written? A) Sn 4+ (aq) + Fe 3+ (aq) Sn 2+ (aq) + Fe 2+ (s) B) 3Fe(s) + 2Cr 3+ (aq) 2Cr(s) + 3Fe 2+ (aq) C) Sn 4+ (aq) + Fe 2+ (s) Sn 2+ (aq) + Fe(s) D) 3Sn 4+ (aq) + 2Cr(s) 2Cr 3+ (aq) + 3Sn 2+ (aq) E) 3Fe 2+ (s) Fe(s) + 2Fe 3+ (aq) 29) Which of the following reactions will occur spontaneously as written? A) 3Fe 2+ (aq) + Cr 3+ (aq) Cr(s) +3Fe 3+ (aq) B) 2Cr 3+ (aq) + Sn 2+ (aq) 3Sn 4+ (aq) + 2Cr(s) C) Sn 4+ (aq) + Fe 2+ (s) Sn 2+ (aq) + Fe(s) D) Sn 2+ (aq) + Fe 2+ (s) Sn 4+ (aq) + Fe 3+ (aq) E) Cr(s) +3Fe 2+ (s) 3Fe(s) + 2Cr 3+ (aq) 30) The standard cell potential (E cell) of the reaction below is V. The value of ΔG for the reaction is kj/mol. Pb(s) + 2H + (aq) Pb 2+ (aq) + H2(g) A) B) C) D) E) IV. Effect of concentration on Cell EMF/ Nernst Equation 31) The standard cell potential (E cell) for the reaction below is V. The cell potential for this reaction is V when [Zn 2+ ] = 1.0M and [Pb 2+ ] = 2.0x10-4 M Pb 2+ (aq)+ Zn(s) Zn 2+ (aq) + Pb(s) A) 0.52 B) 0.85 C) 0.41 D) 0.74 E) ) The standard cell potential (E cell) for the reaction below is V. The cell potential for this reaction is V when the concentration of [Cu 2+ ] = 1.0x10-5 M and [Zn 2+ ] = 1.0M Zn(s) + Cu 2+ (aq) Cu(s) + Zn 2+ (aq) A) 1.40 B) 1.25 C) 0.95

7 D) 0.80 E) ) Consider an electrochemical cell based on the reaction: 2H + (aq)+ Sn(s) Sn 2+ (aq) + H2(g) Which of the following actions would change the measured cell potential? A) increasing the ph in the cathode compartment B) lowering the ph in the cathode compartment C) increasing the [Sn 2+ ] in the anode compartment D) increasing the pressure of hydrogen gas in the cathode compartment E) Any of the above will change the measure cell potential. 34) Consider an electrochemical cell based on the reaction: 2H + (aq)+ Sn(s) Sn 2+ (aq) + H2(g) Which of the following actions would not change the measured cell potential? A) lowering the ph in the cathode compartment B) addition of more tin metal to the anode compartment C) increasing the tin (II) ion concentration in the anode compartment D) increasing the pressure of hydrogen gas in the cathode compartment E) Any of the above will change the measured cell potential. 35) The standard cell potential (E ) of a voltaic cell constructed using the cell reaction below is 0.76 V: Zn(s) + 2H + (aq) Zn 2+ (aq) + H2(g) With PH2 = 1.0 atm and [Zn 2+ ] = 1.0 M, the cell potential is 0.66 V. The concentration of H + in the cathode compartment is M. A) 2.0x10-2 B) 4.2x10-4 C) 1.4x10-1 D) 4.9x10 1 E) 1.0x ) A voltaic cell is constructed with two silver-silver chloride electrodes, where the halfreaction is AgCl(s) + e - Ag(s) + Cl - (aq) E = V The concentrations of chloride ion in the two compartments are M and 2.22 M, respectively. The cell EMF is V. A) B) C) D) 22.2 E) ) A voltaic cell is constructed with two Zn 2+ -Zn electrodes, where the half-reaction is Zn e - Zn(s) E = V The concentrations of zinc ion in the two compartments are 5.50 M and 1.11x10-2 M,

8 respectively. The cell EMF is V. A) -1.54x10-3 B) -378 C) D) E) ) The standard EMF for the cell using the overall cell reaction below is V: 2Al(s) +3I2(s) 2Al 3+ (aq) + 6l - (aq) The EMF generated by the cell when [Al 3+ ] = 4.5x10-3 M and [l - ] = 0.15 M is V. A) 2.20 B) 2.32 C) 2.10 D) 2.39 E) ) The standard EMF for the cell using the overall cell reaction below is V: Zn(s) + Ni 2+ (aq) Zn 2+ (aq)+ Ni(s) The EMF generated by the cell when [Ni 2+ ] = 2.50 M and [Zn 2+ ] = M is V. A) 0.40 B) 0.50 C) 0.52 D) 0.56 E) 0.44 V. Electrolysis/Electroplating 40) One of the differences between a voltaic cell and an electrolytic cell is that in an electrolytic cell. A) an electric current is produced by a chemical reaction B) electrons flow toward the anode C) a nonspontaneous reaction is forced to occur D) O2 gas is produced at the cathode E) oxidation occurs at the cathode 41) How many minutes will it take to plate out 2.19 g of chromium metal from a solution of Cr 3+ using a current of 35.2 amps in an electrolyte cell? A) 5.77 B) 346 C) 115 D) 1.92 E) ) What current (in A) is required to plate out 1.22 g of nickel from a solution of Ni 2+ in 1.0 hour? A) 65.4

9 B) 4.01x10 3 C) 1.11 D) 12.9 E) ) How many grams of Ca metal are produced by the electrolysis of molten CaBr2 using a current of 30.0 amp for 10.0 hours? A) 22.4 B) 448 C) D) 224 E) ) How many grams of Cu are obtained by passing a current of 12 A through a solution of CuSO4 for 15 minutes? A) B) 3.6 C) 7.1 D) 14 E) ) How many seconds are required to produce 1.0 g of silver metal by the electrolysis of AgNO3 solution using a current of 30 amps? A) 2.7x10 4 B) 3.2x10 3 C) 30x10 1 D) 37x10-5 E) 60 46) How many grams of copper will be plated out by a current of 2.3 A applied for 25 minutes to a 0.50 M solution of copper(ii) sulfate? A) 1.8x10-2 B) 2.2 C) 1.1 D) E) ) How many minutes will it take to plate out 4.56 g of Ni metal from a solution of Ni 2+ using a current of 45.5 amps in an electrolytic cell? A) 2.75 B) 5.49 C) 4.55 D) 155 E) ) How many minutes will it take to plate out g of Al metal from a solution of Al 3+

10 using a current of 12.9 amps in an electrolytic cell? A) 60.1 B) 74.9 C) 173 D) 225 E E) ) The electrolysis of molten AlCl3 for 3.25 hr with an electrical current of 15.0 A produces g of aluminum metal. A) 147 B) C) 4.5x10-3 D) 16.4 E) ) How many seconds are required to produce 4.00 g of aluminum metal from the electrolysis of molten AlCl3 with an electrical current of 12.0 A? A) 27.0 B) 9.00 C) 1.19x10 3 D) 2.90x10 5 E) 3.57x10 3 Extra practice balancing half-reactions 51) What is the coefficient of the permanganate ion when the following equation is balanced? A) 1 B) 2 C) 3 D) 5 E) 4 MnO4 - + Br - Mn 2+ + Br2 (acidic solution) 52) What is the coefficient of the dichromate ion when the following equation is balanced? Fe 2+ + Cr2O7 2- Fe 3+ + Cr 3+ (acidic solution) A) 1 B) 2 C) 3 D) 5 E) 6 53) What is the coefficient of Fe 3+ when the following equation is balanced? CN - + Fe 3+ CNO - + Fe 2+ (basic solution) A) 1 B) 2

11 C) 3 D) 4 E) 5 54) Given the two half reactions and their potentials, which net reaction is spontaneous? Ni 2+ (aq) + 2e - Ni(s) E = V Mg 2+ (aq) + 2e - Mg(s) E = V A) Ni(s) + Mg 2+ (aq) Mg(s) + Ni 2+ (aq) B) Ni 2+ (aq) + Mg(s) Mg 2+ (aq) + Ni(s) C) Ni(s) + Mg(s) Mg 2+ (aq) + Ni 2+ (aq) D) Mg 2+ (aq) + Ni 2+ (aq) Mg(s) + Ni(s) E) Mg 2+ (aq) + Mg(s) Ni(s) + Ni 2+ (aq) 55) Calculate E for the following reaction: Sn 4+ (aq) + 2K(s) Sn 2+ (aq) + 2K + (aq) A) V B) V C) V D) V E) V 56) Calculate E for the following reaction: 2Al 3+ (aq) + 3Cd(s) 2Al(s) + 3Cd 2+ (aq) A) V B) V C) V D) V E) V 57) Using data from the reduction potential table and the reaction calculate the standard reduction potential of the half-reaction 2Ag(s) + Pt 2+ (aq) Pt(s) + 2Ag + (aq) E = 0.38 V A) V B) V C) 0.40 V D) 1.18 V E) 2.00 V Pt 2+ (aq) + 2e - Pt(s)

12 58) Using data from the reduction potential table, predict which of the following is the best oxidizing agent. A) F2 B) Ag C) Sn 4+ D) Ag + E) Al 3+ 59) How long would it take to deposit 1.36 g of copper from an aqueous solution of copper(ii) sulfate by passing a current of two amperes through the solution? A) 2070 sec B) 1.11 x 10-5 sec C) 2570 sec D) 736 sec E) 1030 sec 60) If a current of 6.0 amps is passed through a solution of Ag + for 1.5 hours, how many grams of silver are produced? A) 0.60 g B) 36 g C) 0.34 g D) 3.0 g E) 1.0 g 61) is reduced in the following reaction. A) Cr B) S2O3 2- C) H + D) Cr 3+ E) S4O6 2- Cr S2O H + 2Cr S4O H2O 62) Which of the following transformations could not take place at the cathode of an electrochemical cell? A) NO NO3 - B) CO2 C2O4 2- C) VO2 + VO 2+ D) H2AsO4 H3AsO3 E) O2 H2O2 63)The standard reduction potentials in V for Ag + and Ni 2+ are and -0.28, respectively. Electrons in the cell flow through the toward the. A) Wire, silver electrode

13 B) Wire, nickel electrode C) Salt bridge, nickel electrode D) Salt bridge, silver electrode E) Salt bridge, voltmeter 64)Given the following information Half-reaction E red Sn 4+ (aq) + 2e - Sn 2+ (aq) V Fe 2+ (aq) + 2e - Fe (s) Fe 3+ (aq) + e - Fe 2+ (s) Cr 3+ (aq) + 3e - Cr (s) Determine the standard potential (in V) of a cell based on the reaction: Sn 2+ (aq) + 2 Fe 3+ (aq) 2Fe 2+ (aq) + Sn 4+ (aq) A) +.46 B) C) D) E) ) Respective standard reduction potentials in V for Pb 2+ and Ni 2+ are and Which substance will be oxidized in the voltaic cell below? A) Pb 2+ B) Pb C) Ni 2+ D) Ni E) No substance will be oxidized because the reaction is nonspontaneous 66)E for the following reaction is 0.13 V. what is the value of ΔG (in kj) for the reaction? (F = 96,500 J/mol) Pb (s) + 2H + (aq) Pb 2+ (aq) + H2 (g) A) -25 B) 25 C) -12 D) 12 E) )Given the following information Half-reaction Sn 4+ (aq) + 2e - Sn 2+ (aq) E red V

14 Fe 2+ (aq) + 2e - Fe (s) Fe 3+ (aq) + e - Fe 2+ (s) Cr 3+ (aq) + 3e - Cr (s) Which of the following reactions will occur spontaneously as written? A) 3 Fe 3+ (aq) + Cr 3+ (aq) Cr (s) + 3 Fe 3+ (aq) B) 2 Cr 3+ (aq) + 3 Sn 2+ (aq) 3 Sn 4+ (aq) + 2 Cr (s) C) Sn 4+ (aq) + Fe 2+ (aq) Sn 2+ (aq) + Fe (s) D) Sn 2+ (aq) + Fe 2+ (aq) Sn 4+ (aq) + Fe 3+ (aq) E) 2 Cr (s) + 3 Fe 2+ (aq) 3 Fe (s) + 2 Cr 3+ (aq) 68)How many seconds will be required to produce 1.0 g of silver metal by the electrolysis of a AgNO3 solution using a current of 30 amps? A) 2.7 x 10 4 B) 3.2 x 10 3 C) 30 D) 3.7 x 10-5 E) 60 Answers 1) B 2) D 3) D 4) B 5) A 6) A 7) C 8) A 9) B 10) A 11) B 12) C 13) D 14) A 15) D 16) D 17) C 18) E 19) C 20) E 21) A 22) D 23) E 24) B 25) C 26) A 27) B 28) D 29) E 30) A 31) A 32) C 33) E 34) B 35) A 36) B 37) C 38) B 39) C 40) C 41) A 42)C 43) D 44) B 45) C 46) C 47) B 48) D 49) D 50) E 51) B 52) A 53) B 54) B 55) C 56) E 57) D 58) A 59) A 60) B 61) A 62) A 63) A 64) B 65) D 66) A 67) E 68) C

15 Oxidation numbers: Assign oxidation numbers to all of the elements in each of the compounds or ions below. HCl H2SO3 KNO3 H2SO4 OH- BaO2 Mg3N2 KMnO4 KClO3 LiH Al(NO3)3 MnO2 S8 OF2 H2O2 SO3 PbO2 NH3 NaHSO4 Na

16 Free response questions: 1) Answer the following questions that relate to electrochemical reactions. (a) Under standard conditions at 25 C, Zn(s) reacts with Co 2+ (aq) to produce Co(s). (i) Write the balanced equation for the oxidation half reaction. (ii) Write the balanced net-ionic equation for the overall reaction. (ii) Calculate the standard potential, E, for the overall reaction at 25 C. (b) At 25 C, H2O2, decomposes according to the following equation. 2H2O2(aq) 2H2O(l) + O2(g) E = 0.55 V (i) Determine the value of the standard free energy change, G, for the reaction at 25 C. (ii) Determine the value of the equilibrium constant, Keq, for the reaction at 25 C. (iii) The standard reduction potential, E, for the half reaction O2(g) + 4H + (aq) + 4e 2H2O(l) has a standard value of 1.23 V. Using this information in addition to the information above, determine the value of the standard reduction potential, E, for the half reaction below. O2(g) + 2H + (aq) + 2e H2O2(l) (c) In an electrolytic cell, Cu(s) is produced by the electrolysis of CuSO4(aq). Calculate the maximum mass of the Cu(s) that can be deposited by a direct current of 100 amperes passed through 5.00 L of 2.00 M CuSO4(aq) for a period of 1.00 hr. 2) Answer the following questions that refer to the galvanic cell shown in the diagram below. (a) Identify the anode of the cell and write the half reaction that occurs there. (b) Write the net ionic equation for the overall reaction that occurs as the cell operates and calculate the value of the standard cell potential, E cell.

17 (c) Indicate how the value of Ecell would be affected if the concentration of Ni(NO3) 2(aq) was changed from 1.0 M to 0.10 M and the concentration of Zn(NO3) 2(aq) remained at 1.0 M. Justify your answer. (d) Specify whether the value of Keq for the cell reaction is less than 1, greater than 1, or equal to 1. Justify your answer. 3) Answer parts (a) through (e) below, which relate to reactions involving silver ion, Ag +. The reaction between silver ion and solid zinc is represented by the following equation. 2Ag + (aq) + Zn(s) Zn 2+ (aq) + 2Ag(s) (a) A 1.50 g sample of Zn is combined with 250 ml of M AgNO3 at 25. (i) Identify the limiting reactant. Show your calculations to support your answer. (ii) On the basis of the limiting reactant that you identified in part (i), determine the value of [Zn 2+ ] after the reaction is complete. Assume that volume change is negligible. (b) Determine the value of the standard potential, E, for the galvanic cell based on the reaction between AgNO3(aq) and solid Zn at 25 C. Another galvanic cell is based on the reaction between Ag + (aq) and Cu(s), represented by the equation below. At 25 C, the standard potential, E, for the cell is 0.46 V. 2Ag + (aq) + Cu(s) Cu 2+ (aq) + 2Ag(s) (c) Determine the value of the standard free energy change, ΔG, for the reaction between Ag + (aq) and Cu(s) at 25 C. (d) The cell is constructed so that [Cu 2+ ] is M and [Ag + ] is M. Calculate the value of the potential, E, for the cell. (e) Under the conditions specified in part (d), is the reaction in the cell spontaneous? Justify your answer. 4) Answer the following about electrochemistry.

18 (a) Several different electrochemical cells can be constructed using the materials shown below. Write the balanced net ionic equation for the reaction that occurs in the cell that would have the greatest positive value of E cell. (b) Calculate the standard cell potential, E cell, for the reaction written in part (a). (c) A cell is constructed based on the reaction in part (a) above. Label the metal used for the anode on the cell shown below. (d) Of the compounds NaOH, CuS, and NaNO3, which one is appropriate to use in a salt bridge? Briefly explain you answer, and for each of the other compounds, include a reason why it is not appropriate. (e) Another standard cell is based on the following reaction. Zn + Pb 2+ Zn 2+ + Pb If the concentration of Zn 2+ is decreased from 1.0 M to 0.25M, what effect does this have on the cell potential? Justify your answer? 5) An electrochemical cell is constructed with an open switch as in the below diagram. A trip of Sn and a strip of an unknown metal, X are used as electrodes. When the switch is closed, the mass of the Sn electrode increases. The half reactions are shown below. Sn(aq) + 2e - Sn(s) E = V X(aq) + 3e - X(s) E =?

19 a) In the diagram above, label the electrode that is the cathode. Justify your answer. b) Draw an arrow indicating the electron flow in the circuit when the switch is closed. c) if the standard cell potential, E, is +0.60V, what is the standard reduction potential, in volts, for the X 3+ /X electrode? d) identify the metal X. e) write a balanced net ionic equation for the cell reaction. f) In the cell, the [Sn 2+ ] is changed from 1.0M to 0.5M, and the [X 3+ ] is changed from 1.0M to 0.10M. i) Substitute all the appropriate values in the Nernst equation. (No Calculation needed) ii) On the basis of your response in part (f) (i), will the cell potential Ecell be greater than, less than, or equal to the original E cell? Justify your response. 6) How many seconds are required to produce 4.00 g of aluminum metal from the electrolysis of molten AlCl3 with an electrical current of 12.0 A? 7) a) A voltaic cell is created using a zinc electrode immersed in a 1.0 M solution of Zn 2+ ions and a silver electrode immersed in a solution of 1.0 M Ag+ ions. The two electrodes are connected with a wire, and a salt bridge is placed between the two half-cells. The temperature of the cells is 25 C. i) Write the balanced half-reactions that are occurring at the cathode and the anode. ii) Calculate the standard cell voltage for the cell. iii) Calculate ΔG for the cell. iv) How will the voltage be affected if 2.5 g of KCl is added to each half cell? Justify your response.

20 b) An electrolytic cell contains molten NaCl. i) Gas is seen bubbling at one of the electrodes. Is this the cathode or anode? Explain. ii) If a current of 0.5 A is applied to the cell for 30 minutes, how many grams of sodium metal will be collected? iii) If the current is raised to 5.0 A, how long will it have to be applied to produce 10.0 g of sodium metal? iv) If the molten NaCl is replaced by a 1.0 M aqueous sodium chloride solution, how much sodium can be collected if a current of 1.0 A is applied for 1 hour? Answers 1. a) i) Zn(s) Zn 2+ (aq) + 2e - ii) Co 2+ (aq) + Zn(s) Co(s) + Zn 2+ (aq) ii)-0.28 (-0.76) = = 0.48V b) i) ΔG = -nfe = -2(96500)(0.55V) = -1.1x10 5 J ii) ΔG = -RTlnK -1.1x10 5 J = (8.31 J mol -1 K -1 )(298K)(ln(K)) K = 2.0x10 19 iii) O2 + 2H2O 2H2O V O2 + 4H + + 4e - 2H2O 1.23 V 2O2 + 4H + + 4e - 2H2O V»» O2 + 2H + + 2e - H2O V log(k) = ne / c) 3,600sec (100C/1sec) (1mol e - /96,500C) (1mol Cu/2mol e - ) (63.55g Cu/1mol Cu) = 119g Cu 2. a) Electrode on the right, Zn is the anode Half reaction: Zn Zn e - b) Zn + Ni 2+ Zn Ni E cell = (-0.25V) (-0.76V) = 0.51V c) Ecell would decrease Since Ni 2+ is a reactant. A decrease in its concentration decreases the driving force for the forward reaction OR Ecell = E = (RT/n^)ln(Q), where Q = [Zn 2+ ]/[Ni 2+ ] : Decreasing the [Ni 2+ ] would increase the value of Q, so a larger number would be subtracted from E, thus decreasing the value of E cell d) K > 1 E is positive, so K > 1

21 3) a) i) nzn = 1.50 g Zn (1 mol Zn/65.4 g Zn) = 2.29x10 2 mol Zn nag + = L (0.110 mol Ag + /1L) = 2.75x10 2 mol Ag + nag + = 1.50 g Zn (1 mol Zn/65.4 g Zn)(2 mol Ag + /1 mol Zn) = 4.59x10 2 mol Ag + required Since only 2.75x10 2 mol Ag + available, Ag + is the limiting reactant. OR nag + = L (0.110 mol Ag + /1L) = 2.75x10 2 mol Ag + nzn = 2.75x10 2 mol Ag + (1 mol Zn/2 mol Ag + ) = 1.38x10 2 mol Zn required Since 2.29x10 2 mol Zn are available, more is available than required, so Zn is in excess and Ag + is limiting. ii) nzn 2+ = 2.75x10 2 mol Ag + (1 mol Zn 2+ /2 mol Ag + ) = 1.38x10 2 mol Zn 2+ (1.38x10 2 mol Zn 2+ /0.250L) = M Zn 2+ OR [Ag + ]initial = 0.110M, therefore [Zn 2+ ] = ½(0.110M) = M OR if Zn is identified as limiting reagent 1.50 g Zn (1 mol Zn/65.4 g)(1 mol Zn 2+ /1 mol Zn) = 2.29x10 2 mol Zn 2+ formed (2.29x10 2 mol Zn 2+ /0.250L) = M Zn 2+ b) E cell = E (reduction) - E (reduction) = (0.80V) (-0.76V) = 1.56V 2Ag + (aq) + Zn(s) Zn 2+ (aq) + 2Ag(s) OR Ag + (aq) + e - Ag(s) Zn(s) Zn 2+ (aq) + e - 2Ag + (aq) + Zn(s) Zn 2+ (aq) + 2Ag(s) +1.56V E +0.80V +0.76V +1.56V c) ΔG = -nfe = (-2mol e - )(96,500 J V -1 mol -1 )(0.46V) = -89,000J or -89kJ d) Ecell= E (RT/nF)(lnQ)= E (RT/nF)(ln[Cu 2+ ]/[Ag + ] 2 )= E (.0592/n)(log[Cu 2+ ]/[Ag + ] 2 ) Ecell= +0.46V {(8.314 J K -1 mol -1 ) (298K)}/{(2mol e - ) (96,500 J V -1 mol -1 )} (ln[0.045]/[0.010] 2 ) Ecell = (0.0128V)(ln450) = (0.0128V)(6.11) = V = +0.38V e) Ecell = +0.38V The cell potential under the non-standard conditions in part (d) is positive. Therefore the reaction is spontaneous under the conditions stated in part (d). A correct reference (from answer in part (d)) to a negative ΔG (not ΔG ) is acceptable. If no answer to (d) is given, students must make an assumption or a general statement about Ecell, not E. 4) a) Al(s) Al 3+ (aq) + 3e - Cu 2+ (aq) + 2e - Cu(s) 2Al(s) + 3Cu 2+ (aq) 2Al 3+ (aq) + 3Cu(s)

22 b) Al 3+ (aq) + 3e - Al(s) E = -1.66V Cu 2+ (aq) + 2e - Cu(s) E = E E cell = E cathode - E anode = +0.34V (-1.66V) = +2.00V c) the metal is aluminum solid d) NaOH is not appropriate. The anion, OH -, would not migrate towards the anode. The OH - would react with the Al 3+ ion in solution. CuS is not appropriate. It is insoluble in water, so no ions would be available to migrate to the anode and cathode compartment to balance the charge. NaNO3 is appropriate. It is soluble in water, and neither the cation nor the anion will react with the ions in the anode or cathode compartment e) Ecell = E cell (0.059)(ln[Zn 2+ ]/[Pb 2+ ]) If [Zn 2+ ] is reduced, then the ratio [Zn 2+ ]/[Pb 2+ ] < 1, therefore ln[zn 2+ ]/[Pb 2+ ] < 0. Thus Ecell increases (becomes more positive). 5) a) The Sn (tin) electrode is the cathode. The increase in mass indicates that reduction occurs at Sn electrode: Sn 2+ (aq) + 2e - Sn(s) Reduction occurs at cathode. b) Diagram should have arrow showing electrons flowing from the anode towards the cathode. c) E cell = E cathode - E anode +0.60V = -0.14V - E anode E anode = -0.74V d) Cr e) 3Sn Cr 3Sn + 2Cr 3+ f)i) Ecell = E cell (0.059/n)(log[Cr 3+ ] 2 /[Sn 2+ ] 3 ) = +0.60V - (0.059/6)(log[0.10] 2 /[0.50] 3 ) ii) Ecell will be greater (more positive). Since the Q ratio is a number less than 1, the log of the ratio will be negative. A negative times a negative is positive. Thus, - (0.059/6)(log[0.10] 2 /[0.50] 3 ) increases Ecell. 6) moles Al = 4/ = mol x ( 3 Faradays / 1 mol) = Faradays F = F ( C / F) = C C / 12.0 C/s = s 7) a)i) cathode: 2Ag + + 2e - 2Ag anode: Zn Zn e - ii) 2Ag + + 2e - 2Ag E = 0.80V Zn Zn e - E cell = (0.80V) (-0.76V) = 1.56V iii) ΔG = -nfe = -2(96500)(1.56) = 301 kj iv) voltage will drop b)i) E = -0.76V

23 Review Problems: 1. Which of the following will be the best oxidizing agent? A) Cl2 B) Fe C) Na D) Na + E) F - 2. A piece of copper metal is placed into solutions containing each of the following three cations. Which ion will be reduced by the copper metal? i. Sn 2+ ii. Zn 2+ iii. Ag + A) I only B) II only C) III only D) I and II E) I, II, and III 3. When molten CaCl2 is electrolyzed, the calcium metal collects at the charged electrode, which is called the, and the chlorine is given off at the charged electrode, which is known as the. A) Positively, anode, negatively, cathode B) Positively, cathode, negatively, anode C) Negatively, anode, positively, cathode D) Negatively, cathode, positively, anode E) Positively, electrolyte, negatively, EMF 4. If A of current is applied for 1.50 h to an electrolytic cell containing molten lithium chloride, how many grams of lithium would be deposited on the cathode? A) g B) g C) g D) g E) 5.4 x 10-5 g 5. The reduction of 3.00 moles of Ca 2+ to Ca would require how many coulombs? A) 6.22 x 10-5 C B) 3.22 x 10 4 C C) 1.61 x 10 4 C D) 2.90 x 10 5 C

24 E) 5.79 x 10 5 C 6. A voltaic cell is created with one half-cell consisting of a copper electrode immersed in 1.0 M CuSO4 solution and the other half cell consisting of a lead electrode immersed in a 1.0 M Pb(NO3)2 solution. Each half-cell is maintained at 25 C. what is the cell potential, in volts? A) 0.47 V B) 0.21 V C) V D) V E) 2.50 V 7. For the reaction below, what is the equilibrium constant at 25 C? Ag + (aq) + Fe 2+ (aq) Ag (s) + Fe 3+ (aq) A) 1.0 B) 1.6 C) 3.2 D) 6.4 E) If a current of 15.0 A is applied to a solution of Cr 3+ ions, how long will it take to plate out 1.86 g of chromium metal? A) 3.83 minutes B) 5.7 minutes C) 11.5 minutes D) 35.0 minutes E) 690 minutes 9. A voltaic cell is set up with two half-cells. In the first half-cell, a silver electrode is placed in an aqueous solution containing Ag + ions. In the second half-cell, a nickel electrode is placed in an aqueous solution containing Ni 2+ ions. The silver electrode is the and has a charge. A) Cathode, positive B) Cathode, negative C) Anode, positive D) Anode, negative E) Salt bridge, positive and negative

25 10. A voltaic cell contains one half-cell with a zinc electrode in a Zn 2+ (aq) solution and a copper electrode in a Cu 2+ (aq) solution. At standard condition, E =1.10 V. Which condition below would cause the cell potential to be greater than 1.10 V? A) 1.0 M Zn 2+ (aq), 1.0 M Cu 2+ (aq) B) 5.0 M Zn 2+ (aq), 5.0 M Cu 2+ (aq) C) 5.0 M Zn 2+ (aq), 1.0 M Cu 2+ (aq) D) 0.5 M Zn 2+ (aq), 0.5 M Cu 2+ (aq) E) 0.1 M Zn 2+ (aq), 1.0 M Cu 2+ (aq) 11. What is the standard free energy change (ΔG ) for the reaction shown below? Fe 3+ (aq) + Ag (s) Fe 2+ (aq) + Ag + (aq) A) -2.5 kj B) 2.9 kj C) 8.7 kj D) 10.0 kj E) 29 kj 12. An electrolytic cell contains molten CuBr2. What is the minimum voltage that must be applied to begin electrolysis? A) 0.17 V B) 0.34 V C) 0.73 V D) 1.07 V E) 1.41 V Answers: 1. A 2. C 3. D 4. B 5. E 6. A 7. C 8. C 9. A 10. E 11. B 12. C