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:

Cr2O72- + 6S2O3

2- + 14H+ 2Cr3+ + 3S4O62- + 7H2O

A) Cr6+

B) S2+

C) H+ D) O2-

E) S4O62-

6) __________ is the oxidizing agent in the reaction below.

Cr2O72- + 6S2O3

2- + 14H+ 2Cr3+ + 3S4O62- + 7H2O

A) Cr2O72-

B) S2O32-

C) H+

D) Cr3+

E) 264S O

7) Which substance is serving as the reducing agent in the following reaction?

14H+ + Cr2O72- + 3Ni 3Ni2+ + 2Cr3+ + 7H2O

A) H+ B) Cr2O7

2- C) Ni D) H2O E) Ni2+

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?

Fe2+ + H+ + Cr2O72- Fe3+ + Cr3+ + 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) 3Mn2+(aq) + 5Fe3+(aq) + 12H2O(l)

A) MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l)

B) 2MnO4-(aq) + 12H+(aq) + 6e- 2Mn2+(aq) + 3H2O(l)

C) Fe(s) Fe3+(aq) + 3e-

D) Fe(s) Fe2+(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) Pb2+ + 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) +2.87 CI2(g) + 2e- 2CI-(aq) +1.359 Br2(g) + 2e- 2Br-(aq) +1.065 O2(g) + 4H+(aq) + 4e- 2H2O(l) +1.23 Ag+(aq) + e- Ag(s) +0.799 Fe3+(aq) + e- Fe2+(aq) +0.771 I2(g) + 2e- 2I-(aq) +0.536 Cu2+(aq) + 2e- Cu(s) +0.34 2H+(aq) + 2e- H2(g) 0 Pb2+(aq) + 2e- Pb(s) -0.126 Ni2+(aq) + 2e- Ni(s) -0.28

Li+(aq) + e- Li(s) -3.05

17) 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 C2O42-

C) VO2 VO2+ 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) Mn2+ MnO4

-

23) Which transformation could take place at the anode of an electrochemical cell? A) Cr2O7

2- Cr2+

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.

Sn2+(aq) + 2Fe3+(aq) 2Fe2+(aq) + Sn4+(aq) A) +0.46 B) +0.617 C) +1.39 D) -0.46 E) +1.21

25) The standard cell potential (E°cell) for the voltaic cell based on the reaction below is __________ V.

Cr(s) +3Fe3+(aq) 3Fe2+(aq) + Cr3+(aq) A) -1.45 B) +2.99 C) +1.51 D) +3.05 E) +1.57

26) The standard cell potential (E°cell) for the voltaic cell based on the reaction below is __________ V.

2Cr(s) +3Fe3+(aq) 3Fe2+(aq) + 2Cr3+(aq) A) +0.30 B) +2.80 C) +3.10 D) +0.83 E) -0.16

27) The standard cell potential (E°cell) for the voltaic cell based on the reaction below is __________ V.

3Sn4+(aq) + 2Cr(s) 2Cr3+(aq) + 3Sn2+(aq) A) +1.94 B) +0.89 C) +2.53 D) -0.59 E) -1.02

III. Spontaneity of redox reactions /Gibbs Free Energy & EMF Table 20.2

Half Reaction E°(V)

Cr3+(aq) + 2e- Cr(s) -0.740 Fe2+(aq) + 2e- Fe(s) -0.440 Fe3+(aq) + e- Fe2+(s) +0.771 Sn4+(aq) + 2e- Sn2+(aq) +0.154

28) Which of the following reactions will occur spontaneously as written?

A) Sn4+(aq) + Fe3+(aq) Sn2+(aq) + Fe2+(s) B) 3Fe(s) + 2Cr3+(aq) 2Cr(s) + 3Fe2+(aq) C) Sn4+(aq) + Fe2+(s) Sn2+(aq) + Fe(s) D) 3Sn4+(aq) + 2Cr(s) 2Cr3+(aq) + 3Sn2+(aq) E) 3Fe2+(s) Fe(s) + 2Fe3+(aq)

29) Which of the following reactions will occur spontaneously as written?

A) 3Fe2+(aq) + Cr3+(aq) Cr(s) +3Fe3+(aq) B) 2Cr3+(aq) + Sn2+(aq) 3Sn4+(aq) + 2Cr(s) C) Sn4+(aq) + Fe2+(s) Sn2+(aq) + Fe(s) D) Sn2+(aq) + Fe2+(s) Sn4+(aq) + Fe3+(aq) E) Cr(s) +3Fe2+(s) 3Fe(s) + 2Cr3+(aq)

30) The standard cell potential (E°cell) of the reaction below is +0.126 V. The value of ΔG° for the reaction is __________ kJ/mol.

Pb(s) + 2H+(aq) Pb2+(aq) + H2(g) A) -24.3 B) +24.3 C) -12.6 D) +12.6 E) -50.8 IV. Effect of concentration on Cell EMF/ Nernst Equation 31) The standard cell potential (E°cell) for the reaction below is +0.63 V. The cell potential for this reaction is __________ V when [Zn2+] = 1.0M and [Pb2+] = 2.0x10-4M

Pb2+(aq)+ Zn(s) Zn2+(aq) + Pb(s) A) 0.52 B) 0.85 C) 0.41 D) 0.74 E) 0.63

32) The standard cell potential (E°cell) for the reaction below is +1.10 V. The cell potential for this reaction is __________ V when the concentration of [Cu2+] = 1.0x10-5M and [Zn2+] = 1.0M

Zn(s) + Cu2+(aq) Cu(s) + Zn2+(aq) A) 1.40 B) 1.25 C) 0.95

D) 0.80 E) 1.10

33) Consider an electrochemical cell based on the reaction: 2H+(aq)+ Sn(s) Sn2+(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 [Sn2+] 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) Sn2+(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) Zn2+(aq) + H2(g) With PH2 = 1.0 atm and [Zn2+] = 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) 4.9x101 E) 1.0x10-12

36) A voltaic cell is constructed with two silver-silver chloride electrodes, where the half-reaction is

AgCl(s) + e- Ag(s) + Cl-(aq) E° = +0.222 V The concentrations of chloride ion in the two compartments are 0.0222 M and 2.22 M, respectively. The cell EMF is __________ V.

A) 0.212 B) 0.118 C) 0.00222 D) 22.2 E) 0.232

37) A voltaic cell is constructed with two Zn2+ -Zn electrodes, where the half-reaction is

Zn2+ + 2e- Zn(s) E° = -0.763 V

The concentrations of zinc ion in the two compartments are 5.50 M and 1.11x10-2M,

respectively. The cell EMF is __________ V. A) -1.54x10-3 B) -378 C) 0.0798 D) 0.160 E) -0.761

38) The standard EMF for the cell using the overall cell reaction below is +2.20 V:

2Al(s) +3I2(s) 2Al3+(aq) + 6l-(aq) The EMF generated by the cell when [Al3+] = 4.5x10-3M and [l-] = 0.15 M is ________ V.

A) 2.20 B) 2.32 C) 2.10 D) 2.39 E) 2.23

39) The standard EMF for the cell using the overall cell reaction below is +0.48 V:

Zn(s) + Ni2+(aq) Zn2+(aq)+ Ni(s) The EMF generated by the cell when [Ni2+] = 2.50 M and [Zn2+] = 0.100 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 Cr3+ using a current of 35.2 amps in an electrolyte cell? A) 5.77 B) 346 C) 115 D) 1.92 E) 17.3

42) What current (in A) is required to plate out 1.22 g of nickel from a solution of Ni2+ in

1.0 hour? A) 65.4

B) 4.01x103 C) 1.11 D) 12.9 E) 2.34

43) 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) 0.0622 D) 224 E) 112

44) How many grams of Cu are obtained by passing a current of 12 A through a solution

of CuSO4 for 15 minutes? A) 0.016 B) 3.6 C) 7.1 D) 14 E) 1.8

45) 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) 2.7x104

B) 3.2x103

C) 30x101

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) 0.036 E) 0.019

47) How many minutes will it take to plate out 4.56 g of Ni metal from a solution of Ni2+

using a current of 45.5 amps in an electrolytic cell? A) 2.75 B) 5.49 C) 4.55 D) 155 E) 330

48) How many minutes will it take to plate out 16.22 g of Al metal from a solution of Al3+

using a current of 12.9 amps in an electrolytic cell? A) 60.1 B) 74.9 C) 173 D) 225 E E) 13480

49) 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) 0.606 C) 4.5x10-3 D) 16.4 E) 49.1

50) 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) 1.19x103 D) 2.90x105 E) 3.57x103

Extra practice balancing half-reactions 51) What is the coefficient of the permanganate ion when the following equation is

balanced? MnO4

- + Br - Mn2+ + Br2 (acidic solution) A) 1 B) 2 C) 3 D) 5 E) 4

52) What is the coefficient of the dichromate ion when the following equation is

balanced? Fe2+ + Cr2O7

2- Fe3+ + Cr3+ (acidic solution) A) 1 B) 2 C) 3 D) 5 E) 6

53) What is the coefficient of Fe3+ when the following equation is balanced?

CN- + Fe3+ CNO- + Fe2+ (basic solution) A) 1 B) 2

C) 3 D) 4 E) 5

54) Given the two half reactions and their potentials, which net reaction is

spontaneous?

Ni2+(aq) + 2e- Ni(s) E° = -0.25 V

Mg2+(aq) + 2e- Mg(s) E° = -2.37 V

A) Ni(s) + Mg2+(aq) Mg(s) + Ni2+(aq)

B) Ni2+(aq) + Mg(s) Mg2+(aq) + Ni(s)

C) Ni(s) + Mg(s) Mg2+(aq) + Ni2+(aq)

D) Mg2+(aq) + Ni2+(aq) Mg(s) + Ni(s)

E) Mg2+(aq) + Mg(s) Ni(s) + Ni2+(aq)

55) Calculate E° for the following reaction:

Sn4+(aq) + 2K(s) Sn2+(aq) + 2K+(aq)

A) +6.00 V

B) -3.08 V

C) +3.08 V

D) +2.78 V

E) -2.78 V

56) Calculate E° for the following reaction:

2Al3+(aq) + 3Cd(s) 2Al(s) + 3Cd2+(aq)

A) -2.06 V

B) +4.52 V

C) +2.06 V

D) -4.52 V

E) -1.26 V

57) Using data from the reduction potential table and the reaction calculate the

standard reduction potential of the half-reaction

2Ag(s) + Pt2+(aq) Pt(s) + 2Ag+(aq) E° = 0.38 V

Pt2+(aq) + 2e- Pt(s)

A) -1.18 V

B) -0.40 V

C) 0.40 V

D) 1.18 V

E) 2.00 V

58) Using data from the reduction potential table, predict which of the following is the

best oxidizing agent.

A) F2

B) Ag

C) Sn4+ D) Ag+

E) Al3+

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.

Cr2072- + 6S2O3

2- + 14H+ 2Cr3+ + 3S4O6

2- + 7H2O

A) Cr2072-

B) S2O32-

C) H+

D) Cr3+

E) S4O62-

62) Which of the following transformations could not take place at the cathode of an

electrochemical cell?

A) NONO3-

B) CO2C2O42-

C) VO2+VO2+

D) H2AsO4 H3AsO3

E) O2 H2O2

63)The standard reduction potentials in V for Ag+ and Ni2+ are +0.80 and -0.28,

respectively. Electrons in the cell flow through the ____ toward the ____.

A) Wire, silver electrode

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

Sn4+(aq) + 2e- Sn2+(aq) +0.154 V

Fe2+ (aq) + 2e- Fe (s) -0.440

Fe3+ (aq) + e- Fe2+ (s) +0.771

Cr3+ (aq) + 3e- Cr (s) -0.74

Determine the standard potential (in V) of a cell based on the reaction:

Sn2+ (aq) + 2 Fe3+ (aq) 2Fe2+ (aq) + Sn4+ (aq)

A) +.46

B) +0.62

C) +1.39

D) -0.46

E) +1.21

65) Respective standard reduction potentials in V for Pb2+ and Ni2+ are -0.13 and -0.28.

Which substance will be oxidized in the voltaic cell below?

A) Pb2+

B) Pb

C) Ni2+

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) Pb2+ (aq) + H2 (g)

A) -25

B) 25

C) -12

D) 12

E) -50

67)Given the following information

Half-reaction E°red

Sn4+(aq) + 2e- Sn2+(aq) +0.154 V

Fe2+ (aq) + 2e- Fe (s) -0.440

Fe3+ (aq) + e- Fe2+ (s) +0.771

Cr3+ (aq) + 3e- Cr (s) -0.74

Which of the following reactions will occur spontaneously as written?

A) 3 Fe3+ (aq) + Cr3+ (aq) Cr (s) + 3 Fe3+ (aq)

B) 2 Cr3+ (aq) + 3 Sn2+ (aq) 3 Sn4+ (aq) + 2 Cr (s)

C) Sn4+ (aq) + Fe2+ (aq) Sn2+ (aq) + Fe (s)

D) Sn2+ (aq) + Fe2+ (aq) Sn4+ (aq) + Fe3+ (aq)

E) 2 Cr (s) + 3 Fe2+ (aq) 3 Fe (s) + 2 Cr3+ (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 104

B) 3.2 x 103

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

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

Free response questions:

1) Answer the following questions that relate to electrochemical reactions.

(a) Under standard conditions at 25°C, Zn(s) reacts with Co2+(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.

(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) Zn2+(aq) + 2Ag(s)

(a) A 1.50 g sample of Zn is combined with 250 mL of 0.110 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 [Zn2+] 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) Cu2+(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 [Cu2+] is 0.045 M and [Ag+] is 0.010 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.

(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 + Pb2+ Zn2+ + Pb

If the concentration of Zn2+ 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° = -0.14 V

X(aq) + 3e- X(s) E° = ?

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 X3+ /X electrode?

d) identify the metal X.

e) write a balanced net ionic equation for the cell reaction.

f) In the cell, the [Sn2+ ] is changed from 1.0M to 0.5M, and the [X3+] 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 Zn2+ 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.

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) Zn2+(aq) + 2e-

ii) Co2+(aq) + Zn(s) Co(s) + Zn2+(aq)

ii)-0.28 – (-0.76) = -0.28 + 0.76 = 0.48V

b) i) ΔG° = -nFE° = -2(96500)(0.55V) = -1.1x105J

ii) ΔG° = -RTlnK

-1.1x105 J = (8.31 J•mol-1•K-1)(298K)(ln(K)) log(K) = nE°/0.0592

K = 2.0x1019

iii) O2 + 2H2O 2H2O2 -0.55 V

O2 + 4H+ + 4e- 2H2O 1.23 V

2O2 + 4H+ + 4e- 2H2O2 0.68 V

»» O2 + 2H+ + 2e- H2O2 0.68 V

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 Zn2+ + 2e-

b) Zn + Ni2+ Zn2+ + Ni

E°cell = (-0.25V) – (-0.76V) = 0.51V

c) Ecell would decrease Since Ni2+ 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 = [Zn2+]/[Ni2+] : Decreasing the [Ni2+] 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

3) a) i) nZn = 1.50 g Zn (1 mol Zn/65.4 g Zn) = 2.29x10–2 mol Zn nAg

+ = 0.250 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

+ = 0.250 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 Zn2+/2 mol Ag+) = 1.38x10–2 mol Zn2+

(1.38x10–2 mol Zn2+/0.250L) = 0.0550M Zn2+

OR

[Ag+]initial = 0.110M, therefore [Zn2+] = ½(0.110M) = 0.0550M

OR if Zn is identified as limiting reagent

1.50 g Zn (1 mol Zn/65.4 g)(1 mol Zn2+/1 mol Zn) = 2.29x10–2 mol Zn2+ formed

(2.29x10–2 mol Zn2+/0.250L) = 0.0916M Zn2+

b) E°cell = E°(reduction) - E°(reduction)

= (0.80V) – (-0.76V)

= 1.56V

2Ag+(aq) + Zn(s) Zn2+(aq) + 2Ag(s) +1.56V

OR

E°

Ag+(aq) + e- Ag(s) +0.80V

Zn(s) Zn2+(aq) + e- +0.76V

2Ag+(aq) + Zn(s) Zn2+(aq) + 2Ag(s) +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[Cu2+]/[Ag+]2)= E°– (.0592/n)(log[Cu2+]/[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 = +0.46 – (0.0128V)(ln450) = +0.46 – (0.0128V)(6.11) = +0.46 – 0.0782V = +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) Al3+(aq) + 3e-

Cu2+(aq) + 2e- Cu(s)

2Al(s) + 3Cu2+(aq) 2Al3+(aq) + 3Cu(s)

b) Al3+(aq) + 3e- Al(s) E° = -1.66V

Cu2+(aq) + 2e- Cu(s) E° = +0.34 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 Al3+ 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[Zn2+]/[Pb2+])

If [Zn2+] is reduced, then the ratio [Zn2+]/[Pb2+] < 1, therefore ln[Zn2+]/[Pb2+] < 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: Sn2+(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) 3Sn2+ + 2Cr 3Sn + 2Cr3+

f)i) Ecell = E°cell – (0.059/n)(log[Cr3+]2/[Sn2+]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/ 26.98 = 0.148

0. 148 mol x ( 3 Faradays / 1 mol) = 0.444 Faradays

0.444 F = 0.444 F ( 96500 C / F) = 42846 C

42846 C / 12.0 C/s = 3570.5 s

7) a)i) cathode: 2Ag+ + 2e- 2Ag anode: Zn Zn+2 + 2e-

ii) 2Ag+ + 2e- 2Ag E = 0.80V Zn Zn+2 + 2e- E = -0.76V

E°cell = (0.80V) – (-0.76V) = 1.56V iiI) ΔG° = -nFE° = -2(96500)(1.56) = 301 kJ iv) voltage will drop b)i)

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. Sn2+

ii. Zn2+

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 0.500 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) 0.388 g

B) 0.194 g

C) 0.097 g

D) 0.050 g

E) 5.4 x 10-5 g

5. The reduction of 3.00 moles of Ca2+ to Ca would require how many coulombs?

A) 6.22 x 10-5 C

B) 3.22 x 104 C

C) 1.61 x 104 C

D) 2.90 x 105 C

E) 5.79 x 105 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) -0.21 V

D) -0.47 V

E) 2.50 V

7. For the reaction below, what is the equilibrium constant at 25°C?

Ag+ (aq) + Fe2+ (aq) Ag (s) + Fe3+ (aq)

A) 1.0

B) 1.6

C) 3.2

D) 6.4

E) 10.0

8. If a current of 15.0 A is applied to a solution of Cr3+ 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 Ni2+ 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

10. A voltaic cell contains one half-cell with a zinc electrode in a Zn2+ (aq) solution

and a copper electrode in a Cu2+ (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 Zn2+ (aq), 1.0 M Cu2+ (aq)

B) 5.0 M Zn2+ (aq), 5.0 M Cu2+ (aq)

C) 5.0 M Zn2+ (aq), 1.0 M Cu2+ (aq)

D) 0.5 M Zn2+ (aq), 0.5 M Cu2+ (aq)

E) 0.1 M Zn2+ (aq), 1.0 M Cu2+ (aq)

11. What is the standard free energy change (ΔG°) for the reaction shown below?

Fe3+ (aq) + Ag (s) Fe2+ (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