ChE 323 Compilation of Exam Problems-Electrochemistry 3-5-12
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University of San Carlos-Chemical Engineering Department ChE323 Problems – Electrochemistry Engr. May V. Tampus Page 1 ChE 323 Compilation of Exam Problems Chapter 7: ELECTROCHEMISTRY 1. Sketch a typical galvanic cell. Show direction of electron flow, the direction of ion migration through the porous disk, and identify the cathode and anode. Indicate in which electrode oxidation and reduction occur. Will there be migration of ions in the absence of the porous disk? Next, sketch a galvanic cell with a salt bridge showing the required features as in a typical galvanic cell. Is there a change in the direction of ion migration when a salt bridge is present? Will the diagram change if it were an electrolytic cell? Will the direction of ion migration change? Explain your answer. 2. Discuss the relationship between ionic mobility, molar conductivity, and transport number to ionic conduction. Does electronic conduction also involve the measurement of these quantities? Explain. 3. The standard electromotive force of the cell Pt l H 2 (g) l HCl(aq) l AgCl(s) l Ag has been determined from 0 to 90 o C by R.G. Bates and V.E. Bower, J. Res. Nat. Bur. Stand. 53:283 (1954). Their data may be represented by 0 4 6 2 9 3 () 0.23659 4.8564 10 3.4205 10 5.869 10 V t t t where t is the temperature in o C. Calculate the reaction entropy at 25 o C. (Hint: 0 0 r P S nF T . [ANSWER: -62.297 J/mol-K] 4. One of the few industrial-scale processes that produce organic compounds electrochemically is used by the Monsanto Company to produce 1,4-dicyanobutane. The reduction reaction is 2CH 2 =CHCN + 2H + + 2e - NC – (CH 2 ) 4 – CN The NC – (CH 2 ) 4 – CN is then chemically reduced using hydrogen gas to H 2 N – (CH 2 ) 6 – NH 2 , which is used in the production of nylon. What current must be used to produce 1.0 x 10 3 kg of NC – (CH 2 ) 4 – CN per hour? [ANSWER: 5.0 x 10 5 A] 5. The calomel electrode, Hg(l) l Hg 2 Cl 2 (s) l Cl - (aq), is a component of instruments used to measure pH. What is the half reaction of this electrode? What is the reaction quotient? [ANSWER: Hg 2 Cl 2 (s) + 2e - 2 Hg(l) + 2 Cl - (aq); 2 Cl Q a ] 6. A AgNO 3 solution containing 0.00739 g of AgNO 3 per g of H 2 O is electrolysed between silver electrodes. During the experiment 0.078 g of Ag plate out at the cathode. At the end of the experiment that anode portion contains 23.14 g of H 2 O and 0.236 g of AgNO 3 . What is the transference number of Ag + ? [ANSWER: 0.47] 7. For the oxidation-reduction reaction S 4 O 6 2- (aq) + Cr 2+ (aq) Cr 3+ (aq) + S 2 O 3 2- (aq) The appropriate half reactions are S 4 O 6 2- + 2e- 2S 2 O 3 2- o = 0.17 V Cr 3+ + e - Cr 2+ o = -0.50 V What is the value of the equilibrium constant K at 25 o C? [ANSWER: 4.32 x 10 22 ] 8. The o cell is 0.48 V for the galvanic cell based on the reaction 2Al(s) + 3Mn 2+ (aq) 2Al 3+ (aq)+ 3Mn(s) Consider a cell in which [Mn 2+ ] = 0.50 M and [Al 3+ ] = 1.50M. Will the cell potential at 25 o C for these concentrations be greater or lesser than the o cell ? [ANSWER: o cell > cell ] 9. Can permanganate ion oxidize Fe 2+ to Fe 3+ at 25 o C under the following conditions? [Mn 2+ ] = 1 x 10 -6 M ; [MnO 4 - ] = 0.01 M; [Fe 2+ ] = 1 x 10 -3 M ; [Fe 3+ ] = 1 x 10 -6 M; pH = 4.0 Under standard conditions, the half-cell reactions and standard reduction potential are given below. Fe 3+ + e - Fe 2+ 0.77 V MnO 4 - + 8H + + 5e - Mn 2+ + 4H 2 O 1.51 V [ANSWER: YES] 10. A hydrogen electrode can, in principle, be used to monitor changes in the molar concentration of weak acids in biologically active solutions. Consider a hydrogen electrode in a solution of lactic acid (CH 3 CHOCOOH) as part of an overall galvanic cell at 25 o C and 1 bar. Estimate the change in the electrode potential when the solution is charged from 5.0 mmol/L to 25.0 mmol/L. [ANSWER: 64.2 mV]
Transcript of ChE 323 Compilation of Exam Problems-Electrochemistry 3-5-12
University of San Carlos-Chemical Engineering Department
ChE323 Problems – Electrochemistry Engr. May V. Tampus Page 1
ChE 323 Compilation of Exam Problems
Chapter 7: ELECTROCHEMISTRY
1. Sketch a typical galvanic cell. Show direction of electron flow, the direction of ion migration through the porous disk, and identify the cathode and anode. Indicate in which electrode oxidation and reduction occur. Will there be migration of ions in the absence of the porous disk? Next, sketch a galvanic cell with a salt bridge showing the required features as in a typical galvanic cell. Is there a change in the direction of ion migration when a salt bridge is present? Will the diagram change if it were an electrolytic cell? Will the direction of ion migration change? Explain your answer.
2. Discuss the relationship between ionic mobility, molar conductivity, and transport number to ionic conduction. Does electronic conduction also involve the measurement of these quantities? Explain.
3. The standard electromotive force of the cell Pt l H2(g) l HCl(aq) l AgCl(s) l Ag has been determined from 0 to 90oC by R.G. Bates and V.E. Bower, J. Res. Nat. Bur. Stand. 53:283 (1954). Their data may be represented by
0 4 6 2 9 3( ) 0.23659 4.8564 10 3.4205 10 5.869 10V t t t where t is the temperature in oC. Calculate the
reaction entropy at 25oC. (Hint:
00
r
P
S nFT
. [ANSWER: -62.297 J/mol-K]
4. One of the few industrial-scale processes that produce organic compounds electrochemically is used by the Monsanto Company to produce 1,4-dicyanobutane. The reduction reaction is
2CH2=CHCN + 2H+ + 2e- NC – (CH2)4 – CN
The NC – (CH2)4 – CN is then chemically reduced using hydrogen gas to H2N – (CH2)6 – NH2, which is used in the production of nylon. What current must be used to produce 1.0 x 10
3 kg of NC – (CH2)4 – CN per hour? [ANSWER: 5.0 x 10
5 A]
5. The calomel electrode, Hg(l) l Hg2Cl2(s) l Cl-(aq), is a component of instruments used to measure pH. What is the half reaction of
this electrode? What is the reaction quotient? [ANSWER: Hg2Cl2(s) + 2e- 2 Hg(l) + 2 Cl- (aq); 2
ClQ a ]
6. A AgNO3 solution containing 0.00739 g of AgNO3 per g of H2O is electrolysed between silver electrodes. During the experiment 0.078 g of Ag plate out at the cathode. At the end of the experiment that anode portion contains 23.14 g of H2O and 0.236 g of AgNO3. What is the transference number of Ag+? [ANSWER: 0.47]
7. For the oxidation-reduction reaction S4O6
2-(aq) + Cr2+(aq) Cr3+(aq) + S2O32-(aq)
The appropriate half reactions are
S4O62- + 2e-
2S2O32- o = 0.17 V
Cr3+
+ e- Cr
2+
o = -0.50 V
What is the value of the equilibrium constant K at 25oC? [ANSWER: 4.32 x 1022]
8. The ocell is 0.48 V for the galvanic cell based on the reaction
2Al(s) + 3Mn2+(aq) 2Al3+(aq)+ 3Mn(s) Consider a cell in which [Mn2+] = 0.50 M and [Al3+] = 1.50M. Will the cell potential at 25oC for these concentrations be greater or lesser than
the ocell? [ANSWER: o
cell > cell]
9. Can permanganate ion oxidize Fe2+
to Fe3+
at 25oC under the following conditions?
[Mn2+
] = 1 x 10-6
M ; [MnO4-] = 0.01 M; [Fe
2+] = 1 x 10
-3 M ; [Fe
3+] = 1 x 10
-6 M; pH = 4.0
Under standard conditions, the half-cell reactions and standard reduction potential are given below.
Fe3+
+ e- Fe
2+ 0.77 V
MnO4- + 8H+ + 5e- Mn2+ + 4H2O 1.51 V
[ANSWER: YES] 10. A hydrogen electrode can, in principle, be used to monitor changes in the molar concentration of weak acids in biologically
active solutions. Consider a hydrogen electrode in a solution of lactic acid (CH3CHOCOOH) as part of an overall galvanic cell at 25oC and 1 bar. Estimate the change in the electrode potential when the solution is charged from 5.0 mmol/L to 25.0 mmol/L. [ANSWER: 64.2 mV]
