IST / Departamento de Engenharia Química
Electrochemistry and Energy / 2012-2013, 2nd semester
Prof: Alda Simões June 28 2013, 18:30
Student’s name
IST number
Part 1 To be filled and delivered to at the end of the exam.
Multiple choice questions:
For each of these questions there is only one correct answer. Make a circle around the right answer. Try to fully make
your choice before you write it down.
1.
In the reaction
+
2+
2HNO3 (aq) + Cu (s) + 2H (aq)
2NO2 (g) + Cu (aq) + 2H2O (l)
Which of the following statements is correct?
a. HNO3 is the reducing agent and Cu is the oxidizing agent.
+
b. HNO3 is the reducing agent and H is the oxidizing agent.
+
c. Cu is the reducing agent and H is the oxidizing agent.
d. Cu is the reducing agent and HNO3 is the oxidizing agent.
+
e. H is the reducing agent and Cu is the oxidizing agent.
2.
In Fe2(SO4)3 the oxidation numbers of Fe, S and O are
a. +2, +3, -4
b. +3, +6, -2
c. +2, +4, -8
d. +2, +4, -2
e. +4, +5, -3
3.
An electrochemical cell consists of a left compartment with a zinc electrode in contact with 1.0 mol/L Zn(NO 3)2 (aq) and
a right compartment with a silver electrode in contact with 1.0 mol/L AgNO3 (aq). The standard reduction potentials are:
+
o
Ag + e
Ag
E = 0.80 V
2+
o
Zn + 2e
Zn E = -0.76 V
When this cell is allowed to operate at 25ºC, which of the following statements is true?
a. Electrons will flow from right to left through the wire.
2+
b. Zn ions will be reduced to Zn metal.
+
c. The concentration of Ag ions in the right compartment will increase.
d. The silver electrode will be the cathode.
e. The standard cell potential for this cell will be 0.04 V.
4.
Creating an output voltage of 21V, will require a combination of how many inline cells in which wiring configuration?
a. 30 fuel cells, wired in parallel.
b. 3 fuel cells, wired in parallel.
c. 30 fuel cells, wired in series.
d. 3 fuel cells, wired in series.
e. 10 fuel cells wired in series.
5.
Hydrogen gas burns with a ______ flame, when ignited.
a. Orange-red
b. Colourless
c. Blue
d. Light yellow
e. Green
6.
Given the following standard reduction potentials,
2+
o
Zn + 2e
Zn
E = -0.76 V
2+
o
Fe + 2e
Fe
E = -0.44 V
2+
o
Pb + 2e
Pb
E = -0.13 V
2+
o
Ni + 2e
Ni
E = -0.24 V
which reactions are expected to occur in the beakers below?
a.
b.
c.
d.
e.
I.
Fe (s) + Zn
II.
Fe
III.
Fe
I only
I and II
I and III
II and III
III only
2+
(aq)
2+
(s) + Pb (aq)
2+
(s) + Ni (aq)
Fe
2+
(aq) + Zn (s)
2+
Fe (aq) + Pb (s)
2+
Fe (aq) + Ni (s)
Fe
Fe
Fe
Pb(NO3)2
Ni(NO3)2
Zn(NO3)2
7.
An electrochemical cell is set up with a copper electrode in contact with1.0 mol/L CuSO4 (aq) and a lead electrode in
contact with 1.0 mol/L Pb(NO3)2 (aq). The standard reduction potentials are
2+
o
Pb + 2e
Pb
E = -0.13 V
2+
o
Cu + 2e
Cu
E = +0.34 V
At 25ºC, the standard potential for this cell in volts is
a. 0.13
b. 0.34
c. 0.47
d. 0.21
e. 0.94
8.
A hydrogen fuel cell consumes fuel and oxygen and produces all of the following, except______.
a. Water
b. Electricity
c. Heat
d. Carbon dioxide.
e. None of the above.
9.
The internal combustion engine, the fuel cell, and the battery, all have which of the following in common?
a. They all convert chemical energy into electrical or mechanical energy.
b. They all convert thermal energy into electrical or mechanical energy
c. They all convert chemical energy into electrical energy
d. They all convert electrical energy into mechanical energy
e. None of the above
10. Which of the following is an advantage of a direct methanol fuel cell in comparison with a hydrogen fuel cell?
a. A methanol system produces less direct emissions to the air than a hydrogen system.
b. A methanol system does not need a bulky hydrogen storage system or a reforming subsystem.
c. Less of the expensive platinum catalyst is needed.
d. Methanol fuel crosses through the membrane from the anode to the cathode.
11. The reaction at the cathode of a fuel cell always_______.
a. Consumes electrons in the reaction
b. Produces water
c. Reacts H2 with OH
d. Produces excess electrons.
Instituto Superior Técnico / DEQ - Chemical Engineering Department
Electrochemistry and Energy / 2012-2013, 2nd semester
Prof: Alda Simões
June 28, 2013, 18:30
Part 2
These questions are to be answered in a separate examination form.
I.
What is a fuel cell?
II.
A PE fuel cell system with an active area of 200 cm2 is being operated with hydrogen and air.
Calculate the necessary volume fluxes of air (21 vol % O2) and hydrogen, if the stack is operated at a load of
900mA/cm2. Reactants are considered as ideal gases and the utilization ratios are 40% for oxygen and 60%
for hydrogen.
The measured voltage of the single cell under these conditions is 0.65 V. Calculate the power output of the
system at this operating point.
a.
b.
III.
Balance the following redox reaction and identify the oxidation and the reduction reactions (assume acidic
aqueous medium):
Br2 + Cr3+ Cr2O72- + Br-
IV.
The overall reaction of a zinc-nickel oxide cell is:
Zn + 2 NiO(OH) + 2H2O Zn(OH)2 + 2Ni(OH)2
Calculate the standard potential of the cell using the values of the standard free energies of formation.
If the battery has a total weight of zinc of 10 g and is being recharged at a current of 4.5 A, how long will it
take to fully recharge a totally exhausted battery?
a.
b.
V.
Explain why cars are made using galvanized body parts. Refer to the E-pH diagrams in annex.
VI.
Zinc metal is corroding in de-aerated water at pH=3 at 25ºC. Estimate the corrosion rate in A cm-2,
assuming activation control.
Take: ba= 110 mV/decade; bc = 60 mV/decade; i0,a= 10-8 A cm-2 ; i0,c= 10-6 A cm-2 ; E0 Zn2+/Zn= -0.76 V
Data:
the molar concentration of oxygen in air is 21%.
R= 8.314 J K-1 mol-1
; 1 cal= 4.184 J
Questions 1-9: 0.8 each (-0.2 if wrong)
Questions 10-11: 0.75 each (-0.25 if wrong)
Part 2: I: 0.8; II: 3.0; III: 1.5; IV: 2.0; V: 1.0; VI: 3.0
Parts 1 and 2 are given to the student at the beginning of the exam. Calculators are allowed only after Part 1 is
finished and given to the supervisor.
Other material allowed: Periodic Table
Compound
NiO(OH)
Ni(OH)2
Zn(OH)2
H2O
Atomic weight of Zn: 65.37
Density of zinc metal: 7.14 g/cm3
∆G0f / kJ mol-1
-315
-453
-555
-237