Download Midterm 2 (Spring `06)

1. [7 points] Which substance is the reducing agent in the following reaction?
Fe2S3 + 12HNO3  2Fe(NO3)3 + 3S + 6NO2 + 6H2O
a.
b.
c.
d.
e.
HNO3
S
NO2
Fe2S3
H2O
2. [7 points] What is the oxidation number of manganese in potassium
permanganate, KMnO4?
(a) +1
(b) +2
(c) +5
(d) +4
(e) +7
3. [7 points] What is the coefficient of Fe3+, when the following redox equation
is balanced in basic solution?
a.
b.
c.
d.
e.
1
2
3
4
5
CN- + Fe3+  CNO- + Fe2+
4. [7 points] What are the coefficients of H2S and H2O, when the following
redox equation is balanced in acidic solution?
a.
b.
c.
d.
e.
5, 8
5, 10
2, 4
2, 6
3, 8
MnO4- + H2S  S + Mn2+ + H2O
5. [7 points] In a voltaic cell, electrons flow from ________________ .
a.
b.
c.
d.
e.
The salt bridge to the anode
The anode to the salt bridge
The cathode to the anode
The salt bridge to the cathode
The anode to the cathode
6. [7 points] Which one of the following substances would you expect to be the
best oxidizing agent?
a.
b.
c.
d.
e.
H2
Na
Cl2
Li
Ca
7. [7 points] Which of the following reactions will occur spontaneously as
written (standard reduction potentials are given on the back page)?
a. Sn4+ (aq) + Fe3+ (aq)  Sn2+ (aq) + Fe2+ (aq)
b. 3Fe (s) + 2Cr3+ (aq)  2Cr (s) + 3Fe2+ (aq)
c. Sn4+ (aq) + Fe2+ (aq)  Sn2+ (aq) + Fe (s)
d. 3Sn4+ (aq) + 2Cr (s)  2Cr3+ (aq) + 3Sn2+(aq)
e. 3Fe2+ (aq)  Fe (s) + 2Fe3+ (aq)
8. [7 points] Consider a voltaic cell where one half of the cell contains ZnCl2
(aq) in contact with a Zn electrode, while the other half of the cell contains
FeCl2 (aq) in contact with a Fe electrode. Which electrode is the cathode
and what is the standard cell potential, Eºcell ?
a. Cathode = Zinc, Eºcell = +0.32 V
b. Cathode = Iron, Eºcell = +0.32 V
c. Cathode = Zinc, Eºcell = +1.20 V
d. Cathode = Iron, Eºcell = +1.20 V
e. Cathode = Zinc, Eºcell = -1.20 V
9. [7 points] In the above problem what would be the cell potential if the
temperature were 298 K, the FeCl2 concentration was 0.50 M, and the ZnCl2
concentration was 0.0025 M?
a.
b.
c.
d.
e.
+0.39 V
+0.25 V
+0.46 V
+1.27 V
+1.34 V
10. [7 points] Consider a voltaic cell based on the following reactions:
2Fe (s) + O2 (g) + 4H+ (aq)  2Fe2+ (aq) + 2H2O (l)
Which of the following actions would increase the measured cell potential?
a.
b.
c.
d.
e.
Increasing the size of the iron anode
Adding FeCl2 to the anode compartment
Adding NaOH to the cathode compartment (neutralization rxn)
Adding NaOH to the anode compartment (precipitation rxn)
None of the above actions will increase the cell potential
11. [7 points] The standard cell potential (Eºcell) of the reaction below is +0.126
V. What is the value of Gº for this reaction?
Pb (s) + 2H+ (aq)  Pb2+ (aq) + H2 (g)
a.
b.
c.
d.
e.
–24 kJ
+24 kJ
–12 kJ
+12 kJ
–50 kJ
12. [7 points] One of the differences between a voltaic cell and an electrolytic
cell is that in an electrolytic cell _________ .
a.
b.
c.
d.
e.
an electric current is produced by a chemical reaction
electrons flow toward the anode
a non-spontaneous reaction is forced to occur
O2 gas is produced at the cathode
oxidation occurs at the cathode
13. [7 points] How many grams of copper (atomic weight = 63.55 g/mol) 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.
b.
c.
d.
e.
1.8  10-2 g
2.2 g
1.1 g
3.6  10-2 g
1.8  10-3 g
14. [7 points] Calculate the equilibrium constant at 25 ºC for the following
reaction:
Sn4+ (aq) + Sn (s)  2 Sn2+ (aq)
a.
b.
c.
d.
e.
2.2
7.9  104
6.3  109
1.6  10-10
None of the above
15. [7 points] Construct a molecular orbital diagram for the peroxide ion, O22-.
Based on this diagram what is the oxygen-oxygen bond order and the
magnetic behavior of peroxide?
a.
b.
c.
d.
e.
Bond order
Bond order
Bond order
Bond order
Bond order
= 2, magnetism = diamagnetic
= 2, magnetism = paramagnetic
= 1, magnetism = diamagnetic
= 1, magnetism = paramagnetic
= 1.5, magnetism = paramagnetic
16. [7 points] Which of the following statements is false?
a. The number of molecular orbitals in a molecule is equal to the number
of atomic orbitals in the constituent atoms
b. A molecular orbital can hold 2 electrons,
c. The splitting between bonding and antibonding molecular orbitals
increases as the overlap increases
d. Sigma bonds have a nodal plane intersecting the internuclear axis
e. It’s not possible to form a pi bond through the overlap of two s
atomic orbitals
17. [7 points] Which of the following MO’s is representative of an eg molecular
orbital in a ML6 octahedral transition metal complex?
(a)
(b)
(c)
(d)
18. [7 points] Based on the electron configuration which one of the following
complexes is most likely to be colorless?
a.
b.
c.
d.
e.
[Cu(NH3)4]2+
[Fe(H2O)6]3+
[CrCl6]3[Zn(OH)4]2[Ti(H2O)6]3+
19. [7 points] Which complex ion is most likely to absorb light in the red region
of the visible spectrum?
a.
b.
c.
d.
[CuCl4]2[Cu(H2O)4]2+
[Cu(NH3)4]2+
[Cu(CN)4]2-
20. [7 points] What transition metal ion is responsible for the red color of a
ruby?
a.
b.
c.
d.
e.
Mn2+
Cr6+
Fe2+
Ti3+
Cr3+
21. [7 points] How many unpaired electrons are there on a [Co(CN)6]3- complex
ion?
a.
b.
c.
d.
e.
0
2
4
5
6
22. [7 points] What structure can be described as a hexagonal close packing of
anions with 50% of the tetrahedral holes filled?
a.
b.
c.
d.
e.
Rock salt
Nickel Arsenide
Zinc Blende
Wurtzite
Cesium Chloride
23. [7 points] Nickel sulfide adopts the Nickel Arsenide structure type? What
is the coordination environment of nickel and of sulfur.
a.
b.
c.
d.
e.
Ni  octahedron, S  octahedron
Ni  octahedron, S  trigonal prism
Ni  tetrahedron, S  tetrahedron
Ni  tetrahedron, S  octahedron
Ni  octahedron, S  tetrahedron
24. [7 points] The mineral oldhamite (CaS) adopts the rock salt structure type
with a unit cell edge of 5.689 Å? What is the Ca-S bond distance?
a.
b.
c.
d.
e.
2.84 Å
2.46 Å
4.39 Å
5.69 Å
2.56 Å
25. [7 points] The strongest peak in the X-ray diffraction pattern of cubic ZnS,
which adopts the zinc blende structure type, is the (111) peak. If the
diffraction pattern was collected with Cu K radiation (wavelength = 1.541
Å), and the 2-theta location of the (111) peak is 28.56º, what is the length of
the unit cell edge, a ?
a.
b.
c.
d.
e.
5.65 Å
3.12 Å
2.79 Å
5.41 Å
None of the above
Standard Reduction Potentials
Half Reaction
Eº (V)
Cr3+ (aq) + 3e-  Cr (s)
-0.74
Ag+ (aq) + e-  Ag (s)
+0.80
Fe2+ (aq) + 2e-  Fe (s)
-0.44
Fe3+ (aq) + e-  Fe2+ (s)
+0.77
Cu2+ (aq) + 2e-  Cu (s)
+0.34
Zn2+ (aq) + 2e-  Zn (s)
-0.76
Pb2+ (aq) + 2e-  Pb (s)
-0.13
Sn4+ (aq) + 2e-  Sn2+ (aq)
+0.15
Sn2+ (aq) + 2e-  Sn (s)
-0.14
2H+ (aq) + 2e-  H2 (g)
+0.00
BiO+ (aq) + 2H+ (aq) + 3 e-  Bi (s) + H2O
+0.32
Spectrochemical Series
Weak Field
Ligands
Cl- < F- < H2O < NH3 < NO2- < CN-
Physical Constants & Conversion Factors
h = 6.626  10-34 J-s
c = 2.998  108 m/s
R = 8.314 J/K-mol
1 Å = 10 pm = 1  10-10 m
1 eV = 1.602  10-19 J
1 F = 96,500 C/mol = 96,500 J/V-mol
1 A = 1 C/s
1 eV = 1.602  10-19 J
Strong Field
Ligands