CHEM 251 (Fall-2005)
Final Exam (100 pts)
Name: ------------------------------------------------------------------------------, Student Clid # -------------------LAST NAME, First
(Circle the alphabet segment of your LAST NAME): A-C
D-H
I-L
M-S
T-Z
Please answer the following questions:
Part I: Multiple Choices (30 pts: 10 @ 3 pts each). Circle the ONE best answer:
1. Consider the following two half reactions:
I2 (aq) + 2 e
2 I- (aq)
E° = + 0.54 V
Zn2+ (aq) + 2 e
Zn (s)
E° = - 0.44 V
Which of the above four species is the best oxidizing agent?
a) I2
b) I-
c) Zn2+
d) Zn
2. The best explanation for the fact that nitrogen forms only NF3 with fluorine while phosphorus
forms PF3, and PF5 is that
a) nitrogen is more electronegative than sulfur.
b) nitrogen is smaller than phosphorus.
c) phosphorus has d orbitals available for bonding
d) only nitrogen forms hydrogen bonds
3. Which of the following statements is false?
a) IF6- possesses a distorted octahedral structure.
b) HClO is a weak acid in aqueous solution.
c) ClO2 is used to bleach flour and wood pulp and for water treatment.
d) I3- possesses an angular structure.
4. Which of the following ions contribute greatly to the hardness of water?
a) NO3b) Ca2+
c) Cu2+
5.
6.
d) Al3+
Consider the species Cl2SO, XeOF4 and F5IO2-, what is the hybridization of the central atom,
respectively?
a) sp2, sp3d, and sp3d2
b) sp3d, sp3d2, sp3d3
c) sp3, sp3d2, and sp3d3
d) sp3, sp3d, and sp3d2
In the above problem, what is the molecular shape of the species, respectively?
a) trigonal planar, trigonal bipyramid, and octahedral
b) trigonal pyramid, trigonal bipyramid, and distorted octahedral
c) T-shaped, square pyramid, and octahedral.
d) trigonal pyramidal, square pyramid, and distorted octahedral.
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7. What is the proper order of decreasing F-X-F angle in the species below?
XeF2 (A)
a)
b)
c)
d)
ClF2+ (C)
BF3 (B)
ClF3 (D)
A>B>C>D
D>C>B>A
A>B>C>D
B>C>A>D
8. Which one of the following statements is incorrect?
a) H2S is a toxic gas.
b) The most useful production of Mg involves the electrolysis of molten MgCl2.
c) The B-H-B bond in diborane, B2H6, consists of four electrons shared among three atoms.
d) The Ca2+ salt of SiO44- is the component of cement.
9. Which of the following ions are more likely to form insoluble salts with OH- ion?
Mg2+
10.
Ag+
Fe3+
a) only Fe3+
b) only Ag+
c) Mg2+ and Fe3+
d) only F-
F-
The unit cell of the rulite lattice is illustrated to the right.
The compound is composed of titanium and oxygen. Based
on the unit cell shown here (Ti ions are located at the corners,
and in the body center of the cube and oxide ions are located
as follows: two in opposite quadrants of the bottom face, two
at directly above the first two in the top face and the final two
ions are in the plane of with the body centered titanium ion).
What is the formula of rulite?
a) Ti2O4
b) Ti2O3
c) TiO2
d) TiO
Part II. Reactions & Chemistry of the Main Group elements (40 pts)
1.
Reactions (10 pts: 5 @ 2 pts each) Predict the composition of the product(s), its state and balance
the reactions:
S8 (s)
Heat
a)
Ga (s) +
b)
OF2 (s) + H2O (l)
c)
Ca (s) +
d)
SO2 (g) + H2O (l)
e)
PBr3 (l) + H2O (l)
P4 (s)
Heat
Heat
2
2. (30 pts: 6 @ 5 pts each): Answer ONLY 6 of the following:
a) Explain why BF3 is a weaker Lewis acid than BCl3?
b) Suggest an explanation for the fact that the mobilities of the alkaline metal ions in aqueous
solution follow the sequence: Li+ < Na+ < K+ < Rb+ < Cs+.
c) Briefly discuss the extraction of aluminum from its ores (for full credit, chemical reactions must be
included in your answer).
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d) Write a brief account of the bonding and reactivity of Borazine, N3B3H6 which emphasizes the
ways in which this compound is similar or dissimilar to benzene, C6H6.
e) Stability constants for the formation of [M(18-crown-6)]+ complexes in acetone are given below.
Comment critically on these data.
Li+
Na+
K+
Rb+
Cs+
M+:
log K:
1.5
4.6
6.0
5.2
4.6
f) Write a brief account of the chemistry of the xenon fluorides and their decomposition reactions by
water.
g) Write down, in order, the names and symbols of the elements in group 14. Classify them in terms
of metallic, semi-metallic or non-metallic and then give the general notation showing the ground
state electronic configuration of each element.
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Part III: Bonding (15 pts: 3 @ 5 pts each)
a) The azide ion, N3- is linear with equal N-N bond distances whereas the N-N bond distances in HN3
are different; the terminal N-N bond (113 pm) is shorter than the central N-N distance (124 pm).
Explain this observation.
b) Suggest a likely structure for the dimmer of BeCl2, present in the vapor phase below 1020 K.
What is the appropriate hybridization scheme for the Be center? When BeCl2 dissolves in diethyl
ether to form monomeric BeCl2.2Et2O; suggest a structure for each of the two compounds and
give a description of the bonding.
c) Construct an MO diagram for the formation of O2; show only the participation of the valence
orbitals of the oxygen atoms. (b) Use the diagram to rationalize the following trend in O-O bond
distances: O2, 121 pm; O2+, 112 pm; O2-, 134 pm; O22-, 149 pm. (c) Which of these species are
paramagnetic?
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Part III (20 pts) Show all work for full credit. Please express all answers with proper units and
correct number of significant figures.
1. (7 pts) Given that E° for:
VO2+ (aq) + 2 H+ (aq) + e
V3+ (aq) + H2O (l)
is + 0.34 V, calculate the reduction potential, E in a solution when pH = 2.2 and the ratio of the
concentrations of [VO2+]:[V3+] = 1:2. Comment on this result.
[H+] = 6.31 x 10-3
E = + 0.06 V
2. (6 pts) Use the following data to
a) estimate the bond dissociation enthalpy of ClF: D(F-F) = 158 kJ/mol; D(Cl-Cl) = 242 kJ/mol;
χP (F) = 4.0; χP (Cl) = 3.2.
b) the charge distribution in ClF if the dipole moment is 0.86 D and the bond length is 163 pm.
Which atom carries a negative charge?
.
??? kJ/mol
0.11
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3. (7 pts) Construct Born-Haber cycle for MgO. Use the following thermochemical data to calculate
the lattice energy of MgO: standard enthalpy of formation at 298 K (∆fH°) of MgO = -602 kJ/mol,
∆aH° (Mg, s) = 148 kJ/mol, IE1 (Mg,g) = 738 kJ/mol, IE2 (Mg,g) = 1451 kJ/mol, D(O2, g) = 498
kJ/mol, ∆EAH1 (O, g) = -141 kJ/mol, and ∆EAH2 (O, g) = 880 kJ/mol.
-3930 kJ/mol (3927)
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PREFERENCE SHEET FOR
CHEM 251
Final Exam – Fall 2005
You will have 140 minutes to complete this exam.
The exam has 7 pages plus Periodic Table and Reference page.
When you are told to do so, tear off the Periodic Table cover sheet and use as required during
exam.
Useful Information:
1.0 eV ≈ 96.5 kJ/mol & J = CV
1 D = 3.336 x 10-30 Cm
R = 8.314 x 10-3 kJ/mol
1pm = 10-12 m
F = 96,485 C/mol
e = 1.60 x 10-19 C
Useful Equations:
∆G° = - RT ln K = - zFE°
Nernst Eqn: E = E° - (RT/zF) ln [reduced form]/[oxidized form]
√∆D = χP (F) – χP (Cl)
µ=qxexd
1
1
2
H
Periodic Table of the Elements
He
1.01
3
2
3
4
5
6
4.00
4
5
6
7
8
9
10
Li Be
B
C
N
O
F
Ne
6.94
9.01
10.81 12.01 14.01 16.00 19.00 20.18
11
12
13
14
15
16
17
Na Mg
Al Si
P
S
Cl Ar
22.99 24.30
26.98 28.08 30.97 32.06 35.45 39.95
21
19
20
K
Ca Sc Ti V
39.1
40.08 44.96 47.88 50.94 52.00 54.94 55.85 58.93 58.69 63.55 65.38 69.72 72.59 74.92 78.96 79.90 83.80
37
38
39
Rb Sr Y
22
40
23
41
24
25
26
27
28
29
30
31
32
33
34
18
35
36
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
42
43
44
45
46
47
48
49
50
51
52
53
Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I
54
Xe
85.47 87.62 88.91 91.22 92.91 95.94 (98)
101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.1
55
76
56
57
72
73
74
75
77
Cs Ba La Hf Ta W Re Os Ir
78
79
80
81
82
83
84
85
86
Pt Au Hg Tl Pb Bi Po At Rn
132.9 137.3 138.9 178.5 181.0 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222)
87
7
88
89
104 105 106 107 108 109 110 111 112
Fr Ra Ac Rf Db Sg Bh Hs Mt Uun Uuu Uub
(223) 226.0 227.0 (261) (262) (263) (262) (265) (266) (269) (272) (277)
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
140.1 140.9 144.2 (145) 150.4 152.0 157.2 158.9 162.5 164.9 167.3 168.9 173.0 175.0
90
91
92
Th Pa U
93
94
95
96
97
98
99
100 101 102 103
Np Pu Am Cm Bk Cf Es Fm Md No Lr
232.0 231.0 238.0 237.0 (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)
8
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