1
Chemistry 64
Winter 1994
NAME: ____________________
FIRST EXAMINATION
•
THIS EXAMINATION IS WORTH 100 POINTS
AND CONTAINS 4 (FOUR) QUESTIONS
•
THEY ARE NOT EQUALLY WEIGHTED!
• YOU SHOULD ATTEMPT ALL QUESTIONS
AND ALLOCATE YOUR TIME ACCORDINGLY
• ALL BOOKS AND PAPERS OTHER THAN TABLES
WHICH HAVE BEEN HANDED OUT SHOULD BE PLACED
ON THE FLOOR
DURING THIS EXAMINATION
•
IF YOU DO WORK ANYWHERE OTHER THAN THE SPACE PROVIDED
FOR EACH QUESTION, INDICATE CLEARLY WHERE IT IS LOCATED
2
FOR GRADING USE ONLY
Question 1 (36 pts)........................__________
Question 2 (18 pts)........................__________
Question 3 (16 pts).........................__________
Question 4 (30 pts)........................__________
TOTAL (100 pts)......................===========
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Question 1
a)(10 pts)
(36 pts)
• State the point group to which the molecule belongs. In some
cases you will need to draw the ideal structure, based o n
VSEPR predictions.
(IN EACH CASE THE CENTRAL ATOM IS LISTED FIRST. ALL OTHER ATOMS ARE
BONDED DIRECTLY TO THE CENTRAL ATOM.)
TeF5–
Cl
ICl4–
H
C C
H
Cl
IF7 (pentagonal bipyramid)
IO2F2–
ICl2+
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b) (6 pts)
i. Heating a mixture of B 2S 3 and sulfur to 570 K in a sealed tube under
vacuum gives crystals of the planar molecule B8S16, whose structure is
shown below.
To which point group does this molecule belong?
ii. The compound N 3S 3Cl3 (yellow needles) can be made from S 4N 4 and
Cl2. Its structure is shown below and features a planar 6-membered ring with
equal S-N distances; the S-Cl bonds are also equivalent.
To which point group does this molecule belong?
iii. Oxidation of Te with AsF5 in AsF3 solvent yields brown crystals
containing the cation [Te6]4+, whose trigonal prismatic structure is shown
below.
To which point group does this molecule belong?
= Te
5
f) (6 pts)
i. Write the symbols for the following elements: Rhenium, Tungsten,
Antimony, Astatine
ii. [X]3+ has the electron configuration [Xe]. Which element is X?
iii. Give the symbol for one element which is a liquid at room temperature.
iv. Write the name of the element whose atomic number is one less than
that of tin.
v. Give the name of the element which is the least electronegative halogen.
g) (2 pts) Define the terms asymmetric and dissymmetric.
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(h) (6 pts) Chiral molecules can occur in only a few point groups. List 3 of these
groups, and sketch an example of a molecule for each group. DO NOT use C as the
central atom.
(i) (6 pts) Molecules having dipole moments can occur in only a few point groups. List
3 of these point groups, and sketch an example of a molecule for each group.
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Question 2 (18 pts)
a) (3 pts)
Predict whether the Xe-F bond length in XeF4 will be longer, shorter, or
the same length as the I-F one in IF4–. Explain your answer.
b) (3 pts) Explain why the first ionization energy of C is less than that of N.
c) (3 pts) Predict the relative sizes of the following atoms and ions: N3–, Na+, Ne, O2–.
Explain your answer.
d) (3 pts) Predict the structure of H2F+. Explain your answer.
e) (3 pts) Explain why ethylene is a common molecule, but the analogous H 2 Si=SiH 2
cannot be isolated.
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f) (3 pts)
Draw a cartoon picture showing a π-bonding interaction between a p orbital
and a d-orbital. (Your choice of orbitals, but say which ones you use). Be
sure to label the axes and indicate the nodes.
Question 3 (16 pts)
An MO energy level diagram for CO is shown below. Use it to answer the
following questions, assuming the diagram is also applicable to other
heteronuclear diatomic molecules.
4σ
2π
2p
3σ
2p
2s
1π
2σ
2s
1σ
C
CO
O
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a) (4 pts) Make a cartoon sketch of the HOMO of CO.
c) (4 pts) The first ionization energy of CO is less than that of the isoelectronic N2.
Rationalize this experimental observation. [Hint, see question 2, part b]
d) (4 pts)
Explain why the bond dissociation energy in CF is less than that in CF+.
e) (4 pts) Explain why ionization of NO to NO+ occurs readily.
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Question 4 (30 pts)
A molecular orbital energy level scheme for homonuclear diatomic
molecules is given below.
1 σ g < 2σ
σ u < 1π
π u < 3σ
σ g < 2π
π g < 4σ
σu
Use this information to rationalize the following experimental observations:
a) (3 pts) The bond in Cl2 is longer and weaker than the bond in Cl2+. [Assume that
F2 is a good model for Cl2.]
b) (3 pts) The electron affinity of O2 is much smaller than that of C2.
c) (3 pts) The molecule Be2 has not been observed experimentally.
d) (3 pts)
The internuclear distance in O2 decreases when an electron is removed,
but the internuclear distance in N2 increases when an electron is removed.
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A correlation diagram for AH3 molecules (planar or pyramidal) is shown below.
AH3 Planar/Pyramidal
Correlation Diagram
H
A
H
A
z
H
H
x
2e'(*)
2e(*)
3a1(*)
2a1'(*)
1a2''
2a1
1e
1e'
1a1'
1a1
H
H
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(e) (3 pts) Using this diagram and Walsh's rules, predict the structure of the first excited
state of NH3.
(f) (3 pts) Discuss the relevance of your answer in part (e) to the dynamic process
occurring in ammonia.
(g) (3 pts) Predict the structure of CH3– and make a cartoon sketch of the HOMO of this
molecule. Is your description consistent with the behavior of this anion in
organic reactions? Explain your answer.
(h) (4 pts) Predict the structure of LiH3.
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(i) (5 pts) Another correlation diagram for AH3 molecules is shown below. It includes
another possible geometry, T-shaped (C2v).
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Using this diagram and Walsh's rules, predict which AH3 molecules will adopt the C 2v
geometry. [Suppose A can only be one of the first row elements from Li to Ne].
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DETACHABLE, HANDY–DANDY
PERIODIC TABLE OF THE ELEMENTS
H
1
He
1.0079
Li
3
4.00260
Be
4
Na
K
39.0983
37
Ca
40.08
38
Rb
Sr
85.4678
87.62
55
Cs
132.905
87
Fr
(223)
10.81
13
Mg
24.305
20
56
Ba
21
Sc
44.9559
39
Y
Ra
22
23
V
Cr
47.88
50.9415
51.996
40
Zr
41
Nb
91.224
92.9064
57
72
73
89
Ac
226.025 227.028
25
26
Mn
Fe
54.9380
55.847
42
43
Mo
88.9059
La
24
Ti
95.94
74
Ru
(98)
101.07
75
58.9332
44
Tc
27
Co
45
Rh
76
28
29
6
C
N
7
O
8
9
F
Si
P
S
Cl
32.06
34
26.9815 28.0855 30.9738
30
31
32
33
Cu
Zn
Ga
Ge
As
Se
Br
58.69
63.546
65.39
69.72
72.59
74.9216
78.96
79.904
47
48
49
50
Pd
Ag
Cd
In
Sn
102.906
106.42
107.868
112.41
114.82
118.71
77
78
79
80
81
51
52
Sb
82
Ar
35.453 39.948
35
36
Ni
46
10
Ne
12.011 14.0067 15.9994 18.9984 20.179
14
15
16
17
18
Al
137.33 138.906
88
5
B
6.941 9.01218
11
12
22.9898
19
2
Kr
83.80
53
Te
I
121.75
127.60
126.905
83
84
85
54
Xe
131.29
86
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
178.49
180.948
183.85
186.207
190.2
192.22
195.08
196.967
200.59
204.383
207.2
208.980
(209)
(210)
(222)
104
105
106
107
108
109
Une
Unq
Unp Unh
Uns Uno
(261)
(262)
(262)
58
Ce
(263)
59
Pr
140.12 140.908
90
91
Th
Pa
60
(265)
(266)
61
62
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
144.24
92
(145)
93
150.36
94
151.96
95
157.25
96
158.925
97
162.50
98
164.930
99
167.26
100
168.934
101
173.04
102
174.967
103
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
(237)
(244)
(243)
(247)
(247)
(251)
(252)
(257)
(258)
(259)
(260)
232.038 231.036 238.029
63
64
65
66
67
68
69
70
71
16
17
18
19
20
21
22
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