Chem 301
Spring 2004
Exam 2
March 24, 2004
Name
KEY
This exam has 6 questions on 6 pages.
1.
(25 pts.) Draw the best Lewis structures for the following species. Show all valence electrons
and formal charges for all atoms. Provide the information requested.
a.
SnCl3–
molecular shape trigonal pyramid
hybridization of central atom sp
b.
26e–
0
Sn
Cl
Cl
Cl
0
0
3
NO2–
molecular shape bent
-
18e–
0
0
0
O
N
O
-
hybridization of central atom sp2
c.
SeOCl2
molecular shape trigonal pyramid
26e–
O
0
hybridization of central atom sp3
d.
ClF3
molecular shape T-shape
28e–
e.
PO43–
molecular shape Td
hybridization of central atom sp3
Se
Cl
F
0
F
32e–
Cl
O
O
P
O
0
0
F
0
hybridization of central atom dsp2
Cl
-
O
0
-
O
N
O
0
2
2.
(8 pts.) Rank in order of increasing bond strength (weakest on the left, strongest on the right).
a.
N–H
P–H
As–H
As–H < P–H < N–H
b.
N–NH2
P–NH2
As–NH2
As–NH2 < N–NH2 < P–NH2
3.
(15 pts.) Give the point groups for the following:
a.
XeF4
D4h
N
N
b.
C3h
____________
N
c.
HCN
C∞v
H
d.
C
C
C
H
Cl
H
H
D2d
____________
Cl
C
e.
H
C
Cl
Cs
____________
3
4.
(27 pts.) Follow the steps below to derive the M.O. diagram for SH4.
a.
(12 pts.) Show the symmetry adapted linear combinations (SALCs) of the hydrogen
orbitals that would overlap with the sulfur orbitals. Use the axes and atom labels shown.
Show both pictorial and mathematical forms. Give the correct symmetry labels for all
SALCs. Key Hint: The four hydrogens are not all symmetry equivalent.
z
H4
x
b.
H1
S
H3
H2
y
H3 + H4
a1
H3 - H4
b2
H1 + H2
a1
H1 - H2
b1
(8 pts.) Draw the M.O. diagram. Show pictorial forms of all MOs. Use correct symmetry
labels. Specify the bonding, antibonding, and nonbonding orbitals.
4a1
4a1
3a1
2b2
a1, b1, b2
3a1
antibonding
2b1
2b2
3p
2b1
3s (a1)
S
b1
b2
a1
a1
1b1
1b1
H
1b2
1b2
bonding
2a1
2a1
1a1
1a1
c.
(4 pts.) Show the electron occupancy in your M.O. diagram. Is the molecule paramagnetic
or diamagnetic?
diamagnetic
d.
(3 pts.) Briefly comment on what the MO diagram reveals about the average strength of the
S-H bonds. What is the approximate S-H bond order?
Four bonding and one antibonding orbitals are filled. Thus, the average bond order is 0.75
and the S–H bonds do not have full single-bond strength.
4
5.
(15 pts.) Indicate if the following species are chiral or achiral.
H
CN
a.
b.
H3C
O
H
Cl
achiral
H
d.
Cl
H3N Os CO
H3N
NH3
H2N
Cl
e.
P
F
H
H
chiral
achiral
NH2
Cl
Hf Cl
Cl
achiral
chiral
6.
c. H
(10 pts.) The H-bonding between NH4+ and NH3, [H3N⋯H⋯NH3]+ can be readily explained by
MO Theory. Consider the lone pairs on each ammonia to be sp3 hybrids. With these two
orbitals and the hydrogen orbital, derive the MO energy level diagram for the N⋯H⋯N
interaction. Show the electron occupation of this MO diagram. Draw pictorial representations
of all molecular orbitals.
•
antibonding
•
nonbonding
•
bonding
5
Data Pages
Periodic Table of the Elements
8A
18
1A
1
1
H
1.01
3
Li
6.94
11
Na
23.0
19
K
39.1
37
Rb
85.5
55
Cs
133
87
Fr
(223)
2A
2
3A
13
4A
14
5A
15
6A
16
7A
17
2
He
4.00
4
Be
9.01
12
Mg
24.3
20
Ca
40.1
38
Sr
87.6
56
Ba
137
88
Ra
226
5
B
10.8
6
C
12.0
8
O
16.0
16
S
32.1
34
Se
79.0
9
F
19.0
14
Si
28.1
32
Ge
72.6
50
Sn
119
82
Pb
207
7
N
14.0
15
P
31.0
33
As
74.9
51
Sb
122
83
Bi
209
17
Cl
35.4
35
Br
79.9
10
Ne
20.2
18
Ar
39.9
36
Kr
83.8
52
Te
128
84
Po
(209)
53
I
127
85
At
(210)
54
Xe
131
86
Rn
(222)
67
Ho
165
99
Es
(252)
68
Er
167
100
Fm
(257)
69
Tm
169
101
Md
(258)
70
Yb
173
102
No
(259)
71
Lu
175
103
Lr
(260)
8B
3B
3
4B
4
5B
5
6B
6
7B
7
21
Sc
45.0
39
Y
88.9
57
La
139
89
Ac
227
22
Ti
47.9
40
Zr
91.2
72
Hf
178
104
Rf
(261)
23
V
50.9
41
Nb
92.9
73
Ta
181
105
Ha
(262)
24
Cr
52.0
42
Mo
95.9
74
W
184
106
Unh
(263)
25
Mn
54.9
43
Tc
(98)
75
Re
186
107
Uns
(262)
58
Ce
140
90
Th
232
59
Pr
141
91
Pa
231
60
Nd
144
92
U
238
Lanthanides
Actinides
10
1B
11
2B
12
28
Ni
58.7
29
Cu
63.5
44
Ru
101
76
Os
190
108
Uno
(265)
27
Co
58.9
45
Rh
103
77
Ir
192
109
Une
(266)
46
Pd
106
78
Pt
195
47
Ag
108
79
Au
197
30
Zn
65.4
48
Cd
112
80
Hg
201
13
Al
27.0
31
Ga
69.7
49
In
115
81
Tl
204
61
Pm
(145)
93
Np
(237)
62
Sm
150
94
Pu
(244)
63
Eu
152
95
Am
(243)
64
Gd
157
96
Cm
(247)
65
Tb
159
97
Bk
(247)
66
Dy
162
98
Cf
(251)
8
26
Fe
55.8
9
6
Start
yes
yes
no
Linear?
yes Two or
no
more Cn,
n>2?
i?
yes
no
yes
i?
yes
no
C∞v
C5? no
Ih
σ?
yes Are nC2 ⊥ to
principle Cn?
Oh
i?
yes
σh ?
no
yes
no
yes σ ? no
h
D∞h
no
Cn ?
no
yes
no
Td
yes
σd?
no
σv ?
yes
no
S2n ?
yes
Cnh
no
Dnh
C2v
A1
A2
B1
B2
E
1
1
1
1
C2
1
1
–1
–1
σv(xz)
1
–1
1
–1
Dnd
σv(yz)
1
–1
–1
1
Dn
z
Rz
x, Ry
y, Rx
Cn
S2n
Cnv
x2, y2, z2
xy
xz
yz
Cs
Ci
C1