NAME:___________________________
Spring 2008
INSTRUCTIONS:
Section:_____
Student Number:__________________
Chemistry 2000 Midterm #1B
____/ 50 marks
1) Please read over the test carefully before beginning. You should have 5
pages of questions and a periodic table.
2) If you need extra space, use the bottom of the periodic table page, indicate
that you are doing so next to the question and clearly number your work.
3) If your work is not legible, it will be given a mark of zero.
4) Marks will be deducted for incorrect information added to an otherwise
correct answer.
5) You have 90 minutes to complete this test.
1.
A small amount of arsenic is added to a sample of silicon to improve its ability to conduct
electricity.
[5 marks]
(a)
Is this an example of a p-type semiconductor or an n-type semiconductor?
n-type
(b)
Explain how the addition of arsenic increases conductivity.
•
Arsenic has one more valence electron than silicon.
•
As a result, it provides a donor band (containing electrons) close in energy to the conduction
band of the silicon.
•
Electrons in the donor band require less energy to be excited into the conduction band of silicon
(compared to the electrons in the valence band of silicon).
•
The electrons excited into the conduction band can conduct electricity.
For comparison: p-type semiconductors (not expected to be mentioned on test)
•
Dopant (Al, Ga, etc.) has one less valence electron than silicon.
•
As a result, it provides an acceptor band close in energy to the valence band of the silicon.
•
Electrons in the valence band of silicon require less energy to be excited into the acceptor band
(compared to the conduction band of silicon).
•
The holes left behind in the valence band (by the excited electrons) can conduct electricity.
NAME:___________________________
Section:_____
Student Number:__________________
2.
[10 marks]
Energy Values for the Atomic Orbitals of Oxygen and Hydrogen Atoms
1s
(a)
Hydrogen
Oxygen
-1.00 Ry
-39.9 Ry
2s
-2.38 Ry
2p
-1.17 Ry
Use the energy values in the table above to help you develop a valence molecular orbital
diagram for the hydroxide ion (OH-). Label all orbitals on your diagram and include electrons.
It is not necessary to draw pictures of the orbitals for this question.
1σ is the core orbital on oxygen (from 1s).
(b)
According to your MO diagram, what is the bond order for this ion?
1
(c)
Identify any nonbonding valence molecular orbitals.
2σ and 1π
(d)
Would you expect OH- to be paramagnetic or diamagnetic?
diamagnetic
NAME:___________________________
3.
(a)
Section:_____
Student Number:__________________
Calcium carbide, CaC2, is a reactive salt used in a number of industrial processes including the
production of acetylene. Use molecular orbital theory to describe the bonding of the carbide
anion ( C 22− ).
[17 marks]
Complete the valence molecular orbital diagram below by:
i.
drawing and naming the atomic orbitals,
ii.
drawing and naming the molecular orbitals, and
iii.
adding electrons to the appropriate molecular orbitals
3σ*
1π*
2p
2p
Energy
3σ
1π
2σ*
2s
2s
2σ
C
(b)
C22-
C
Write the complete orbital occupancy (i.e. electron configuration) for C 22− .
(1σ)2 (1σ*)2 (2σ)2 (2σ*)2 (1π)4 (3σ)2
(c)
Would you expect the carbide anion to have a larger or smaller bond dissociation energy than
C2? Justify your answer.
Larger
The bond order for C22- is 3. The bond order for C2 is 2. Bond dissociation energy increases
with increasing bond order.
(d)
Give formulas for two neutral diatomic molecules that are isoelectronic with C 22− .
and
CO
N2
NAME:___________________________
4.
Section:_____
Student Number:__________________
The valence π molecular orbital diagram shown below can be used to describe the π bonding in
C3 H 3 , C3 H 3+ or C3 H 3− . Each of these compounds consists of a triangle of carbon atoms with
one hydrogen attached to each carbon.
[10 marks]
E
π∗
π
(a)
i.
Draw all valid resonance structures for the C3 H 3− anion.
H
H
C
(b)
C
C
C-
H
H ..
-C
C
C-
C
C
H
H
H
ii.
What is the average bond order for each C-C bond in the C3 H 3− anion according to your
resonance structures?
1⅓
i.
Fill the π-MOs in the diagram above with the appropriate number of π electrons for the
C3 H 3− anion.
ii.
iii.
(c)
..
.. H
H
See MO diagram
What is the average π bond order for each C-C bond in the C3 H 3− anion according to the
MO diagram?
0
What is the average bond order for each C-C bond in the C3 H 3− anion according to MO
theory?
1
On the π-MO diagram above, draw a picture of the lowest energy π-MO.
See picture next to MO diagram
NAME:___________________________
Section:_____
Student Number:__________________
5.
Consider the bonding in COCl2 according to valence bond theory.
(a)
Draw a Lewis structure for COCl2.
: O:
..
: Cl
..
C
..
Cl
.. :
sp2
(b)
What is the hybridization of the carbon atom in COCl2?
(c)
Clearly indicate which atomic orbitals combine to make each σ bond in COCl2.
: O:
σ:C(sp2)+Cl(3p)
..
: Cl
..
(d)
C
σ:C(sp2)+O(2p)
..
Cl
.. :
σ:C(sp2)+Cl(3p)
Clearly indicate which atomic orbitals combine to make each π bond in COCl2.
π:C(2p)+O(2p)
: O:
..
: Cl
..
C
..
Cl
.. :
[8 marks]
NAME:___________________________
Section:_____
Student Number:__________________
CHEM 1000 Standard Periodic Table
1
18
4.0026
1.0079
H
He
2
13
14
15
16
17
6.941
9.0122
10.811
12.011
14.0067
15.9994
18.9984
Li
Be
B
C
N
O
F
Ne
3
22.9898
4
24.3050
5
26.9815
6
28.0855
7
30.9738
8
32.066
9
35.4527
10
39.948
1
2
20.1797
Na
Mg
11
39.0983
12
40.078
3
4
5
6
7
8
9
10
11
12
44.9559
47.88
50.9415
51.9961
54.9380
55.847
58.9332
58.693
63.546
65.39
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
19
85.4678
20
87.62
21
88.9059
22
91.224
23
92.9064
24
95.94
26
101.07
27
102.906
28
106.42
29
107.868
30
112.411
31
114.82
32
118.710
33
121.757
34
127.60
35
126.905
36
131.29
Rb
Sr
37
132.905
38
137.327
Cs
Ba
55
(223)
56
226.025
Fr
87
Ra
Y
39
La-Lu
Ac-Lr
88
P
S
Cl
Ar
15
74.9216
16
78.96
17
79.904
18
83.80
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
41
180.948
42
183.85
43
186.207
44
190.2
45
192.22
46
195.08
47
196.967
48
200.59
49
204.383
50
207.19
51
208.980
52
(210)
53
(210)
54
(222)
Hf
Ta
W
Re
Os
Ir
Pt
Au
72
(261)
73
(262)
74
(263)
75
(262)
76
(265)
77
(266)
78
(281)
79
(283)
Rf
Db
Sg
105
106
138.906
140.115
140.908
144.24
La
Ce
Pr
Nd
57
227.028
58
232.038
59
231.036
60
238.029
Ac
Si
14
72.61
40
178.49
104
89
25
(98)
Al
13
69.723
Th
90
Pa
91
U
92
Bh
107
Hs
Mt
Dt
Hg
Tl
Pb
Bi
Po
At
80
81
82
83
84
85
174.967
Rg
108
109
110
111
(145)
150.36
151.965
157.25
158.925
162.50
164.930
167.26
168.934
173.04
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
61
237.048
62
(240)
63
(243)
64
(247)
65
(247)
66
(251)
67
(252)
68
(257)
69
(258)
70
(259)
71
(260)
Np
93
Pu
94
Am
95
Cm
96
Rn
86
Bk
97
Cf
98
Es
99
Fm
100
Md
101
No
102
Lr
103
Developed by Prof. R. T. Boeré