CHM 134 General Chemistry I
Exam 3, Fall 2007 – Dr. Steel
Name
SOLUTIONS
Place your answers to section II of the exam on this sheet. Be certain your name appears on both
this sheet and the exam.
2.
E
3.
D
4.
C
5.
E
6.
C
7.
D
8.
E
9.
C
10.
A
11.
D
12.
B
13.
C
14.
D
15.
D
16.
A
CHM 134 General Chemistry I
Exam 4, Fall 2007 – Dr. Steel
Name
SOLUTIONS
1. For each compound in the table draw a reasonable Lewis dot structure (2 points),
name the VSEPR geometry (1 point), and indicate the type of orbital hybridization
present on the central atom (1 point).
Compound
Lewis structure
Geometry
Orbital
Hydrization
T-shaped
sp3d
tetrahedral
sp3
square planar
sp3d2
trigonal planar
sp2
bent
sp3
Cl
ICl3
I
Cl
Cl
F
PF41+
P
F
F
F
F
F
XeF4
Xe
F
F
O
COH2
C
H
H
S
SF2
F
F
Section II: Multiple Choice. Write the letter of the best answer to each question on the
answer sheet provided. Each question in this section is worth 2 points.
2. Which of the following is a correct Lewis structure for ozone, O3?
..
..
.O
.
(d)
(c)
..
O
..
O
..
O
O
..
O
..
O
..
(b)
(a)
..
O
..
O
..
..
.O
.
O..
..
..
..
..
.O
.
..
..
O
..
O..
..
..
..
..
.O
.
(e)
3. Which of the following are possible Lewis structures for C2H6O?
H
H
H C H
H
..
C O
..
H
H
..
H C O
.. C H
H
H
H
b) 2
..
H C C O
.. H
H H
(2)
(1)
a) 1
H H
(3)
c) 3
d) 2 and 3
e) 1, 2, and 3
4. Which of the following are resonance structures for the formate ion, HCO2-?
..
O
.. .
C O
.. .
H
-
..
O
..
C O..
H
..
O
.. .
C O
.. .
H
(2)
(1)
a) 1 and 2
-
(3)
b) 2 and 3
c) 3 and 4
-
..
O
..
C O..
H
-
(4)
d) 1, 3, and 4
e) 2, 3, and 4
5. How many sigma (σ) bonds and pi (π) bonds are in the following molecule?
H
H
a) five σ and two π
b) five σ and three π
c) five σ and five π
d) seven σ and two π
e) seven σ and three π
C
C
H
C
C
H
6. How many sigma (σ) bonds and pi (π) bonds are in acetic acid?
O
H
H
a) six σ and one π
b) six σ and two π
c) seven σ and one π
d) eight σ and zero π
e) eight σ and one π
C
C
O
H
H
7. How many sigma (σ) bonds and pi (π) bonds are in acetylene, C2H2?
a) one σ, one π
b) two σ, two π
c) three σ, one π
d) three σ, two π
e) four σ, one π
8. What is the maximum number of hybridized orbitals formed by a sulfur atom?
a) 2
b) 3
c) 4
d) 5
e) 6
9. What is the maximum number of hybridized orbitals formed by a carbon atom?
a) 2
b) 3
c) 4
d) 5
e) 6
10. What is the hybridization of the carbon atom is carbon disulfide, CS2?
a) sp
b) sp2
c) sp3
d) sp3d
e) sp3d2
d) sp3d
e) sp3d2
11. What is the hybridization of the sulfur atom in SF4?
a) sp
b) sp2
c) sp3
12. What is the hybridization of the central oxygen atom in ozone, O3?
a) sp
b) sp2
c) sp3
d) sp3d
e) sp3d2
13. What is the hybridization of the central atom in a molecule with a trigonal pyramidal
molecular geometry?
a) sp
b) sp2
c) sp3
d) sp3d
e) sp3d2
14. What is the hybridization of the central atom in a molecule with a see-saw geometry?
b) sp2
a) sp
c) sp3
d) sp3d
e) sp3d2
15. Atomic orbitals combine most effectively to form molecular orbitals when
a) electrons in the orbitals have no spins.
b) electrons in the orbitals have the same spin.
c) the atomic orbitals are hybridized.
d) the atomic orbitals have similar energies.
e) metals combine with nonmetals.
16. Carbon dioxide reacts with an aqueous solution of sodium hydroxide to form
carbonate ion. What change in the hybridization of carbon occurs in this reaction?
a) sp to sp2
b) sp2 to sp3
c) sp3 to sp3d
d) sp3 to sp3d2
e) no change
Section III: Answer each question in the space provided. Points per question are
indicated in parentheses.
17. What type of bond is most likely to form between the atoms in each pair? Choose
from ionic, polar covalent, and nonpolar covalent. (6 points)
Al and F
IONIC
POLAR COVALENT
IONIC
S and O
Cs and Br
POLAR COVALENT
P and H
NONPOLAR COVALENT Cl and Cl
POLAR COVALENT
C and O
18. For the two resonance structures shown below, indicate the formal charge on each of
the carbon and nitrogen atoms and circle the better structure. (8 points)
-
H
C
H
-1
H
C
N
C
C
N
0
0
-1
H
0
0
Formal
Charges
The structure on the right is the better one because the negative
charge resides on nitrogen, the more electronegative element.
19. When heated, azomethane decomposes into nitrogen gas and ethane gas. (8 points)
H
H
H
C
N
N
H
C
N
H
N
+
H
H
Bond
C-H
C-N
C-C
Bond Energy
(kJ/mol)
413
305
346
Bond
N-N
N=N
N≡N
H
H
C
C
H
H
H
Bond Energy
(kJ/mol)
163
418
945
Using average bond energies, calculate the enthalpy of reaction.
o
Δ H rxn
= ∑ Δ H (bonds broken) − ∑ Δ H (bonds formed)
= [2(C − N) + (N = N)] − [(N ≡ N) + (C − C) ]
= [2(305) + 418] − [945 + 346] kJ
= −263 kJ
20. The thiocyanate ion, SCN1-, has three resonance structures. In each case the carbon
atom is the central atom. Draw all three resonance structures and circle which is the
most favorable structure. (8 points)
1N
C
S
N
C
S
-2
0
+1
-1
0
0
N
0
C
S
0
-1
Formal
Charges
The middle structure is the best because it has the -1 charge on the nitrogen,
the most electronegative element in the structure.
The following molecular orbital diagram may be used for problems 21-23. For oxygen
and fluorine, the σ2p orbital should be lower in energy than the π2p. However, the diagram
will still yield correct bond order and magnetic behavior for these molecules.
Energy
________
________
________ π*2p
________
________
σ*2p
σ2p
________ π2p
________
σ*2s
________
σ2s
21. According to molecular orbital theory, what is the bond order in the C22- ion?
(3 points)
The C22- ion has 10 valence electrons: σ2s2 σ∗2s2 π2p4 σ2p2
The bond order is ½(8-2) = 3.
22. According to molecular orbital theory, what is the bond order of fluorine, F2?
(3 points)
The F2 molecule has 14 valence electrons: σ2s2 σ∗2s2 π2p4 σ2p2 π∗2p4
The bond order is ½(8-6) = 1.
23. According to molecular orbital theory, is F2 diamagnetic or paramagnetic? Why?
(4 points)
There are no unpaired electrons in the MO diagram of F2, therefore it is
diamagnetic.
24. What second-row diatomic molecule will have the following valence molecular
orbital energy level diagram? (3 points)
σ*2p
π*2p
σ2p
π2p
↑↓
↑↓
σ*2s
↑↓
σ2s
↑↓
This molecule has 8 valence electrons, therefore it is C2.
25. What ion with a +1 charge will have the following valence molecular orbital energy
level diagram? (3 points)
σ*2p
π*2p
π2p
↑↓
↑
↑↓
↑↓
σ2p
↑↓
σ*2s
↑↓
σ2s
↑↓
This ion has 13 valence electrons. F2 has 14, therefore, the F21+ ion
would have only 13 valence electrons. This is the MO diagram of F21+.
26. For each of the pairs of orbitals shown below, considering their types and orientation,
what type of bond is the most likely to form? (4 points)
pi bond
(side-to-side overlap)
sigma bond
(head-to-head overlap)