Name
CHM 115 EXAM #4
December 8, 2004
Circle the correct answer. (numbers 1-8, 2.5 points each)
1.
Which of the following bonds should be the least polar?
a. N—Cl
2.
d. Al
e. S
b. -1780 kJ
c. +180 kJ
d. +1780 kJ
e. none of the above
b. NO3-
c. CH4
d. PCl5
e. none of the above
b. C≡C
d. C≡O
c. O=O
_
e. C C
b. H2S, HCN, CO2
c. H2O, CO, H2S
d. H2S, CO, CO2
Which of the following is a tetrahedral molecule?
a. CO2
8.
c. O
Which of the following sets contains the greatest number of bent molecules?
a. CO2, HCN, O2
7.
b. Be
Which of the following bonds should be the shortest and strongest?
a. C=C
6.
e. N—Br
Which of the following species has a central atom with sp2 hybridization?
a. CO2
5.
d. H—I
What is the change in energy of a reaction in which 980 kJ of energy is required to break bonds
and 800 kJ of heat is generated by bond formation
a. -180 kJ
4.
c. F—F
Choose the element that should have the lowest electronegativity.
a. K
3.
b. N—F
b. NO3-
c. CH4
d. PCl5
e. none of the above
Which of the following is a molecule that contains at least one π bond?
a. CO2
b. NO3-
c. CH4
Complete the following as directed (point values as indicated).
(12 points)
9. (a) Draw the Lewis structure of ozone
(O3) using resonance if needed.
d. PCl5
e. none of the above
2
(b) Calculate the AVERAGE bond energy in ozone by using the information provided about the
following reaction and the fact that diatomic oxygen (O=O) has a bond energy of 498 kJ/mole.
O3 (g)
+
O
Æ
∆H = -392 kJ
2 O2 (g)
(c) Discuss the “reasonableness” of your answer to part (b) based on the structure drawn
in part (a).
(18 points)
10.
+
(a) In the spaces provided, do the following for SF5
Draw the best Lewis structure of the species and
show formal charge calculations for each type of atom
and give the names of the electron pair
geometry and of the molecular shape,
finally, specify the hybridization of the S AND
draw the energy diagram for the hybridized orbitals
(you know - the boxes with electrons in them).
Is this species polar or nonpolar?
+
(b) In the spaces provided, do the following SF3
Draw the best Lewis structure of the species and
show formal charge calculations for each type of atom
and give the names of the electron pair
geometry and of the molecular shape,
finally, specify the hybridization of the S AND
draw the energy diagram for the hybridized orbitals
(you know - the boxes with electrons in them).
Is this species polar or nonpolar?
3
(20 points)
11.
For the following two species complete the tasks specified below in a - d.
ii. C2H4
i. H3PO4
a. Draw the best Lewis
structures.
b. Specify the hybridization
(ex. sp2) of both
the P atom and the O atoms
specified in (i)
and the C atoms in (ii)?
P in H3PO4
c. Show the energy level
diagram for both the P atom
and the O atoms in (i) and
the C atoms in (ii), fill the
diagrams with the correct
number of valence electrons,
and identify the types of
electrons present (i.e. σ, π,
lone pair)
P in H3PO4
C in CH2
O in the P-O-H groups
C in CH2
O in the P-O-H groups
P in H3PO4
d. What are the bond angles
around both the P atom and
the O atoms in (i) and the C
atoms in (ii)?
C in CH2
O in the P-O-H groups
(15 points)
12. (a) Draw Lewis structures for CN , CO, N2, and NO+. Label all atoms with their formal charges.
(b) Select the species from part (a) that you would expect to have the shortest, strongest bond. In
order to receive credit you must EXPLAIN the reasoning for your selection.
4
(15 points)
12. Fill in the MO energy diagrams for the species listed below. The answer the questions that appear
under the MO energy diagram of each species.
Oxygen
Ο2
Peroxide
σ∗2p
Ο2−2
σ∗2p
π∗2p
π∗2p
π∗2p
π∗2p
π2p
π2p
π2p
π2p
σ2p
σ2p
σ∗2s
σ∗2s
σ2s
σ2s
Show the calculation of the bond order for
oxygen.
Show the calculation of the bond order for the
peroxide ion.
Is oxygen diamagnetic or paramagnetic?
Explain your answer.
Is the peroxide ion diamagnetic or paramagnetic?
Explain your answer.
Draw the Lewis structure of oxygen.
Draw the Lewis structure for the peroxide ion.
Does the bond order for oxygen from the
Lewis structure match the bond order from
the MO diagram?
Does the bond order for the peroxide ion from
the Lewis structure match the bond order from
the MO diagram?
Which should have the longer, weaker bond; oxygen or the peroxide ion? Explain your choice!
5
BONUS (for up to 8 points)
The formation of NaCl from the elements is a violent EXOthermic reaction.
This process may be viewed as the sum of the following steps (a “Born-Haber cycle”):
sublimation, bond dissociation, first ionization process, first electron affinity,
and lattice formation.
Create an energy diagram illustrating this cycle. Include the physical state of all species involved and
make sure to indicate which of these steps are endothermic or exothermic!
Discuss which one step in the diagram is the primary factor that makes the overall process of NaCl
formation exothermic.