1. (3 pts) The relative size of one of the Van der Waals constants, a, correlates well with boiling
point; that is, the larger a is, the higher the boiling point. The reason for this correlation is:
a) The constant a is a measure of intermolecular interactions
b) The constant a is determined experimentally
c) The constant a varies with temperature
d) The constant a accounts for molecular shape
e) None of the above
2. (3 pts) The value of the constant b in the Van der Waals equation accounts for what incorrect
assumption made by the ideal gas law?
a) gaseous collisions are not completely elastic
b) gas molecules interact with each other
c) gas molecules do not take up space
d) velocity of gas molecules changes with temperature
e) none of the above
3. Using the potential energy plot below for the two molecules, A2 and B2, answer the following
questions:
a) (2 pts) Which molecule has the more stable bond?
b) (2 pts) What is the equilibrium B-B bond length?
1
4. (3 pts) Which molecule is the most polar?
a) NO
b) O2
c) HF
d) HCl
e) These molecules are all equally polar
5. (5 pts) What is the best Lewis structure for phosphoric acid, H3PO4?
6. (5 pts) Sketch the best Lewis structure(s) for the nitrate ion, NO3− ?
2
7. (18 pts) Complete the following table. CO2 has been completed as an example.
Electron group
Central atom
Shape
Dipole moment?
geometry
hybridization
CO2
linear
sp
linear
no
XeF4
NH4+
SeCl4
H3O+
H2CO
H2O
8. The normal C-C single bond length is 1.54Å. The normal C=C double bond length is 1.34Å
6
4
5
3
2
1
a) (2 pts) Estimate the length of the bond between atoms 4 and 5 in the line drawing for
hexatriene given above.
b) (4 pts) Explain your answer.
3
9. (14 pts total) In this problem you will examine the relative bond strengths of N2 and N2–.
a) (8 pts) First, sketch the valence molecular orbital energy diagrams for N2 and for N2–.
For full credit, be sure to include all of the valence electrons and to label each molecular
orbital with its bond type (σ, σ*, π, or π*) and the atomic orbitals it was generated from (s
or p).
b) (4 pts) Calculate the bond order for each molecule.
c) (2 pts) Does N2– have a stronger or weaker bond than N2?
10. (8 pts) Arrange the following materials in order of decreasing conductivity.
a) C (diamond)
b) Si
c) Si doped with 0.0000002 % As
d) Cu
Best Conductor
Worst Conductor
4
11. (10 pts) For each compound, indicate each type of intermolecular interaction that has a
significant effect on the boiling point. Also, give the boiling point for each compound by
selecting from the following list (in K):
4.2, 87, 273, 335, 410
Water is provided as an example.
Water
Dispersion
Dipolar
Hydrogen bond
Boiling Point (K)
X
X
X
373
Pentanol
Pentane
Fluoropentane
Argon
Helium
12. (5 pts) On the blank phase diagram below label the following five items:
regions of solid, liquid, and gas phases, the triple point, and the critical point.
5
13. (6 pts) Methylene carbene (CH2) is an organic reagent that is unusually stable for a small
molecule that doesn’t satisfy the octet rule. It has two electronic forms: a “singlet” state that is
diamagnetic and a “triplet” state that is paramagnetic. Use this information to predict bond
angles for the “singlet” and “triplet” states of CH2. Be sure to include diagrams and words to
explain your predictions.
14. (10 pts) Large buildings (like the Hope College science center) utilize evaporative cooling
for their summer air conditioning (remember summer?). If the air in the science center has a heat
capacity of 30.0 J/(mol K) and it needs to be cooled by 5 K, how much water (in kg) must be
evaporated? Assume the science center has a volume of 64,000,000 L and is at standard
temperature and pressure. The ∆Hvap for water is 40.79 kJ/mol.
exam3c_fall2005
6