CHEMISTRY 102B
Hour Exam III
April 30, 2015
Dr. D. DeCoste
Name ______________________________
Signature ___________________________
T.A. _______________________________
This exam contains 17 questions on 6 numbered pages. Check now to make sure you have a
complete exam. You have one hour and thirty minutes to complete the exam. Determine the
best answer to the first 15 questions and enter these on the special answer sheet. Also, circle
your responses in this exam booklet.
Show all of your work and provide complete answers to questions 16 and 17.
1-15
(30 pts.)
_________
16
(15 pts.)
_________
17
(15 pts.)
_________
(60 pts)
_________
Total
Useful Information:
CHEMISTRY 102B
Hour Exam III
1.
April 30, 2015
Page No. 1
How many of the following statements are true?
I.
II.
In general, larger atoms have larger electronegativity values.
The smaller the difference in electronegativity values between bonding atoms, the
greater the covalent character of the bond.
III. The oxygen atom is more electronegative than the phosphorus atom.
IV. It is possible for a molecule with polar bonds to have no overall dipole moment.
a) 0
2.
b) 1
e) 4
b) N3–
c) N–
d) N5+
e) N
Arrange the following bond from most to least polar.
a)
b)
c)
d)
e)
4.
d) 3
Use the information given below to choose the most stable species of nitrogen in an
evacuated container.
a) N+
3.
c) 2
P-F > O-F > S-F
S-F > O-F > P-F
S-F > P-F > O-F
P-F > S-F > O-F
O-F > S-F > P-F
Consider the following reaction:
C2H4(g) + F2(g) → CH2FCH2F(g)
ΔH = –549 kJ
Estimate the carbon-fluorine bond energy given that the C-H bond energy is 413 kJ/mol,
the C-C bond energy is 347 kJ/mol, the C=C bond energy is 614 kJ/mol, and the F-F bond
energy is 154 kJ/mol. Note: you will need to draw Lewis structures for the reactants and
products to determine the possible presence of multiple bonds.
a) 198 kJ
b) 352 kJ
c) 485 kJ
d) 704 kJ
e) 970 kJ
CHEMISTRY 102B
Hour Exam III
5.
Which of the following is the best Lewis structure for the hydrogen thiocyanate molecule
(HSCN) taking formal charge into account? Note: the lone pairs are not included in these
skeletal structures. Before answering the question, complete the Lewis structures.
a)
b)
c)
d)
e)
6.
April 30, 2015
Page No. 2
H-S-C≡N
H-S=C=N
H-S≡C-N
H-S-C-N
At least two of the above structures are equally valid when considering formal charge.
How many of the following can correctly be described as a “bent” molecule?
O3
H2S
CO2
XeCl2
a) 0
b) 1
c) 2
d) 3
e) 4
____________________________________________________________________________
7.-8.
Formic acid is an organic compound that red ants inject into their victims with their
bites. In fact, the name formic acid comes from the Latin name for ant. Formic acid
has the following skeletal structure:
O
H
C
O* H
This structure does not include lone pairs of electrons or possible multiple bonds.
Complete the Lewis structure of formic acid to answer the following questions.
7. What is the hybridization of the oxygen atom marked with the asterisk (*)?
a) sp
b) sp2
c) sp3
d) dsp3
e) d2sp3
8. How many π bonds are included in the Lewis structure for formic acid?
a) 0
b) 1
c) 2
d) 3
e) 4
_____________________________________________________________________________
9.
How many of the following molecules is/are nonpolar while having polar bonds?
BF3
a) 1
10.
b) 2
N2
CO2
c) 3
XeF4
CCl4
d) 4
SF4
e) 5
What type(s) of intermolecular force(s) is/are exhibited by methane (CH4)?
a)
b)
c)
d)
e)
hydrogen bonding and London dispersion forces
hydrogen bonding
dipole-dipole and London dispersion forces
dipole-dipole
London dispersion forces
CHEMISTRY 102B
Hour Exam III
11.
Which of the following statements is false?
a)
b)
c)
d)
e)
12.
April 30, 2015
Page No. 3
Ethanol (CH3CH2OH) has a higher boiling point than dimethyl ether (CH3OCH3).
Propane (C3H8) has a higher boiling point than dimethyl ether (CH3OCH3).
Br2 has a higher boiling point than Cl2.
HCl has a higher vapor pressure than HF at 0°C.
NaCl has a higher melting point than HCl.
How many of the following statements are true considering chemical equilibrium?
I.
A system that is disturbed from an equilibrium condition responds in a manner to restore
equilibrium.
II. Because K is an equilibrium constant, its value for a given equation can never change.
III. When two opposing processes are proceeding at identical rates, the system is at equilibrium.
IV. For a given reaction at a given temperature, there is only one set of equilibrium
concentrations for the reactants and products.
V. The value of the equilibrium constant at a given temperature for a given reaction
mixture is the same regardless of the direction from which equilibrium is attained.
a) 1
13.
b) 2
e) 5
Addition of hydrogen gas at constant volume and temperature.
Decreasing the volume at constant temperature.
Decreasing the temperature of the system at constant volume.
Addition of helium gas at constant volume and temperature.
a) 0
b) 1
c) 2
d) 3
e) 4
You add an excess of solid PbCl2 to water (Ksp for PbCl2 = 1.6 x 10-5). Calculate the concentration
of Cl– at equilibrium.
a) 0.032 M
15.
d) 4
Consider the system represented by 2H2(g) + O2(g)
2H2O(g) at equilibrium. How
many of the following changes would shift the equilibrium position to the right?
I.
II.
III.
IV.
14.
c) 3
b) 0.016 M
c) 0.0080 M
d) 0.0040 M
e) 1.6 x 10-5 M
A 0.030-mol sample of Na2SO4 is added to 1.0 L of each of two solutions. One solution
contains 1.5 x 10-3 M Ba(NO3)2; the other contains 1.5 x 10-3 M Ca(NO3)2. Given that Ksp
for BaSO4 = 1.5 x 10-9 and Ksp for CaSO4 = 6.1 x 10-5:
a)
b)
c)
d)
e)
BaSO4 would precipitate but CaSO4 would not.
CaSO4 would precipitate but BaSO4 would not.
Both BaSO4 and CaSO4 would precipitate.
Neither BaSO4 nor CaSO4 would precipitate.
Not enough information is given to determine if precipitation would occur.
CHEMISTRY 102B
Hour Exam III
16.
April 30, 2015
Page No. 4
An ionic compound made from the metal M and the diatomic gas X2 has the formula MaXb,
in which a = 1 or 2 and b = 1 or 2. That is, the formula for the ionic compound is either
MX (the charges for the ions could be ±1 or ±2), MX2 (M2+ and X–), or M2X (M+ and X2–).
Relevant data are given below:
Enthalpy of sublimation for M(s) → M(g): 110 kJ/mol
Bond energy of X2: 250 kJ/mol
Successive ionization energies of M(g): 480 kJ/mol, 4750 kJ/mol
Successive electron affinity values for X(g): –175 kJ/mol, 920 kJ/mol
Lattice energy for MX (M+ and X–): –1200 kJ/mol of MX formed
Lattice energy for MX2 (M2+ and X–): –3500 kJ/mol of MX2 formed
Lattice energy for M2X (M+ and X2–): –3600 kJ/mol of M2X formed
Lattice energy for MX (M2+ and X2–): –4800 kJ/mol of MX formed
Use the data provided to determine the formula for the ionic compound and the charges
of each ion. Justify your answer and eliminate other potential answers. Show all work.
[15 points]
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CHEMISTRY 102B
Hour Exam III
17.
April 30, 2015
Page No. 5
You mix an equal number of moles of nitrogen gas and hydrogen gas in a rigid container
such that the total pressure is 2.000 atm. The gases react at constant temperature to form
ammonia and the system reaches equilibrium according to the equation:
N2(g) + 3H2(g)
2NH3(g)
a. At equilibrium, the total pressure is 1.500 atm at the given temperature. Determine the
equilibrium partial pressures of each gas and the value of Kp for this reaction at this
temperature. Show all work. [8 points]
b. You now have N2(g), H2(g), and NH3(g) in equilibrium as calculated in part “a”.
Suppose you decrease the volume of the container by a factor of two at constant
temperature (that is, the volume is ½ of its initial value).
i. Discuss (without numbers) what will happen to the equilibrium position (shift to
the left, shift to the right, or no change) and what will happen to the value of Kp
(increase, decrease, or stay the same) and why. Use LeChâtelier’s principle in
your answer. [3 points]
CHEMISTRY 102B
Hour Exam III
17.
April 30, 2015
Page No. 6
(con’t)
ii. Support your answer to “22bi” with numbers. Determine the value of the reaction
quotient (Q) when the volume is changed, and explain how it is consistent with
your answer to “22bi”. Show all work. [4 points]
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