Exam III

CHEMISTRY 102B
Hour Exam III
April 30, 2015
Dr. D. DeCoste
Name ______________________________
Signature ___________________________
T.A. _______________________________
This exam contains 22 questions on 7 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 20 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 21 and 22.
1-20
(40 pts.)
_________
21
(20 pts.)
_________
22
(20 pts.)
_________
(80 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
b) 1
c) 2
d) 3
e) 4
______________________________________________________________________________
2, 3. For each of the following use the information given below and your knowledge of atomic
theory and chemical bonding.
2.
Choose the most stable species of nitrogen in an evacuated container.
a) N+
3.
b) N3–
c) N–
d) N5+
e) N
Choose the most stable species of nitrogen in an ionic compound.
a) N+
b) N3–
c) N–
d) N5+
e) N
_____________________________________________________________________________
4.
Arrange the following bond from most to least polar.
a)
b)
c)
d)
e)
5.
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
6.
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)
7.
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
a) 0
8.
H2S
CO2
b) 1
XeCl2
c) 2
d) 3
e) 4
How many of the following can correctly be described as a “linear” molecule?
OF2
KrF2
SF2
MgF2
a) 0
b) 1
c) 2
d) 3
e) 4
____________________________________________________________________________
9.-10. 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.
9. What is the hybridization of the oxygen atom marked with the asterisk (*)?
a) sp
b) sp2
c) sp3
d) dsp3
e) d2sp3
10. How many π bonds are included in the Lewis structure for formic acid?
a) 0
b) 1
c) 2
d) 3
e) 4
_____________________________________________________________________________
11. How many of the following molecules is/are nonpolar while having polar bonds?
BF3
a) 1
b) 2
N2
CO2
c) 3
XeF4
CCl4
d) 4
SF4
e) 5
CHEMISTRY 102B
Hour Exam III
12.
What type(s) of intermolecular force(s) is/are exhibited by methane (CH4)?
a)
b)
c)
d)
e)
13.
hydrogen bonding and London dispersion forces
hydrogen bonding
dipole-dipole and London dispersion forces
dipole-dipole
London dispersion forces
Identify the strongest intermolecular force for the given molecule
SO2
dipole-dipole
dipole-dipole
dipole-dipole
LDF
LDF
a)
b)
c)
d)
e)
14.
BH3
LDF
H-bond
H-bond
H-bond
LDF
CO2
LDF
dipole-dipole
LDF
dipole-dipole
LDF
Which of the following statements is false?
a)
b)
c)
d)
e)
15.
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
16.
b) 2
c) 3
d) 4
e) 5
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.
a) 0
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.
b) 1
c) 2
d) 3
e) 4
CHEMISTRY 102B
Hour Exam III
April 30, 2015
Page No. 4
17. Choose the correct equilibrium expression for the equation
2KClO3(s)
2
18.
[KCl] [O 2 ]
a)
K=
b)
K=
c)
K = [3O2]3
d)
K=
e)
K = [O2]3
2KCl(s) + 3O2(g)
3
[KClO 3 ] 2
[2KCl][3O 2 ]
[2KClO 3 ]
[2KCl] 2 [3O 2 ]3
[2KClO 3 ] 2
You have a saturated solution of aqueous NaCl (that is, the solution contains the maximum
concentration of NaCl at the given temperature) and add 0.10 M HCl(aq) dropwise at
constant temperature. What will happen? The Ksp value for NaCl = 38.
a) The NaCl will precipitate out of solution and continue to do so as more 0.10 M HCl is
added.
b) At first NaCl will precipitate out of solution, but it will stop doing so after several
milliliters of 0.10 M HCl are added. As more 0.1 M HCl is added, the NaCl solid will
remain.
c) At first nothing will seem to happen, but as more 0.10 M HCl is added, a precipitate
will form.
d) At first NaCl will precipitate out of solution, and then the NaCl will begin to redissolve. After several milliliters of 0.10 M HCl are added, there will be no solid left.
e) No NaCl will ever precipitate out of solution.
19.
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
20.
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
21.
April 30, 2015
Page No. 5
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.
[20 points]
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CHEMISTRY 102B
Hour Exam III
22.
April 30, 2015
Page No. 6
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. [9 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. [5 points]
CHEMISTRY 102B
Hour Exam III
22.
April 30, 2015
Page No. 7
(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. [6 points]