Chemistry 1110 Dr. Shane Phillips
Fall
1999 Dec.
8, 1999
EXAM 4
Name
Student I.D. Number
Please wait
for the instruction to begin before starting to work on this exam.
This is a closed book exam. It should contain 6 pages including this
cover sheet. The multiple choice should
be answered on a scantron form (882-ES).
Please be sure your exam is complete and your name and student I.D.
number is entered in the space provided above.
You may use an approved calculator.
|
Problems |
Pts. Poss. |
Pts. Earned |
|
Mult. Choice |
80 |
|
|
Work-out |
20 |
|
|
Total |
100 |
|
Potentially Useful Information
Gas
Law Constant: 8.314 J / K mol
0.0821 L atm / K mol
Faraday Constant: 96,500 C / mol
DG = DH – TDS DU = q + w
DG = RT ln(Q) – RT ln(K) DS = q / T
E = – (RT / nF) ln(Q) + (RT / nF) ln(K) ln([Parent]o/[Parent]t)
= kt
DG = –nFE
Choose
the best answer for the following questions.
Questions requiring multiple answers will have responses listed in the
respective order. (4 points each)
1. How many protons and
neutrons, respectively, are in 238U?
a)
146, 92 b) 238, 92
c) 92, 146 d)
92, 330 e) 238, 330
2. If 238U undergoes a emission, what other
nuclide is produced?
a)
234Th b) 236Ac c) 234U d)
234Pa e) 236Np
3. 116In
disintegrates (decays) to produce 116Sn and _________.
a) a positron b) an electron c) alpha particle d) a neutron e) gamma rays
4. Elements with an even number
of protons and neutrons ________________.
a)
never undergo a emission.
b) are
usually stable.
c) do not
have a magic number of protons or neutrons.
d) always have
a proton to neutron ration of about one.
e) make
great candidates for fission reactions.
5. The decay constant for 60Co
is 0.132 yr-1. What mass
of 60Co remains if 2.50 grams of 60Co decays for 10
years?
a)
2.38 g
b)
1.83
g
c)
0.67
g
d)
0.120
g
e)
None
of these.
6. Calculate DU for a system that does 300
kJ of work on the surroundings when 150 kJ of heat are absorbed by the system.
a)
+450 kJ b) 0 kJ
c) -450 kJ d)
-150 kJ e) +150 kJ
7. The disorder of a system is
measured by .
a)
enthalpy b) Gibbs free
energy c) entropy d) equilibrium constant
8. The enthalpy of vaporization
of liquid diethyl ether is 26 kJ/mol at the boiling point of 35.0oC. Calculate DS for the molar conversion
of gaseous to liquid diethyl ether at 35.0oC.
a)
743 J/K b) -1346 J/K c)
910 J/K d) 8008 J/K
e) -84
J/K
9. The entropy of the universe
is always increasing. This is a
statement of .
a)
1st
law of thermodynamics
b)
2nd
law of thermodynamics
c)
3rd
law of thermodynamics
d)
Hess’
law
e)
Gibbs’
law
10. The formation of one mole
of gaseous NO from its elements has a standard free energy change, DGo, of +87 kJ/mol
at 25oC. What is Kp
at this temperature?
a) 419
b) 1.8 x 1015
c) 5.7 x 10-16 d)
3.6 x 104 e) 35
11. Which of the following
would you expect to have the largest entropy?
a)
Ne (g) b) CaCO3 (s) c)
C4H10 (g)
d) LiCl (s) e)
HNO3 (l)
12. Calculate the standard free
energy change for one mole of methanol
(CH3OH) for the
unbalanced reaction below.
a) DGo = -229 kJ b) DGo = -457 kJ c) DGo = -686 kJ d) DGo = -1372
13. If DG is spontaneous at all temperatures, then DH is and DS is .
a)
positive,
negative b) negative, negative c) negative, positive
d)
positive,
positive e) zero, zero
14. What is the minimum
temperature a given reaction would have to be heated above to become
spontaneous if DH = +119 kJ and DS = +263 J/K?
a)
452 K b) 2210 K
c) 382 K d) 2.2 K
e) 363 K
15. The electrochemical cell
below generates 1.30V under standard conditions. What is DGo for this cell?
Zn(s)
+ I2(aq) 
Zn2+(aq) + 2I-(aq)
a)
+125 kJ b) +74 kJ
c) -251 kJ d)
-125 kJ e)None of these.
16. Given the two half cell reactions below,
which net cell reaction is spontaneous?
Mg2+(aq) + 2
e- -----> Mg(s) Eo = -2.37
V
Ni2+(aq) + 2
e- -----> Ni(s) Eo = -0.25 V
a) Mg2+(aq) +
Ni(s) -----> Mg(s)
+ Ni2+(aq)
b)
Ni2+(aq) +
Mg(s) -----> Ni(s)
+ Mg2+(aq)
c)
Ni(s) +
Mg(s) -----> Ni2+(aq) + Mg2+(aq)
d)
Ni2+(aq) + Mg2+(aq) ----->
Ni(s) + Mg(s)
e)
Mg(s) + Mg2+(aq) ----->
Ni(s) + Ni2+(aq)
17. If a current of 6 amps is
passed through a solution of Ag+ (aq) for 1.5 hours, how many
grams of elemental silver are produced?
a) 0.604 g
b) 36.2 g c)
0.335 g d) 3.04 g
e) 1.00 g
18. Which of the following is indicative of a
spontaneous reaction?
a) DG > 0
b) E > 0 c)
K < Q d) H > 0 and S < 0 e)
None of these.
19. For the reaction, 2 Na (s) + F2
(g) -----> 2 F-
(aq) + 2 Na+ (aq) , is and would be found in the compartment of
a voltaic cell.
a)
sodium,
oxidized, anode
b)
fluorine,
reduced, anode
c)
sodium,
oxidized, cathode
d)
fluorine,
oxidized, anode
e)
none
of the above are correct
20. In an electrolytic cell,
oxidation occurs at the _________ which has a _________ polarity.
a) anode, negative
b) cathode, negative
c) anode, positive
d) cathode, positive
e) None of these.
Show all work for the following questions
in the space provided to receive full credit.
Remember to state and check any assumptions you make.
21.
(8
pts) Balance the reaction below, which
occurs in basic, aqueous solution, by the method of half reactions. Clearly label the (i) oxidation
half reaction, (ii) reduction half reaction, and (iii) the net balanced
equation.
Ag +
CN- + O2 ---> Ag(CN)2- +
H2O
22.
(12
pts.) A voltaic cell is constructed of
one half-cell in which a silver wire dips into a 1.0M aqueous solution of AgNO3. The other half-cell consists of an iron
electrode in a solution of 1.0M Fe(NO3)2.
|
Reduction Half-Reaction |
Eo (V) |
|
Ag+(aq) +
e- ----->
Ag(s) |
+0.80 |
|
Fe2+(aq) +
2e- ----->
Fe(s) |
-0.44 |
a)
Provide
the net reaction that occurs in the cell and Eo for this cell at 25oC.
b)
What
is the equilibrium constant for the net cell reaction at 25oC?
c)
If
the concentration of AgNO3 (aq) is only 0.25M and the concentration
of Fe(NO3)2 (aq)
remains at 1.0M, what is DG for this cell at 25oC?
Answers
- c
- a
- b
- b
- c
- d
- c
- e
- b
- b
- c
- c
- c
- a
- c
- b
- b
- b
- a
- c
- 4Ag + 8CN- + O2 + 2H2O à
4Ag(CN)2- + 4OH-
- a) 2Ag+ +
Fe à 2Ag
+ Fe2+ Eo = +1.24
b) K = 7.9 x
1041
c) DG = -232 kJ