CHEMISTRY 104
Hour Exam I
1.
Summer 2012
Page 1
Consider the following reactions for which the sign of the enthalpy change is given.
Which of the reactions can never be spontaneous at any temperature?
a.
2 H202(l) - > 2 H20(l) + 02(g)
b. 302(g) ^
AH(-)
203(g)
AH(+)
c. CaC03(s)
CaO(s) + C02(g)
d. 4 Fe(s) + 3 02(g)
2 Fe203(s)
2.
Which of the following statements is false?
a.
b.
c.
d.
e.
3.
AH(+)
AH(-)
The sign of AG for a reaction indicates whether the reaction is spontaneous.
I f AG° is positive for a reaction, then K is less than 1 (K < 1).
When Q = K for a reaction, then AG = 0.
The entropy of the universe is increasing.
The equilibrium position represents the highest free energy state available to a
reaction.
Which of the following statements is false regarding electrochemical cells?
a. Galvanic cells utilize spontaneous oxidation-reduction reactions.
b. Electrolytic cells are based on oxidation-reduction reactions having positive AG
values.
c. The cell potential (Eceii) in a concentration cell is produced by constructing a
galvanic cell where both the anode and cathode compartments contain the same
components but at different ion concentrations.
d. When the concentration of a reactant ion in a galvanic cell is increased, the cell
potential (Eceii) decreases.
e. Batteries are examples of galvanic cells.
4.
Hydrogen-oxygen fuel cells are utilized in some cities to produce electricity. The fuel
cell reaction and the standard cell potential are:
2 H2(g) + 02(g) ^
2 H20(l)
E^ = 1.23 V
Calculate the maximum amount of work this fuel cell reaction could produce for the
production of 2 mol of H20(l) assuming standard concentrations and T = 25° C.
a. -948 kJ
b. -237 kJ
c. -475 kJ
d. -119 kJ
e. -82.5 kJ
CHEMISTRY 104
Hour Exam I
5.
Summer 2012
Page 2
Consider the following standard reduction potentials:
E° (volts)
F2 + 2e"
2F~
Ag^ + e" ^ A g
Cu2^ + 2e" ^ C u
Pb^^ + 2 e " - ^ P b
Ni^* + 2e" ^ N i
2.87
0.80
0.34
-0.13
-0.23t
Assuming standard conditions, copper (Cu) will spontaneously reduce which of the
following?
6.
a. Ag and F~
b. Ag"^ and F2
d. Pb^ and Ni^"^
e. none of these
Cadmium sulfide (CdS) is used in some semiconductor applications. Calculate the value
of the solubility product (Ksp) for CdS given the following standard reduction potentials.
CdS + 2e"
Cd^^ + 2e" ^
7.
c. Pb and Ni
Cd + S^
Cd
E° = - 1 . 2 1 V
E° =-0.40 V
a. 3.9x10"^^
b. 3.3x10"^^
d. 3.0x10^^
e.
c. 2.1 x 10~"^
2.6x10^^
Consider the following reduction potentials:
E°
Cl2 + 2e"
2Cr
O2 + 4
+ 2e" -> 2 H2O
2 H2O + 2e" ^ H2 + 2 0H~
Na* + e"
Na
1.36 V
0.82 V
-0.41 V
-2.71 V
When molten NaCl is electrolyzed, Na(s) and ClaCg) are produced. When aqueous NaCl
is electrolyzed, H2(g) and Cl2(g) are produced. Which of the following statements
best explains why H2(g) is produced instead of Na(s) in an aqueous solution of NaCl?
a)
b)
c)
d)
H2O
H2O
H2O
H2O
is more
is more
is more
is more
easily
easily
easily
easily
reduced relative to Na.
oxidized relative to Na.
reduced relative to Na^.
oxidized relative to Na"^.
CHEMISTRY 104
Hour Exam I
8.
Summer 2012
Page 3
Which of the following statements if false?
a. Corrosion involves the oxidation of iron.
b. Corrosion is an example of an electrolytic cell.
c. Sacrificial metals (or cathodic protection) utilize metals that are more easily oxidized
than iron.
d. Dry climates tend to have less of a problem with corrosion as compared to climates
with ample moisture.
e. Paint or protective oxides can help prevent corrosion by eliminating contact of the
iron with 02(g).
^
9.
How many of the following four relationships describe a basic solution?
[ H i < 1.0
a.
X
O(None)
10"^ M ;
b. 1
pOH < 7.00;
pH > 7.00;
c. 2
[OIT] > 1.0 x 10"^ M
d. 3
e. 4 (All describe a basic solution.)
10.
Consider several 1.0 M solutions each containing a different acid, HA. How many of the
following four statements (I-IV) is/are true?
I.
As the Ka value of the acid increases, the pH of the solution increases.
II.
As the Ka value of the acid increases, the percent dissociation of the acid increases.
III. As the Ka value of the acid increases, the [H"^] of the solution increases.
IV. As the Ka value of the acid increases, the Kb value for the conjugate base decreases.
a. 0 (none)
11.
b. 1
c. 2
d. 3
e. 4 (All statements are true.)
One mole of a weak base, B, is dissolved in 1.0 L of water and no other substance is
added. Which of the following, to a close approximation, is true?
a. [ B H 1 > [ B ]
b. [B] = [ O H ]
d. [ H l > [ O H - ]
e. [ B H l > [ O H " ]
c. [ B H ^ = [ O H ]
CHEMISTRY 104
Hour Exam I
12.
Summer 2012
Page 4
H2SO4 is a diprotic acid with the following Ka values:
H2SO4 ^ HSO4" + H^
Kai = 1 X 10^
HSO4"
Ka2= 1.2x 10"r2
S04^" + H^
Which of the following statements (a-c) is true?
13.
a.
The pH of a 0.10 M H2SO4 solution should be peater than one (pH > 1.00).
b.
HSO4" is a stronger acid than H2SO4.
c.
HSO4" is a stronger base than S04^~.
d.
None of the above statements (a-c) are true.
Which of the following compounds will not contribute to the acid rain problem, i.e., will
not form an acidic solution when dissolved in water?
a.
14
Na20
b.
SO2
c.
SO3
d. NO2
e.
CO2
The equilibrium constants Ka and Kb refer to specific reactions. Which of the following
four reactions (a-d) is incorrectly matched with the equilibrium constant?
a. H C N ^ H * + CN~
Ka for HCN
b. HONH2 + H2O ^ HONHs^ + OET
KbforHONH2
c. F + H2O
Kbforl^
OH" + HE
d. NH4* i : ; NH3 + H^
Ka for NEU^
e. A l l the above reactions are correctly matched with the equilibrium constant.
CHEMISTRY 104
Hour Exam I
Summer 2012
Page 5
The next three questions refer to the titration of 100.0 mL of 0.40 M HOBr by 0.20 M KOH. The
Ka value for HOBr is 2.0 x 10"^
15.
Calculate the pH after 50.0 mL of KOH has been added.
a. 3.19
16.
d. 8.22
e. 7.00
b. 9.18
c. 8.70
d. 8.22
e. 7.00
Calculate the pH after 200.0 mL of K O H has been added.
a. 12.20
18.
c. 8.70
Calculate the pH after 100.0 mL of K O H has been added.
a. 3.19
17.
b. 9.18
b. 3.09
c. 10.91
d. 4.79
e. 9.21
Arrange the following 0.10 M solutions in order of increasing pH:
H I , HF, NaF, Nal
19.
a. H I < HE < NaF < Nal
b. H I < HF < Nal < NaF
d. HF < H I < NaF < Nal
e. NaF < Nal < H I < HF
c. HF < H I < Nal < NaF
When first studying acid-base chemistry, students sometimes assume that the conjugate
base of a weak acid is a strong base. However, this assumption is false. Which of the
following best explains why the conjugate base of a weak acid is not a strong base?
a.
b.
c.
d.
The conjugate
The conjugate
The conjugate
The conjugate
bases of weak acids all have Kb values greater than 1.
bases of weak acids all have Kb values equal to 1.
bases of weak acids all have Kb values less than 1.
bases of weak acids all have Kb values less than Kw (= 1.0 x 10"^"^).
CHEMISTRY 104
Hour Exam I
20.
Summer 2012
Page 6
Consider 0.25 M solutions of each of the following potassium salts:
KCl, K I , KBr, KNO3
How many of the solutions are basic?
a. O(none)
b. 1
c. 2
d. 3
e. 4 (All are basic solutions.)
21.
Consider the following reaction at some constant temperature:
12(g)+ H2(g)
^2HI(g)
K = 0.75
Initially, 2.0 mol of I2, 2.0 mol of H2 and 2.0 mol of H I are mixed together in a 1.0 L
rigid container and are allowed to react to reach equilibrium. Which of the following
statements is true once this reaction reaches equilibrium?
a.
b.
c.
d.
e.
22.
23.
At equilibrium,
At equilibrium,
At equilibrium,
At equilibrium,
At equilibrium,
[I2] > [HI].
[I2] = 2.0 M .
[I2] > [H2].
[H2] < 2.0 M.
[HI] = 2.5 M.
Which of the following solutions has pOH = 3.00?
a. 3.0MNaOH
b. 5.0 x 10"^MCa(OH)2
d.
e.
3.0MHC1
c.
1.0 x 10"^^ M H C l
1.0 x 10"^^ M N a O H
The [H""] in a buffer solution consisting of HONH3CI and HONH2 is equal to the Ka value
for HONHs^ ([H^] = Ka = 9.1 x 10"^ M). Which of the following statements about this
solution is true?
a. In this solution, the concentration of HONHs"^ equals the concentration of HONH2
([HONH3I = [HONH2]).
b. I f NaOH is added to this solution, the pH will decrease.
c. If HCl is added to this solution, the [H^] will decrease.
d. If HCl is added to this solution, the concentration of HONHs"^ will decrease and the
concentration of HONH2 will increase.
e. The pH of this solution would be lower than the pH of a solution where the
concentration of HONHs"^ is greater than the concentration of HONH2
([HONH3I > [HONH2]).
CHEMISTRY 104
Hour Exam I
24.
25.
Summer 2012
Page 7
A solution is to be buffered at pH = 4.5. Which of the following pairs of substances
would be the best choice for buffering this solution?
a.
HNO2 (Ka for HNO2 = 4.0 x 10"^) and KNO2
b.
HOCl (Ka for HOCl = 3.5 x 10"^) and Ca(OCl)2
c.
HCIO4 (Ka for HCIO4 - 10^) and NaC104
d.
CH3NH2 (Kb for CH3NH2 = 4.4 x 10""^) and CH3NH3NO3
e.
C6H5NH2 (Kb for C6H5NH2 = 3.8 x 10"^^) and C6H5NH3CI
^
Consider the following reaction at equilibrium:
N2(g) + 3 H2(g) ^ 2 NH3(g)
AH = -92.0 kJ (exothermic)
Which of the following changes will shift the equilibrium to the right (to products)?
1. increasing the temperature
2.
3.
4.
5.
6.
7.
8.
a. 1, 4, 6, 7
decreasing the temperature
increasing the volume of the reaction container
decreasing the volume of the reaction container
removing some NH3
adding some NH3
removing some N2
adding some N2
b. 2, 3, 5, 8
c. 1, 6, 8
d. 2, 4, 5, 8
e. 1, 3, 5, 7
CHEMISTRY 104
Hour Exam I I
Summer 2012
Page 8
Written out problems - Show all work for partial credit.
26.
Consider a galvanic cell at 25''C based on the following half-reactions:
(12pts)
Al^^ + 3e" ^
Al
E° = -1.66 V
Ni^^ + 2e" -> Ni
E° = -0.23 V
a) Draw the cell under standard conditions labeling the anode, the cathode, the
^ direction of electron flow, the concentration of ions, the electrodes, and the
direction of flow of cations and anions through the salt bridge.
b) To the standard cell above, OH" is added to the aluminum compartment causing
Al(OH)3(s) to precipitate. After precipitation of Al(OH)3(s) has ceased, the concentration of OH" is 1.0 X 10""^ M and the measured cell potential is 1.82 V.
Calculate the Ksp value for Al(OH)3(s).
Al(OH)3(s) ^
Ar^(aq) + 3 OHr(aq)
Ksp = ?
CHEMISTRY 104
Hour Exam 11
27.
Summer 2012
Page 9
Consider an aqueous solution of copper nitrate, Cu(N03)2. It took 1380 seconds using a
current of 2.00 A to plate out all the copper from 25.0 mL of the copper nitrate solution.
What was the original concentration of copper nitrate in the solution? Calculate the
concentration in units of molarity (= mol/L).
(4 pts)
28.
Consider a weak acid, HX. I f a 0.10 M solution of HX has a pH of 5.83 at 25°C, what is
AG° for the acid's dissociation reaction at 25°C?
(6 pts)
HX(aq) ^
ir(aq) + X~(aq)
AG° = ?
CHEMISTRY 104
Hour Exam I I
Summer 2012
Page 10
29.
Consider the following solutions for the next three questions.
(13 pts)
I.
100.0 mL of 0.50 M (C2H5)3N (Kb for (C2H5)3N = 4.0 x 10"^)
I I . 50.0 mL of 0.30 MHCIO4
I I I . 50.0 mL of 0.30 M K O H
IV. 100.0 mL of 0.50 M (C2H5)3NHI
a. Calculate the pH of solution IV.
b. Calculate the pH of the resulting solution when solutions I , I I I , and I V are mixed
together.
c. Calculate the pH of the resulting solution when solutions I , I I , I I I , and IV are mixed
together.
CHEMISTRY 104
Hour Exam 11
Summer 2012
Page 11
30.
Consider the following two titrations:
(18 pts)
a. 50.0 m L of 0.100 M HCIO4 titrated by 0.0500 M KOH.
b.
80.0 m L of 0.250 M HONH2 (Kb = 1.1 x 10"^) titrated by 1.00 M HBr.
For each of the two titrations, determine the pH at the halfway point to equivalence and
the pH at the equivalence point. Also sketch the general shape of each titration curve
showing the pH of the two points that you calculated. In order to receive credit, you must
show all work. An answer With no supporting work will be marked as incorrect.
Determine all pH values to 2 decimal places.
a. 50.0 m L of 0.100 M HCIO4 titrated by 0.0500 M KOH
b.
80.0 m L of 0.250 M HONH2 (Kb = 1.1 x 10"') titrated by 1.00 M HBr