CHEMISTRY
C317:
EQUILIBRIA
AND
ELECTROCHEMISTRY
Spring, 2008
Practice
Examination 2
1.
Suppose that you wish to attempt the separation of Cu
2+
from SbO
+
by controlledpotential deposition of copper
metal onto a platinum cathode.
Assume that the original sample solution contains 1.00 M H
+
, 0.0350 M Cu
2+
,
and 0.500 M SbO
+
, and that [H
+
] remains at 1.00 M throughout the course of the electrolysis.
Cu
2+
+
2 e
–
Cu
;
E
º
=
+0.337 V vs. SHE
SbO
+
+
2 H
+
+
3 e
–
Sb
+
H
2
O
;
E
º
=
+0.212 V vs. SHE
(a)
At what
potential
(versus SHE) must the platinum cathode be controlled to cause 99.9% deposition of
elemental copper?
(b)
What
fraction
of the SbO
+
will be reduced to the elemental (Sb) state under the experimental
conditions imposed in part (a)?
(c)
If the volume of the original sample solution is 100.0 mL, what will be the total weight gain (
in milligrams
)
of the platinum cathode under the experimental conditions imposed in part (a)?
[Atomic masses:
Cu
=
63.55;
Sb
= 121.76]
2.
An unknown Cd
2+
solution was analyzed by means of polarography.
A 30.00mL sample of the unknown
solution was found to yield an average diffusion current, (
i
d
)
average
, of 2.06 µA.
Next, a 4.00mL aliquot of a
standard 2.59 × 10
–3
M Cd
2+
solution was added to the 30.00mL unknown sample, and the resulting mixture
gave an average diffusion current, (
i
d
)
average
, of 5.41 µA.
Calculate the
molar concentration
of Cd
2+
in the
original unknown solution.
3.
For a cyclic voltammetry experiment, one can use the RandlesSevčik equation
i
p
=
(2.67 × 10
5
)
n
3/2
D
1/2
v
1/2
A
C
to calculate the peak current (
i
p
) in amperes, as a function of the number of electrons transferred (
n
), the diffusion
coefficient (
D
) in cm
2
/sec, the scan rate (
v
) in volts/sec, the area of the electrode (
A
) in cm
2
, and the concentration
(
C
) in mol/cm
3
.
In a certain set of experiments, the reversible twoelectron oxidation of
o
dianisidine was
explored.
NH
2
H
2
N
OCH
3
H
3
CO
o
dianisidine
(a)
When a carbon electrode with an area of 0.0273 cm
2
was used to study the twoelectron oxidation of a 2.27
×
10
–3
M solution of
o
dianisidine at a scan rate of 0.500 volt/sec, the peak current was found to be 8.19
µA.
Calculate the
diffusion coefficient
(
D
) for
o
dianisidine.
(b)
If the carbon electrode used in part (a) was employed to investigate the cyclic voltammetric behavior of a
8.20
×
10
–3
M solution of
o
dianisidine, what
peak current
(
i
p
) would be expected for a scan rate of 0.100
volt/sec?
(c)
Suppose that a solution containing an unknown concentration of
o
dianisidine was analyzed by means of
cyclic voltammetry.
For a carbon electrode with an area of 0.0775 cm
2
, the observed peak current (
i
p
) was
21.9 µA at a scan rate of 0.200 volt/sec.
Calculate the
molar concentration
of
o
anisidine?
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4.
A mixture of Ni
2+
and Co
2+
was analyzed by means of controlledpotential electrolysis.
Exactly 10.00 mL of the
Ni
2+
–Co
2+
mixture was pipetted into an electrolysis cell containing an electrolyte solution.
Quantitative reduction
of Ni
2+
to elemental nickel was carried out at a cathode potential of –0.95 V vs. SCE, and 72.85 coulombs of
electricity was consumed in this process.
Next, the potential was readjusted to –1.20 V vs. SCE, whereupon the
Co
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 Spring '09
 Peters
 Electrochemistry, anodic peak, Selective Electrode Coatings

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