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Experiment V
Voltaic Cells and the Nernst Equation
Outline of Experiment
You will review concepts of voltaic cells from high school
and apply the Nernst equation to obtain metal ion
concentrations in a number of test solutions.
E = E°- 0.0592
log Q
n
Q
E°
n
Reaction Quotient
potential difference under
standard conditions
# of electrons exchanged
Background
e-
V
eAnode
(oxidation)
negative
Cathode
(reduction)
positive
Salt Bridge
Example: Zn|Zn
2+||Cu 2+|Cu
anode
Zn
Zn
cathode
Cu 2+ + 2e-
2+
+ 2eCu
2
I. Cell Potentials at Standard Conditions
A displacement series will be constructed in order of increasing
reduction potentials, by comparing the potential differences
obtained from cells made by combining a series of half-cells:
Each solution will be placed in a 24 - well plate as shown below
and the electrode added to form the half-cell. The lines
represent filter paper salt bridges.
volt
meter
leads
side view
filter paper
aligator
clips
Zn
Cu
Zn(NO3 )2
KNO3
CuSO4
For the example shown, a voltage difference of 1.10V
might be obtained.
II. Halogen Displacement Series
The reactivity between the halogens iodine and bromine are
compared. To do this, a halide is mixed with a halogen, and
then hexane is added to check whether a reaction occurred.
For example, suppose that I-(aq) is mixed with Br2(aq).
There are only two possibilities,
2I-(aq) + Br2(aq) => no reaction
or
3
2I-(aq) + Br2(aq) => I2(aq) + Br-(aq)
After the solution is shaken, hexane (which is immiscible
with water) is added and the mixture is re-shaken. If the color
of the hexane is that due to Br2, then no reaction has
occurred. If the color of the hexane is that due to I2, then the
I-(aq) has displaced the Br2 from solution.
III: Eº and Equilibrium Constant
You will
. determine K for:
2Ag + + Cu(s) → 2Ag(s) + Cu 2+
To do this, you will measure the voltage of a series of cells,
Ag|Ag + ( 1, 0.01, or 0.0001 M ) || Cu 2+ (1M) | Cu
and prepare a graph of Ecell against log(1/[Ag + ]).
x
x x
Ecell
x x
0
0
2
4
6
log (1/ [Ag +] )
8
The intercept at Ecell = 0 will be used to calculate K using
K = [Cu 2+ ] / [ Ag + ]
2
4
IV: Formation Constant of a Complex Ion
You will
. determine Kf for
+ + 2S ! 2 _
Ag
2 3
Ag (
3
S2 !3 )2
_
The test solution will be prepared by adding 2.0 mL of
0.10M Ag + to 2.0 mL of 0.50 M sodium thiosulfate. The
reference solution will be 1.0 M Ag + . An Ag strip will
be added to each solution, and the half-cells are connected
with filter paper/KNO3 salt bridges.
. calculate, from the measured cell potential and the Nernst
Equation, [Ag + ] in the test solution at equilibrium.
. calculate, from knowing the initial amounts of reagents
mixed and the stoichiometry of the reaction, the
concentrations at equilibrium of (S2O3) 2and
Ag(S2O3)2 3. calculate the product of these concentrations to get
Kf =
_
3
[Ag( S O )
]
2 3 2
_
+
2
[A g ][ S 2O3
]2