Electrochemistry
Revised 04/29/15
INTRODUCTION TO ELECTROCHEMISTRY: CURRENT, VOLTAGE,
BATTERIES, & THE NERNST EQUATION
Experiment partially adapted from J. Chem. Educ., 2008, 85 (8), p 1116
Introduction
Electrochemical cell
In this experiment, a Cu/CuSO4 reference electrode will be used in conjunction with a silver
wire working electrode. The oxidation of copper takes place at the anode while the reduction of
silver takes place at the cathode. See Figure 1 for a diagram of an electrochemical cell.
e–
Cu2+
Cl –
Figure 1. Scheme of an electrochemical set-up, containing a Cu/CuSO4 reference electrode and
an Ag wire working electrode. (See Figure 2.) The arrows represent the electron and ion flow;
the textboxes highlighted in blue indicate the half-cell reactions.
The balanced electrochemical reaction is as follows:
Cu0(𝑠) + 2Ag+(𝑎𝑞)→2Ag0(𝑠) + Cu2+(𝑎𝑞)
(1)
The line notation for the cell is:
Cu0|Cu2+(0.100 M)ǁ‖Ag+, AgCl(s)ǁ‖Ag0
(2)
The overall standard electrochemical potential, Eocell is:
𝐸ocell = 𝐸ored (Ag+/Ag) – 𝐸ored(Cu2+/Cu)
(3)
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The Nernst Equation
An electrode potential measures the magnitude of the difference between the concentrations of
the species in the half-cell and their equilibrium values. The Nernst equation provides a
quantitative relationship between the concentration of species and the electrode potential. A
reversible half-reaction can be defined as follows:
aA + bB + … + ne– ⇌ cC + dD + …
(4)
where the capital letters represent atoms, molecules, or ions, e– represents electrons, and the
lower-case italic letters represent the number of moles of each species. The electrode potential
corresponding to equation (8) is given by the following equation:
!"
𝐸 = 𝐸 ! − !" ln
! ! ! !…
(5)
! ! ! !…
where,
𝐸 ! = standard electrode potential
R = ideal gas constant, 8.314 J/Kmol
F = Faraday’s constant, 96,485 C/mol eT = temperature, K
n = number of moles of electrons
ln = natural logarithm
The equation can be rewritten by substituting numerical values for the constants, converting to
base 10 logarithms, and defining the temperature as 25°C (roughly room temperature):
𝐸 = 𝐸 ! −
!.!"#$
!
log
! ! ! !…
(6)
! ! ! !…
The letters in brackets technically represent activities, but usually molar concentrations are
substituted into the equation. For example, if species A is a solute, [A] is the concentration of A.
If A is a pure liquid, pure solid, or solvent, the activity is one. In this experiment, Ag+ gets
reduced to Ag in a one-electron process:
Ag+ + e- à Ag (s)
(7)
The Nernst equation describing the electrochemical potential in this experiment is:
!.!"#$
!
!
𝐸!"## = 𝐸!"##
− ! log [!"! ]
(8)
In this portion, students will be monitoring the electrochemical potential at various
concentrations of Ag+, and gain a better understanding of the Nernst Equation by plotting Ecell
versus log
!
!"!
!
, where the intercept is the 𝐸!"##
and the slope is equal to
!.!"#$
!
.
Electrochemical Plating (Electroplating)
Electrochemical plating is a method used to apply a metallic coating on surfaces (e.g. chrome
plating of fenders, grills, and toasters) to protect them against corrosion and passivation, by using
an electric current. The first step in electroplating is to negatively charge the object (to be
coated) by connecting it to an electric circuit and applying a bias voltage. The object is then
dipped into a solution containing a metal salt. The metal cations, in the solution, partake in
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Electrochemistry
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redox reactions and eventually are reduced to a neutral metal which is deposited onto the object’s
surface to become a protective coating. In this experiment, the object is a copper strip and the
metal salt is ZnSO4. The positive pole of a current probe will be connected to the copper strip
and the negative pole to a battery. The positive pole of the battery will be connected to an
aluminum strip (Figure 2). The aluminum strip is used as the anode since it is made out of metal
and therefore has the ability to conduct electrons and transfer electrons to species in solution. In
practice the anode can be any type of small object made out of metal (e.g. Zn, Cu, Fe).
Battery
_
+
_
Current
Probe
+
Figure 2. Scheme of the electrochemical set-up.
The following reaction is observed at the cathode (copper):
ZnSO4(aq) + 2 e– à Zn(s) + SO42-(aq)
As seen in the reaction above the zinc ions in the solution are reduced at the cathode to Zn(s),
which is deposited onto the copper.
A current probe is used to measure the current (I) passing through the system as a function of
time (t, in sec). The total electrical charge passed can then be calculated (qT) using the following
equation (Faraday’s Law):
qT = I · t
(9)
If the electrical charge is known, the mass of zinc (mZn) plated on the copper can be calculated
using the following equation:
qT M
,
(10)
mZn =
nF
where M is the molar mass of Zn (65.39 g/mol), n is the stoichiometry for the number
of electrons in the half reaction, and F is Faraday’s constant (96,485 C/mol)
In this portion of the experiment the students will determine the mass of zinc plated on the
copper strip, theoretically, using the equations above, and experimentally, from the mass of the
copper strip before and after plating.
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Students will also calculate Avogadro’s number, NA, by using the experimentally determined
values of the mass of Zn(s) plated onto the copper strip and the total amount of electrical charge
passed, qT,
qT M
NA =
n mZn qe ,
(11)
where qe is the charge of one electron, and the other variables are as defined in
equation (10).
Safety
Safety goggles and aprons must be worn in lab at all times.
Part A. Silver nitrate solutions are irritating to skin and eyes. It may create black stains on
clothes or discoloration of the skin, and is harmful if swallowed. Store all the waste in the correct
labeled waste container.
Part B. ZnSO4 is harmful and can cause severe irritation upon contact with eyes. When
handling ZnSO4, wear gloves and if it contacts your eyes or skin, flush with water for at least 15
min.
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Procedures
Part A. Understanding the Nernst Equation
The stockroom has provided a reference electrode that includes the following:
• A glass reference electrode with the bottom submerged in 0.1 M CuSO4.
• A 0.1 M CuSO4 solution.
• Silver and copper wires, both connected to electrical wires at one end.
• An NMR cap to seal the chamber.
Figure 3. Image of the Cu and Ag electrode set-ups. The inner (reference) electrode contains a
copper wire dipped into 0.1 M CuSO4 solution. The bottom of this reference electrode contains a
Vycor frit, which must be in solution at all times. A Vycor frit is a porous piece of glass and is
used to allow for ionic conductivity between the filling solution of the reference electrode and the
solution in which the electrode is submerged. After putting the reference electrode together,
place it back in CuSO4 solution until ready for the experiment. The silver wire is placed outside
as the working electrode. One electrical wire is connected to the silver wire while the other
electrical wire is connected to the copper wire.
1. Prepare the computer for data collection by opening "Exp 28" from the Chemistry with
Vernier experiment files of Logger Pro. The computer is now set to monitor potential in
volts. The potential will appear in the Meter window when the leads are connected to a
cell. Verify that when the voltage probe leads are touched together, the voltage displays
0.00 V. When the two leads are not in contact with a cell (or each other), a meaningless
voltage may be displayed.
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2. Calibrate the voltage probe: Go to the "Experiment" menu and choose "Calibrate". In
the window that appears make sure the "Calibration" tab is chosen. Click on "Calibrate
Now". Connect the two ends of the voltage probe together. When the voltage reading in
the calibration window stabilizes enter 0.00 in the field beneath "Enter Value".
3. Use a multimeter to measure the potential of a 9V battery. Record the potential. Connect
the voltage probe leads to the battery. Make sure the voltage is a positive value. When the
voltage reading in the calibration window stabilizes enter the potential you measured with
the multimeter in the second field beneath "Enter Value". Save this calibration for the
rest of the voltage measurements.
4. Measure out exactly 100 mL of nanopure water and pour into a clean, dry 250 mL beaker
that contains a small magnetic stir bar. Place the beaker on a stir plate.
5. Rinse the reference electrode thoroughly with DI water and position it in the beaker so
the stir bar will not hit the electrode. Make sure the silver wire is submerged in solution.
Connect the black lead of the voltage probe to the reference electrode and the red lead of
the voltage probe to the silver wire. Begin collecting data in Logger Pro by clicking
“Start.”
6. Obtain approximately 25 mL of a 0.10 M stock AgNO3 solution. Add exactly 2 mL of
AgNO3 solution to the reaction beaker and wait for the potential to stabilize. Once
stabilized, click “Keep” to record the data point.
7. Repeat this procedure, adding 2 mL aliquots of AgNO3 solution at a time to the beaker
until you’ve added a total of 20 mL of AgNO3. Record the stabilized potential after each
2 mL aliquot added. Click “Stop” once finished and save your data.
8. In the end, the total volume in the beaker should be 120 mL. Calculate the concentration
of Ag+ ion after each step.
Part A Cleanup
1. DO NOT throw any solution in the sink. All waste should be placed in the appropriate
container in the fume hood.
2. Rinse the reference electrode thoroughly with DI water.
3. Place the reference electrode back into its container making sure the Vycor frit is in
solution.
4. Wash and dry the copper wire and silver wire. Return the entire package to the
stockroom.
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Part B. Electrochemical Plating of Zn on Copper
1. Mass a copper strip on a watch glass and record the mass.
2. Check to make sure the voltage of your 9.0 V battery is at least 7 V. Attach the copper strip
(cathode) and the positive pole of the current probe to each end of one electrical wire. Using
a second electrical wire, attach an aluminum strip (anode) and the positive pole of a 9.0 V
battery. Finally attach the negative pole of the current probe and the 9.0 V battery with a
third wire (Figure 2). Connect the current probe to channel 1 of the Logger Pro interface.
Go to the “Experiment” menu and choose “Data Collection”. Change the length to 60 sec
and click “Done”.
a)
b)
Figure 4. a) An aluminum electrode and b) the electrochemical plating setup
3. Pour ~40 mL of 2.0 M ZnSO4 solution into a 100 mL beaker. Click “Collect” and
immerse the copper and aluminum electrodes (make sure the electrodes do not touch)
into the solution at the same time. Using plastic tongs, remove the copper from the
solution after (exactly) 60 sec. Click “ Done”. The current probe cannot measure current
higher than 0.6 A. (If your current is flat-lined at 0.6 A, add a resistor (680 ohms or 470
ohms) in series in your circuit until you observe a peak current that is less than 0.6 A,
followed by a decay in the current. Rerun the 60 second trial with this new copper
electrode and the resistor in the circuit. An alternative is to dilute the concentration of
ZnSO4 from 2 M to 1 M, or more, and rerun the 60 second trial.)
4. Using the air jets in the hood, dry the copper carefully, place it on a watch glass, and
record the final mass, (mfinal – minitial = mass of zinc plated on the copper). After
obtaining a satisfactory plating current line (i.e. one where the current peaks at first and is
then followed by a decay), determine the charge passed by selecting only data under that
curve. Go to the “Analyze” menu and choose “Statistics” and record the mean of the
current. Another way to determine the charge passed is go to the “Analyze” menu and
chose “Integral”. Use both of these methods in order to calculate the charge passed.
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Compare your results. Repeat the above plating experiments for 120 sec and 180 sec.
(No need to change the ZnSO4 solution but new copper and aluminum electrodes is
needed).
5. Pour the zinc solution into the waste bottle in the hood. Do not flush it down the drain.
The aluminum strip should be placed in the collection beaker in the hood. Do NOT,
under any circumstances, throw the solid electrodes in the trash. Wash your hands
thoroughly before leaving the lab.
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