Electrochemistry
Revised 3/6/14
INTRODUCTION TO ELECTROCHEMISTRY:
CURRENT, VOLTAGE, & BATTERIES
INTRODUCTION
Electrochemical Cells
In this part of the experiment, four “half cells” are created by immersing metal strips of zinc,
copper, aluminum, and magnesium in aqueous solutions containing cations of the same element
(Zn2+, Cu2+, Ni2+, and Mg2+). An electrochemical cell is created when two of these “half cells”
are connected by a KCl salt bridge and a wire (the leads from the voltage probe). Six different
electrochemical cells can be created from the four half cells above. A positive cell potential
is measured when the black lead is connected to the anode and the red lead is connected to the
cathode. Cell notation will be used to describe the electrochemical cells. A reduction table will
be created by designating copper as the standard electrode.
Cu2+(aq) + 2 e− à Cu(s)
E = 0.00 V
Cu2+(aq) + 4 NH3(aq) D [Cu(NH3)4]2+(aq)
Keq = 1.2 x 1012
Electrochemical Plating (Electroplating)
Electrochemical plating is different method used to apply a metallic coating on the surfaces
(fenders, grills, and toasters) to protect against corrosion 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 current. The object is then dipped into a solution containing a metal salt. The metal
cation, in the solution, is eventually reduced to a neutral metal and deposited onto the object’s
surface to become a protective coating. In this experiment, a copper strip will be the object and
ZnSO4 is the metal salt. The positive pole of a current probe will be connected to the copper and
the negative pole to a battery. The positive pole of the battery will be connected to a paper clip
(Figure 2). The paper clip is used as the anode since it is made out of metal and therefore has the
ability to transfer electrons into the solution. In practice the anode could be any type of small
object made out of metal (Zn, Cu, Fe etc).
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Electrochemistry
Revised 3/6/14
Figure 2. Scheme of the electrochemical set-up.
The following reaction is observed at the cathode (copper):
ZnSO4(aq) + 2e- 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 electrical charge can then be calculated using the following equation
(Faraday’s Law):
(1)
q=It
If the electrical charge is known, the mass of zinc plated on the copper can be calculated from
the following equation:
(2)
m(Zn) = q M / n F
M = atomic mass of Zn: 65.39 g/mol
n = number of electrons in the half reaction
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Electrochemistry
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F = Faraday’s constant: 96 485 C/mol
In this experiment the students will determine the mass of zinc plated on the copper; theoretically
using the equations above and experimentally from the weight of the copper before and after the
plating.
Students will also calculate Avogadro’s number, N0. The experimentally determined values of
the mass of Zn (s) plated on the copper strip and the amount of charge used to do so will be used
to obtain Avogadro’s number. Equation (1) can be used to find the total charge consumed.
N0 =
(3)
qM
n m qe
where q is the charge, M is the atomic mass of Zn, n is the number of electrons in the half
reaction, m is the mass of Zn (s) plated onto the copper strip, and qe is the charge of one electron.
SAFETY
Safety goggles and aprons must be worn in lab at all times.
Part A. Nickel can cause contact dermatitis and solutions containing nickel ions may be
carcinogenic. Wear gloves when handling the metal or solution.
Part B. Solutions containing NH3 must be prepared in the hoods. Ammonia and sulfuric
solutions are corrosive and can cause burns and respiratory problems; wash all affected areas
thoroughly with cold water.
Part C. ZnSO4 is harmful and can cause severe irritation when in contact with eyes. When
handling ZnSO4 wear gloves and if in contact with eyes and skin flush with water for at least 15
min.
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Electrochemistry
Revised 3/6/14
PROCEDURES
Part A: Creating Electrochemical Cells
The stockroom will provide the following 0.1 M solutions: ZnSO4, CuSO4, NiSO4, MgSO4,
and their metal electrodes (solid Zn, Cu, Ni, and Mg). (A key to help you identify the metal
electrodes should be displayed somewhere in lab.) Obtain a spot plate from the plastic tub in the
hood and clean it before use. Use sandpaper to remove any impurities from the metal electrodes;
rinse with water and dry after sanding. Put about 25 drops of CuSO4 and a Cu electrode in
one of the wells to create a Cu2+/Cu half cell. Repeat the same procedure with the remaining
solutions and electrodes, recording their location on the spot plate in your notebook. As shown
below in Figure 3, place the wells adjacent to each other, forming a square so they can easily
be connected by a salt bridge. To make the salt bridge, take a piece of filter paper and soak it in
saturated KCl solution.
Figure 3. Electrochemical Cell Set Up
Prepare the computer for data collection by opening "Exp 28" from the Chemistry with
Computers 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.
When the voltage probe leads are touched together, the voltage should display 0.00 V. When
the two leads are not in contact with a cell (or each other), a meaningless default voltage may be
displayed.
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".
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Electrochemistry
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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". Connect the Mg and Cu half
cells with a salt bridge (a small strip of filter paper saturated with 1 M KCl(aq)). Connect the
red voltage probe lead to the Cu electrode, and the black voltage probe lead to the Mg electrode.
When the voltage reading in the calibration window stabilizes enter 2.71 in the second field
beneath "Enter Value". Save this calibration set-up for the rest of the voltage measurements.
Select any two cells and connect them by the salt bridge (e.g. place one end of the salt bridge in
the Cu cell and the other end in the Zn cell). Determine the potential by touching the voltage
probes to the electrodes in the cells. Do this by bringing the black lead of the probe in contact
with one metal electrode and red lead in contact with the other electrode. If the voltage reads
0.00 V, then reverse the leads until you have a positive voltage. Wait about 5 seconds to take a
voltage reading and record the value in your notebook. If the potential fluctuates considerably,
sandpaper the electrode gently to remove oxides and impurities.
Determine which cell was the anode and which was the cathode: If the measured voltage is
positive, the cell connected to the black lead is the anode and the cell connected to the red lead is
the cathode. Once you have recorded this information, measure the potentials for the remaining
cells, making as many combinations of two cells possible with the solutions provided. Be sure to
note the anode and cathode for each combination and use a new salt bridge for each set of cells.
Part B: Measure Concentration Effect on an Electrochemical Cell
Measure the voltage again for the Cu/Zn cell. Add 1 drop of 6 M NH3 solution to the Cu well
(stir with toothpick) and record the voltage. Place a piece of white paper under the spot plate to
observe the color of the Cu(NH3)42+ complex ion that is formed. (This color test is one that is
frequently used to determine the presence of copper (II) ion in a solution.) Add one more drop
of NH3 and measure the voltage again. (Did you see any voltage change?)
Use a disposable pipet and carefully transfer each solution from its well into the collection bottle
in the hood. Place the empty spot plate into the large plastic tub in the hood. Do this carefully as
a dilute bleach solution is in the tub, which can spot clothing.
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Electrochemistry
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Part C: Electrochemical Plating Zn on Copper
Weigh the copper electrode on a watch glass and record the mass.
If electrical wires with alligator clips are not available, follow these instructions to create them:
Use a wire cutter to cut off three ~10 cm pieces from the electrical wire. Strip off about 1 cm of
the coating of each wire with the cutter. Attach each end of the wire to an alligator clip (through
the hole in the back, figure 5).
Attach one of the electrical wires to the copper and the positive pole of the current probe and
attach a second wire to the Al used in Part A and the positive pole of a 9.0 V battery. Finally
attach the negative poles of the current probe and the 9.0 V battery with a third wire (Figure
6). 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”.
copper (cathode) à
Figure 5. The penny (copper) electrode
ß Al (anode)
Figure 6. Electrochemical set up.
and Al electrode.
Pour ~40 mL of 2.0 M ZnSO4 solution into a 100 mL beaker. Click “collect” and immerse the
electrodes (the copper and the Al, make sure the electrodes don’t 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 flatlined
at 0.6 A, add resistor (680 ohms or 470 ohms) in series circuit to the set up until you observe a
peak current, followed by a decay in current. Rerun the 60 second- trial with this new copper
electrode connecting to resistor. Another way is to rerun with diluted concentration of ZnSO 4
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Electrochemistry
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from 2M to 1M).
Using the air jets in the hood, dry the copper carefully, place it on a watch glass, and record the
final mass, (mf – mi = mass of zinc plated on the copper). After finishing the experiment with a
good plating current line (a current peak at first then decay), select only data under that curve.
Go to the “Analyze” menu and choose “Statistics” and record the mean of the current. Other
way to find the charge is go to “Analyze” menu and chose “ Integral”. Use both of the method
above for calculation and compare the results. Repeat the above plating experiments for 120 sec
and 180 sec. (No need to change the ZnSO4 solution but a new copper and aluminum is needed).
Pour the zinc solution into the waste bottle in the hood. Do not flush it down the drain. The
paper clip should be placed in the beaker in the collection beaker in the hood. Do NOT, under
any circumstances, throw the solid electrodes in the trash. Wash hands thoroughly before
leaving lab.
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