University of Puget Sound
Department of Chemistry
Chem 230
EXP. 8 - ELECTROCHEMISTRY®
LABORATORY OBJECTIVES AND ASSESSMENTS
1. Be familiar with the construction and properties of electrochemical cells.
a. Demonstrate construction of a working electrochemical cell.
b. Create a table ranking reduction half-cell potentials from experimental
measurements.
c. Convert measured half-cell reduction potentials to values relative to S.H.E.
d. Predict new cell potentials from measurements of half-cell potentials.
INTRODUCTION
There are two parts to this lab. In the first part of the lab you will develop a reduction
potential table for five metal ions. One metal will be chosen as a reference and the other
four will have their voltage, or potential difference, measured against that standard using
a voltmeter. In the second part of the lab you will put your knowledge to practical use by
building a battery to produce electricity. All of these electrochemical reactions are
reduction-oxidation (REDOX) reactions. In these types of reactions one species is oxidized
and loses an electron(s) and another species is reduced and gains an electron(s). When
you join these two types of reactions together and make the electrons do work you have
an electrochemical cell. The oxidation is considered half of the overall reaction and is
called a half-cell, just as the reduction is the other half reaction or half-cell.
PART 1
Half-cells are normally made by placing a metal into a solution containing a cation of
the metal, for example putting Cu metal in a solution of copper sulfate, CuSO4, which
contains Cu2+ ions. An important property of electrochemical measurements is the fact
that one measures not half-cell potentials, but full-cell potentials, i.e., the difference
between one half-cell and another. The half-cell connected to the black lead is usually
called the “reference”. For all your measurements this lab, you will use the M1 half-cell
(the Cu/Cu2+ half-cell) as the reference, effectively assigning to it E0=0.00 volts. Then the
voltage of all the other half-cells will be just whatever the voltmeter reads. You will make
a table of half-cell potentials (E0) for each metal/metal ion half-cell, relative to M1.
EXPERIMENTAL PROCEDURE
Some of the metals and solutions are toxic. Take care not to purposely handle the
metals or solutions. If you have contact with the metals or solutions wash your hands
with soapy water.
1. Obtain a piece of filter paper and draw five circles with connecting lines as
shown in Figure 1. Using a pair of scissors cut wedges between the circles as
shown. Label the circles M1, M2, M3, M4, and M5. Place the filter paper on top
of the Petri dish or glass plate.
Page 1 of 6
Experiment 8- Electrochemistry
2.
3.
4.
5.
6.
Figure 1. Layout of half-cells on filter paper
Obtain one piece of each of the 5 metals– M1, M2, M3, M4, and M5. If needed
sand each piece of metal so there is a clean surface on both sides. Carefully
look at each metal and describe its characteristics, e.g., color, shape, etc. Write
your descriptions in Table 1.
Note: Do one pair of metals at a time. Place 3 drops of the appropriate metal
ion salt solutions on its circle. Then place the corresponding pieces of metal
on the wet spots. The top side of the metal should be kept dry. Now add
several drops of 1 M NaNO3 to the line drawn between each circle and the
center of the filter paper (this is the salt bridge). Be sure there is a continuous
trail of NaNO3 between each circle and the center.
Determine the potential of the four cells by connecting the black lead to M1
and the red lead to M2, M3, etc. Record the voltages you measure in Table 2.
Now measure the potentials of all cell combinations indicated in the
“Vmeasured” column of Table 3.
When you have finished, use forceps to remove each of the pieces of metal
from the filter paper. Rinse each piece of metal with tap water, dry the metal
and return it to the correct container. Use forceps to remove the filter paper
from the glass plate, and discard it as directed. Rinse the glass plate with tap
water, making sure that your hands do not come in contact with wet spots on
the glass.
PROCESSING THE DATA
1.
Calculate the potential of the cell combinations appearing in Table 3 using the
data from Table 2. Write these values in the “Vcalculated” column of Table 3.
Calculate the % error for each of the potentials and enter in the table.
Percent error =
Vmeasured − Vcalculated
× 100%
Vcalculated
€
Page 2 of 6
Experiment 8- Electrochemistry
2.
Add 0.34 V to the half-cell reduction potential for each of your results and
enter in the second column of Table 4.
Reorder the rows so that the highest E0 appears at the top of the table to the
highest E0 at the bottom. Next, determine the probable identity of metals M2
through M5 by comparing these reduction potentials with the chart in your
text. While you’re at it, enter your text’s values for E0 for each metal.
Table 1.
Description of metal/metal cation unknowns.
METAL
DESCRIPTION
M1
M2
M3
M4
M5
Table 2.
Potentials of metal/metal cation unknowns relative to M1.
Metal Combination
(Black/Red)
Measured Potential
relative to M1 (V)
M1/M2
M1/M3
M1/M4
M1/M5
Table 3.
Predicted and Measured Potentials Between Metal Pairs.
Voltiac Cell
Measured Potential
Calculated Potential
(Black/Red)
(Vmeasured)
(Vcalcuated)
M 2 /M 3
M2/M4
M2/M5
M3/M4
M3/M5
M4/M5
Page 3 of 6
Experiment 8- Electrochemistry
% Error
Table 4.
Metal
(Mi)
Ranked Standard Reduction Potentials.
Reduction Potential E°
relative to SHE
Metal listed in text
with closest E°
(your measurements)
Reduction Potential E°
relative to SHE
(from your text)
PART 2
In Part 1 you investigated various simple cells, under standard conditions, and
established a standard reduction potential table for four unknown metals. Now you will
put your knowledge of standard reduction half-cell potentials to practical use by building
a battery to produce electricity. A battery utilizes a spontaneous oxidation-reduction
reaction to produce electrical energy. From the results of Part 1, decide which two metals
will have the largest chemical potential (volts) and build a larger version of the reaction as
shown in Figure 2. You will then systematically replace the solid metal electrodes with
graphite and see how this affects the cell.
The next step is to design a cell with the largest voltage from the expanded list of
possible metals and solutions below. The ultimate goal of this part of the lab is to produce
the best battery possible (High V and A). Your battery will be monitored using a voltmeter
and will be tested for its maximum voltage and current. The batteries you construct and
test will be set up as Daniell cells.
An example of a Daniell cell is shown in Figure 2. Your Daniell cells will consist of
electrodes of your choice and solutions of your choice, within the available materials listed
below. Evaluations of the best batteries will be done during the last part of the lab period.
EQUIPMENT AND MATERIALS:
In order to maintain safe laboratory conditions, the limitations of equipment and
materials are given below. The equipment available to each lab pair to use to build the
Daniell cell is the following:
2 wires with alligator clips on each end
2 pieces of any two metals
2 solutions (~30 mL) of your choice
1 porous ceramic cups for the Daniell cell
1 100-mLbeaker
Page 4 of 6
Experiment 8- Electrochemistry
Figure 2. Example of a Daniell cell using copper and zinc.
EXPERIMENTAL PROCEDURE (PART 2):
Solutions available include: (all 0.1M)
0.1M CuSO4
0.1M H2SO4
0.1M Al2(SO4)3
0.1M AgNO3
0.1M ZnSO4
0.1M Fe(NO3)2
0.1M SnCl4 (in HCl)
Metals available include:
Cu
Mg Zn
Fe
Al
C
Sn
Ag
Pb
Ni
PART 2A (USING RESULTS FROM PART 1):
1.
2.
3.
Construct and record the voltage of the cell that you think have the largest
measured voltage from the metals in Part one. Draw and label each part of
the cell you have constructed.
Replace the metal attached to the black wire with a piece of graphite. Record
the voltage.
Put the original metal back on the black lead and switch the piece of graphite
to the red wire and record the voltage.
PART 2B:
4. Construct a cell with the largest chemical potential from the expanded list of
metals and solutions listed above. Measure and record the voltage of this
cell. Draw and label each part of this cell. Hint: you may find that what you
learn in Part 2A, step 2 and 3 may be helpful in the design of this cell.
5. You can test your battery to see how well it works when it is connected to a
small motor provided. The current your battery can deliver is proportional to
the speed with which it can drive the motor, or RPMs (revolutions per
minute).
Dispose of the solutions in the appropriate WASTE containers-NOT IN the
SINK! Rinse your equipment with water and return the metals.
Page 5 of 6
Experiment 8- Electrochemistry
QUESTION
1. In Part 2A, steps 2 and 3, you replaced the metal electrodes with graphite. The
two reactions occurring in a cell are:
Ma (s)  Ma+ (aq) + e- (aq)
Mb+ (aq) + e- (aq)  Mb (s)
Which reaction is occurring at the red wire?
WHAT TO DO
To Be Turned in- There is NO assignment to be turned in this week.
For Experiment 8
Before Lab:
Read this experiment and the readings in Harris. Fill in the “Title
bar”. Prepare your notebook. You should also include the following information:
• Completed Title bar.
• The PURPOSE of this experiment.
• Outline of the procedures
• Reagent Table for the following seven reagents as 0.1M solutions
CuSO4
ZnSO4
Fe(NO3)2
H2SO4
Al2(SO4)3 AgNO3
SnCl4 (in HCl)
There will be WASTE containers provided for these solutions. BE SURE TO
NOTE THE DISPOSAL PROCEDURES IN THIS TABLE.
• Prepare Tables 1 through 4 for Part 1 in your notebook. Each Table must
be at least ½ page.
• Prepare a table for the Voltage data for Part 2 and at least ½ page for
each of the two Daniell cells diagrams.
During the Lab:
Carry out the procedure as written. Record all data. Modify your
procedure if you find it necessary, but be sure to record the modifications.
For Part 1 complete Tables 1 through 4 in your notebook. For Part 2 construct at least
two Daniell cell batteries and record their voltages in your table.
To Be Turned In:
• Copy pages of completed Tables 2-4 from Part 1.
• Copy pages with the table and labeled diagrams (with descriptive
captions) of the two batteries you constructed in Part 2, with the
electrodes labeled and the half reaction occurring at each electrode.
• Copy page with the answer for Question 1.
QUESTION
1
In Part 2A, steps 2 and 3, you replaced the metal electrodes with graphite. The
two reactions occurring in a cell are:
Ma (s)  Ma+ (aq) + e- (aq)
Mb+ (aq) + e- (aq)  Mb (s)
Which reaction is occurring at the red wire?
Page 6 of 6
Experiment 8- Electrochemistry