SOTM LAB: C17
I. TEACHER NOTES & GUIDELINES
TITLE OF LAB: Reduction Potentials of Metals
DEVELOPERS OF LAB
Michael Boms JD814, Diana Tennant JD581, Judy Neyhart JD731, Diane McCarron JD729, Thomas Kiszkiel JD 851, Lisa Dellaguardia JD782, SOTM staff
OVERVIEW OF LAB
DESCRIPTION:
The purpose of this laboratory activity is to establish the reduction potentials of five unknown metals relative to an arbitrarily chosen metal. This will be accomplished by measuring the voltage, or potential difference, between various pairs of half-cells.
CURRICULUM CONSIDERATIONS:
This lab could be used in a unit pertaining to:
Redox and electrochemistry
Activity of metals
REAL WORLD APPLICATIONS:
Batteries
Corrosion
Electrolysis
Electroplating
Electrophoresis
SAFETY CONSIDERATIONS
1. Wear an apron and safety goggles while handling or working near chemicals.
2. Follow your teacher’s directions for the handling and disposal of all chemicals and solutions.
BACKGROUND INFORMATION
A. SCIENTIFIC VIEWPOINT
A voltaic cell uses a spontaneous oxidation-reduction reaction to produce electrical energy. Placing a piece of metal into a solution containing a cation of that metal (e.g. copper (Cu) metal in a solution of copper sulfate (CuSO4)) produces a half-cell. In this micro-version of a voltaic cell, the half cell will be a small piece of metal placed into 3 drops of corresponding cation solution on a piece of filter paper. The next figure shows the arrangement of half-cells on a piece of filter paper. A porous barrier or a salt bridge normally separates the two half-reactions. Here, the salt bridge will be several drops of aqueous sodium nitrate (NaNO3) placed on the filter paper linking the half-cells. Using the computer as a voltmeter, the red (+) lead of the voltage sensor makes contact with one metal and the black (–) lead of the voltage sensor with another.
By comparing the voltage values obtained for several pairs of half-cells, and by recording which metal made contact with the red (+) and black (–) leads, it is possible to establish the reduction potential sequence for the unknown metals in this lab.
B. COMMON MISCONCEPTIONS:
1. Water is a good conductor of electricity.
Water is a good solvent for many ionic compounds. Tap water and fresh water typically contain dissolved ions in sufficient concentrations to enable the solution to be conductive. The ions in solution are responsible for carrying the charge, NOT the water itself.
2. Only batteries are electrochemical cells.
Both voltaic and electrolytic cells are electrochemical cells. Voltaic cells produce electrical energy from differences in potential chemical energy. Electrolytic cells use electrical energy to produce products of higher potential chemical energy.
3. Electrons flow through solutions.
Electrons do not pass through solutions. Charge balance is maintained in the solution by movement of cations and anions toward the electrodes where charge transfer takes place at the solution interface.
4. Losing electrons makes the substance more negative.
Loss of negative charge results in the formation of more positive specie.
OBJECTIVES
To create a data table showing how E0 values of different metals are interrelated.
EQUIPMENT/MATERIALS
PROVIDED BY SOTM
Laptop Computer
Science Workshop Interface (500 or higher)
Voltage sensor
Petri Dishes (large)
Metals, 1 cm x 1 cm, M1, M2, … M5
Filter paper, 11-cm diameter
Solutions of M12+, M22+… M52+, 1 M
Sodium nitrate, NaNO3, 1.0 Molar
PROVIDED LOCALLY
Test tube rack,
6 test tubes,
Grease pencil,
6 disposable pipettes
Steel wool
ADVANCE PREPARATION
Prepare filter papers (see diagram A,)
II. PRE-LAB
PRE-LAB EXERCISE TO ELICIT STUDENTS’ PRIOR KNOWLEDGE AND MISCONCEPTIONS
Word splash – using the suggested words from the list below have students create 5 sentences demonstrating their current knowledge Stress that the sentences may not be correct but represent their knowledge before completing the lab
- Oxidation
- Reduction
- Oxidizing agent
- Reducing agent
- Half-cell potential
- Ions
- Half-cell reactions
- Anode
- Cathode
- Salt bridge
- Electrode
- Battery
- Reference electrode
- Galvanized
- Redox
DISCUSSION OF PRECONCEPTIONS
Each student will read one sentence from the pre-lab exercise. The instructor will facilitate a class discussion on the validity of their knowledge. Inform students that they will conduct a lab activity that will give them an opportunity to support or refute their ideas.
III. EXPLORATION OF SCIENTIFIC PRINCIPLE & INTRODUCTION OF EXPERIMENTAL PROTOCOL
PROBLEM
Find the relative reduction potential of 5 unknown metals. Generate a list by measuring the voltage, or potential difference, between various pairs of half-cell reactions. Rank the metals according to their reduction potentials.
EXPERIMENT AND TECHNICAL OPERATION OF EQUIPMENT
PROCEDURE
For this activity, the voltage sensor is used to measure the difference in electric potential of micro-voltaic cells. The Science Workshop program records and displays the data.
PART I: Computer Setup
1. Connect the Science Workshop interface to the computer, turn on the interface, and turn on the computer.
2. Connect the DIN plug of the voltage sensor to Analog Channel A on the interface.
3. Open the Science Workshop file titled: C28_REDU.SWS. It can be found in the Science Workshop folder, under Library/ Chemistry.
The document has a table display of the voltage (V) and the number of the metal in the half-cell. There is a digits display of the voltage (V) which will show the voltage with two decimal places
Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click on its window or select the name of the display from the list at the end of the Display menu. Activate the Experiment Setup window by clicking on the “Zoom” box or the Restore button in the upper right hand corner of that window.
4. The "Sampling Options…" for this experiment are as follows: Periodic Samples = Slow at one measurement per second, and Keyboard sampling with Parameter = [+] Lead and Units = M# (metal number).
PART II: Sensor Calibration and Equipment Setup
It is not necessary to calibrate the voltage sensor.
1. Draw five small circles with connecting lines on a piece of 11 cm circular filter paper, as shown in the diagram.
2. Using a pair of scissors, cut wedges between the circles as shown.
3. Label the circles M1, M2, M3, M4, and M5. Place the filter paper on top of the glass plate
.
Diagram A – Star Chamber
4. Your teacher will supply you with 5 pieces of metal designated M1, M2, M3, M4, and M5. Sand each piece of metal on both sides.
5. Place each metal near the circle with the same number. Place 3 drops of each solution on the appropriate circle (M12+ on M1, etc.). Then place the piece of metal on the wet spot with its respective cation. The top of the metal should be kept dry.
6. Add enough 1 Molar sodium nitrate (NaNO3) solution to make a continuous trail along a line drawn between each circle and the center of the filter paper. You may have to dampen the filter paper with more NaNO3 during the experiment.
PART IIIA: Data Recording – Copper (M1) as the Reference Metal
1. Click on the Digits display to make it active. Move it so you can see it while recording data.
2. Use M1 (which should be copper) as the reference metal. You will measure the potential of four cells by connecting M1 to M2, M1 to M3, M1 to M4 and M1 to M5.
3. When everything is ready, click the “REC” button to begin recording data. The Keyboard Sampling window will appear.
4. Touch the red (+) lead of the voltage sensor to one metal sample (for example, M1) and the black (-) lead to the other metal sample (for example, M2). If the voltage drops to 0.00 V, reverse the leads, that is, switch the red (+) lead and the black (-) lead.
5. Wait about 5 seconds until the voltage stabilizes. Read the voltage in the Digits display and record the value in Data Table 1. Also record the label of which metal (M1 or M2) is touched by the red (+) lead of the voltage sensor and which is touched by the black (–) lead of the voltage sensor.
6. Once you have recorded the voltage of the half-cell and the number of the metal that was touched by the red lead in your Data Table, go the Keyboard Sampling window.
7. In the Keyboard Sampling window, enter the number of the metal sample (either 1 or 2) that was touched by the red (+) lead of the voltage sensor. Click on “Enter”. Your number will replace the default value in the Keyboard Sampling window. (NOTE: In this example, the number is “1”.) This process creates the Table of Voltage (V) and [+] Lead (M#) and makes a record of the potentials measured.
8. Use the same procedure and measure the potential of the other three cells, continuing to use M1 as the reference electrode.
9. Touch the leads to the next pair of metal samples (M1 and M3).
10. Read and record the voltage in Data Table 1.
11. Also record the label (e.g., M1 or M3, etc.) of the metal that was touched by the red (+) lead in the Data Table.
12. Go to the Keyboard Sampling window and enter the number of the metal (e.g., 1 or 2) that was touched by the red (+) lead. Click “Enter”.
13. When you have recorded data for the half-cells using M1 (copper) as the reference metal, click the “Pause” button to halt data recording while you analyze your data for Part IIIA.
ANALYZING THE DATA: PART IIIA - Copper (M1) as the Reference Metal
After finishing Part IIIA of the procedure, arrange the five metals (including M1) in Data Table 2.
Give metal M1, the reference metal, an arbitrary value of 0.00 V.
If the other metal (e.g., M2, M3, or M4) was touched by the negative (black) lead of the voltage sensor, it will be placed above M1 in the chart (with a negative E° value).
The reduction potential is the value that you measured from the Digits display on the computer and put into Data Table 1.
DATA TABLE 1
Voltaic Cell(metals used) / Measured Potential
(V) / Metal Number of
Red (+) Lead / Metal Number of
Black (-) Lead
M1 / M2
M1 / M3
M1 / M4
M1 / M5
DATA TABLE 2
Metal (Mx) / Lowest (-) Reduction Potential, E°, (V)Highest (+) Reduction Potential, E°, (V)
DATA TABLE 3
Half-Cell Combination / Predicted Potential(V) / Measured Potential
(V) / Percent Difference
(%)
M2 / M3
M2 / M4
M2 / M5
M3 / M4
M3 / M5
M4 / M5
PREDICTIONS
1. Calculate a predicted potential difference for each of the remaining half-cell combinations shown in Data Table 3 (M2/M3, M2/M4, M3/M5 and M3/M4, M3/M5 and M4/M5) using the reduction potentials you just determined (in Data Table 2).
2. Record the predicted cell potential differences in Data Table 3.
IV. ELABORATION OF SCIENTIFIC PRINCIPLE: INQUIRY-BASED STUDENT INVESTIGATION
PROBLEM
Design another experiment using the setup and materials of the previous section
HYPOTHESIS OR PREDICTION
In the previous experiment, copper was used as the reference metal. How can you change the experimental design to obtain different voltage potential values?
EXPERIMENTAL DESIGN
Checkpoint (Teacher checks students’ experimental design for feasibility.)
Would you use copper metal as your reference electrode?
What other electrodes can be used instead of copper electrode?
How can you compare the different results using different reference electrodes?
Under what conditions would your voltage potential be zero?
How are the ions conducted from solution to solution?
PLAN FOR DATA COLLECTION & ANALYSIS
In the previous experiment, data was entered on a table to display their results. Design a data table for your own experiment.
Checkpoint (Teacher checks students’ plan for feasibility.)
CONDUCTING THE EXPERIMENT
Checkpoint (Teacher monitors students’ investigations in progress.)
ANALYSIS OF DATA
Find the standard electrode potential chart in your reference table and identify the metals M2 through M5. If your book has an oxidation potential chart, all the reactions will be reversed and the signs will be switched on all the potentials. Reminder: H2 has a reduction potential of 0.00 V on the textbook chart. Locate copper, M1, on the chart, and then determine possible identities of the other metals using your experimental reduction potential sequence in Data Table 2. You must add the difference in potential between H2 and copper to all values in Table 2. Note: One of the metals has a 1+ oxidation state; the remainder of the metals has 2+ oxidation states.
Checkpoint (Teacher checks students’ analysis.)
DISCUSSION OF RESULTS
COMPARE
1. What is the role of electrons in redox reactions?
2. How can chemical reactions be used to produce electricity?
3. How can electricity be used to drive a chemical reaction?
4. How do materials compare in their relative ability to accept or donate electrons?
5. How are electrochemical processes used in business and industry?
6. How are electrochemical devices used in everyday life?
PERSUADE
Did your results confirm or deny the values displayed on Table N? (Support your answers with an explanation of your data)
RELATE
Iron can be protected from corroding by attaching it to zinc.
Iron is frequently used in construction and will corrode (oxidize) unless it is protected. One way to protect iron is to paint it. However, in many applications paint alone will not work because it is easily chipped or scratched. An effective way to protect iron is to attach a piece of zinc to it. This is frequently done to protect buried fuel tanks and hulls of ships. Zinc is easier to oxidize than iron and will lose its electrons (corrode) instead of iron. In such an application, zinc is substituted for the cathode instead of iron.
For further investigation, have the students observe the corrosion of six-penny nails. Start with two identical six-penny nails. Clean the surfaces of both nails with a piece of sandpaper. Wrap one nail with 10 cm of zinc wire, making sure to establish good physical contact between the two metals. Immerse the nails in a beaker containing a 10% sodium chloride solution. Observe the nails over several days. The bare nail will show signs of corrosion while the one wrapped in zinc will be protected from corrosion. (Students should recognize that galvanized construction materials are made up of iron protected zinc).