20 Determination of Electrochemical Series
Purpose
Determine the reduction potential of several metals and the combinations of their half- cells.
Background
The basis for an electrochemical cell is an oxidation-reduction reaction. This reaction can be divided into two half reactions:
• Oxidation (loss of electrons) takes place at the anode.
• Reduction (gain of electrons) takes place at the cathode.
An electrical current is generated due to the difference in potential between the two electrodes. The difference in potential is a result of the difference between the individual potentials of the metal electrodes with respect to the electrolyte. The electric potential also varies with temperature, concentration of electrolyte and pressure. The standard electrode potential (Eo) is the measure of potential of any electrode at standard ambient conditions (temperature 298K, solutes at 1 M and gases at 1 bar). Since the oxidation potential of a half-reaction is the negative of the reduction potential in a redox reaction, it is sufficient to calculate either one of the potentials. The standard electrode potential is commonly written as standard reduction potential.
In this experiment you will compare the potentials of five different metals: copper, zinc, lead, silver, and iron, by measuring combinations of their half cells.
Materials
Equipment• PASPORT Xplorer GLX / • Eyedropper or pipette
• Voltatge probe (supplied) (Chemistry Sensor can be used) / • Forceps
• Scissors / • Glass plate, 15 cm by 15 cm (watch glasses can be substituted)
Consumables
• (5) Metal samples, 1 cm by 1 cm, M1-M5. See table below. / • Filter paper, 11 cm diameter
• Sand paper / • 1.0M Sodium nitrate, NaNO3, (10 mL)
• 1.0M Solutions of M1-M5, (2mL ea). See table below.
Safety Precautions
• Wear safety glasses and follow all standard laboratory safety procedures
• Wear protective gear. Some of the solutions used in this lab are toxic and can stain clothing, skin, etc
Equipment Setup
1) Draw five small circles with connecting lines on a piece of circular filter paper (11 cm diameter), as shown in Figure 1. Label the circles M1, M2, M3, M4, and M5.
2) Using a pair of scissors cut wedges between the circles as shown.
3) Place the filter paper on top of the glass plate.
4) Obtain 1cm X 1cm pieces of each of the five test metals. Sand each piece of metal on both sides so that a good electrical connection can be made.
5) Place three drops of each metal ion solution (see Table 1) on the appropriate circle (M1, M2 etc.). Then place the corresponding piece of metal on the spot with its respective cation. The top side of the metal should be kept dry.
Table 1 Metals and metal ion solutions
Metals and saltsNumber / M1 / M2 / M3 / M4 / M5
Metal / Copper / Zinc / Lead* / Silver / Iron
Solution / Copper sulfate / Zinc sulfate / Lead nitrate / Silver nitrate / Iron sulfate
*Other metals: Mg/ MgSO4 if Pb disposal is a concern.
6) Add enough 1.0M 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.
Procedure
Part 1: Copper as the reference metal
1) Use M1 (copper) as the reference metal. You will measure the potential of four cells by connecting M1 to M2 (copper to zinc), M1 to M3 (copper to lead), M1 to M4 (copper to silver), and M1 to M5 (copper to iron).
2) Turn on the GLX and plug the supplied voltage probe into the voltage port () on the left side of the GLX. A digits display should appear.
3) Touch the tip of the red (+) wire of the voltage probe to one metal sample (for example, M1) and the tip of the black (-) wire to the other metal sample (for example, M2). If the voltage reading is below 0.00 V reverse the ends of the Voltage Sensor, that is, switch the red (+) end of the sensor to M2 and the black (-) end of the sensor to M1.
4) When the voltage reading stabilizes, record the voltage for the half cell combination and the color of the lead or clip that is touching each of the metals.
5) Use the same procedure to measure the potential of the other three half 'cells' with copper, M1, as the reference electrode. If you get a voltage reading of 0.00, try adding more NaNO3 solution along the lines connecting the metal spots.
6) Analyze your data for copper and make predictions about the rest of the half-cell combinations.
Data Table 1: Reduction Potential Data
Half cell combination / Metal contacted by (+) red wire (cathode) / Metal contacted by (-) black wire (anode) / Voltage / Potential Using Standard Reduction TableM1/M2
M1/M3
M1/M4
M1/M5
Analysis
1) The red lead represents the cathode where reduction takes place. The black lead represents the anode where oxidation takes place. By arranging the clips so a positive voltage is obtained, you have determined that the metal at the red clip is more easily reduced than the one at the black clip. Which metals were reduced by copper? Which metals oxidized copper?
2) Arrange the five metals (including copper, M1) in order of reduction potential from the highest reduction potential at the top to the lowest reduction potential at the bottom. Hint: those that were reduced by copper should be reduced. The higher the potential of the cell, the higher the reduction potential of the metal in relationship to copper. The metals that reduced copper are more easily oxidized, so their reduction potentials will be lower. The higher the oxidation potential of a metal, the lower its reduction potential.
3) The voltage of an electrochemical cell is equal to the reduction potential of the cathode half cell minus the reduction potential of the anode half cell. (Ecell = Eanode - Ecathode) You can quantify your rankings by assigning a value of 0.00 V to the reduction of copper and solving for the reduction potential of the other metals. Remember to keep track of whether copper is the anode or cathode. Record the numerical value of the Reduction Potential (voltage) relative to copper, M1, for each of the other metals (zinc, M2, lead, M3, silver, M4, and iron M5)
Data Table 2: Order of Reduction Potentials.
Predictions
Calculate a predicted potential difference for each of the remaining half cell combinations shown in Data Table 3 using the reduction potentials you determined in Data Table 2. (The half cell with the highest reduction potential in Data Table 1 will be the cathode.) Remember that Ecell = Eanode - Ecathode
Data Table 3: Predictions and part 2 results
Half cell combination / Predicted potential / Measured Potential / Potential Using Standard Reduction TableM2/M3
M2/M4
M2/M5
M3/M4
M3/M5
M4/M5
Part 2: Other half cell combinations
1) Go back to data recording. If any of the solutions (metal or NaNO3) have dried, add more to moisten the filter paper.
2) Measure the potential differences of the six remaining half-cell combinations using the same procedure as in Part I. Remember that you want to measure a positive potential. Record each measured Reduction Potential (voltage) in Data Table 3.
Analysis Questions
1) How well did your predictions match the measured values for the potential differences measured using non-copper reference metals?
2) Standard reduction potentials are measured for 1 M solutions at 25°C. Instead of setting the copper reduction potential at 0.00 V, the standard hydrogen electrode, in which hydrogen gas at a pressure of 1 bar is bubbled over a platinum electrode immersed in 1M aqueous H3O+ is used. The reduction reaction is:
2 H3O+ + 2 e- H2 + H2O
Look up a standard reduction table and find the standard reduction potentials for each of the metals you used today (copper, zinc, lead, silver, and iron).
Find the potential of the cells using the values in the table and record them in Data Tables 1 and 3. How do these compare to the ones that you measured?
3) Using the standard reduction table, which combination of reactions would give you the greatest voltage?