ELECTROCHEMICAL CELLS AND CELL POTENTIALS

General CHEMISTRY iiELECTROCHEMICAL CELLS AND CELL POTENTIALSLAB 9

Bezawit Wube7/9/2013

Students will have the opportunity to study a redox reaction with copper and zinc and use a multimeter to measure cell potentials. They will learn the concepts of oxidation reduction by constructing personal electrochemical cells and determining the net cell potentials for several electrochemical cells

Objectives: To study a redox reaction involving copper and zinc species To construct a variety of electrochemical cells To learn to use a digital multimeter to measure electrochemical cell potentials and be able to use the appropriate sign conventions to calculate standard reduction potentials from cell potentials

Materials:1 Paper towels1 Scissors1 Plastic wrap1 Cotton swabs1 TweezersFrom LabPaq 1 Magnifier, dual1 Multimeter-DigitalExperiment BagElectrochemicalCells and CellPotentials1 Aluminum Chloride, 1 M - 1 mL in Pipet1 Aluminum Foil, 1/4" x 1/4" - 2 Pieces in Bag 2"x 3"1 Copper (II) Sulfate, 1 M - 2 mL in Pipet1 Copper Foil, 1/4" x 1/4" - 4 Pieces in Bag 2"x 3"1 Lead Foil, 1/4" x 1/4" - 2 Pieces in Bag 2"x 3"1 Lead Nitrate, 1 M - 2 mL in Pipet1 Magnesium Ribbon, 1/4" x 1/4" - 2 Pieces in Bag2"x 3"1 Magnesium Sulfate, 1 M - 1 mL in Pipet1 Potassium Nitrate, 1 M - 2 mL in Pipet1 Tin Foil, 1/4" x 1/4" - 2 Pieces in Bag 2"x 3"1 Tin II Chloride, 1 M - 1 mL in Pipet1 Zinc Foil, 1/4" x 1/4" - 2 Pieces in Bag 2"x 3"1 Zinc Sulfate, 1 M - 2 mL in PipetSpecial PapersBagSpecial PapersBag-CK-2 3 Filter Paper 9.0 cm

Procedure:Part I: Redox Reaction Investigations1. With your tweezers place one piece each of copper metal (Cu (s)) and zinc metal (Zn (s)) on the paper towel each a few centimeters from the other.2. Place 2 drops of 1 M CuSO4 solution onto the top of the piece of Zn (s).3. Place 2 drops of 1 M ZnSO4 solution onto the top of the piece of Cu (s).4. Study each of the solution-metal interfaces with your hand lens. Record your observations.Note: The discoloration of the Zn (s) is due to the formation of finely divided Cu (s) at the zinc metal surface. The chemical reaction is a spontaneous reaction in which an electron transfer is taking place and it is called a reduction-oxidation or redox reaction:Cu2+ + Zn (s) Cu (s) + Zn2+5. Look at the piece of copper metal with ZnSO4 on it.6. Remove the ZnSO4 from the copper metal with a cotton swab and compare the copper surface with another copper piece. Record your observations. The non-spontaneous reaction, i.e., the redox reaction that did not occur, is:Cu (s) + Zn2+ Cu2+ + Zn (s)7. Dry the metal pieces and your work surface with paper towels and leave the metal pieces on the plastic surface because you will be using them again.Part II: A Small Scale Electrochemical Cell1. Set up your multimeter to measure DC voltage. Set on 2V DC (or 2000 mV which equals 2V in thousandths). Use the two red and black needle probes.2. Cut one piece of filter paper into an approximate 4 x 8 cm rectangle and place it onto the plastic surface.3. See Figure 1 below, and as illustrated place a piece of Cu (s) and a piece of Zn (s) onto the filter paper about 3 cm apart.4. Deliver 2 drops of 1 M CuSO4 to the paper just at the edge of the copper metal so that the solution soaks under the metal piece.5. Deliver 2 drops of 1 M ZnSO4 to the paper so that it soaks under the zinc metal piece.6. Deliver 2 drops 1 M KNO3 to the dry paper in between the wet circles. The KNO3 solution will spread and run into the part of the paper wet with CuSO4 and ZnSO4.7. Switch the multimeter on and touch the red probe (positive terminal) to the piece of copper metal and the black probe (negative terminal) to the piece of zinc metal. Read and record the voltage. Dont forget the sign!8. Reverse the probes. Record what happens to the sign and switch the meter off.9. Write the half-reaction occurring at the anode, Zn (s).10. Write the half-reaction occurring at the cathode, Cu (s).The standard reduction potential Eo is 0.34 V (measured against a standard hydrogen electrode).You can verify this value in the Table of Standard Electrode (Reduction) Potentials in your textbook.Cu2+ + 2e- Cu (s)This value can now be used to calculate the standard reduction potential forZn2+ + 2e- Zn (s)At the anode the half-reaction isZn (s) Zn2+ + 2e-, Eo ox and at the cathode the half-reaction isCu2+ + 2e- Cu (s), Eo red = 0.34 VThe sum of these two half-reactions is the cell reactionCu2+ + Zn (s) Zn2+ + Cu (s), Eo cell = Eo ox + Eo redRemember, you obtained Eo cell for the cell reaction when you measured the voltage (potential) with your multimeter in Step 8.Calculate Eo ox, the standard oxidation potential forZn (s) Zn2+ + 2eand then reverse the sign to obtain the standard reduction potential forZn2+ + 2e- Zn (s)11. Calculate the equilibrium constant for the spontaneous redox reactionCu2+ + Zn (s) Zn2+ + Cu (s) by using the relationship: log10K= nEo cell/0.0592 (at 25C) where K is the equilibrium constant, n is the number of electrons transferred in the redox reaction and Eo cell is the standard cell potential.Part III: An Electrochemical Series from Cell DataThe standard potential data obtained in this section may then be used to interpret corrosion phenomena, design voltaic and electrolytic cells, and arrange reduction reactions into an electrochemical series.1. With one 9-cm filter paper make a cell template similar to the template shown in the diagramthat follows.2. Write on each sector the atomic symbols of the metals as shown.3. Place the cutout paper onto your sheet of plastic.4. Use your tweezers to transfer the appropriate metal pieces to each sector and make sure they are arranged in the order shown.5. Drop 2 drops of the related metal ion solution onto the paper at the edge of each metal piece so that each metal is contacting the solution (i.e., CuSO4 solution with Cu metal square, MgSO4 solution with Mg metal, etc.).6. Drop a few drops of KNO3 salt bridge solution into the middle so that it soaks outwards and contacts all the other wet areas. Try not to use too much!7. Switch the multimeter to measure DC voltage. Take a measurement with the red probe on Mg(s) and the black on Cu (s). Record results and dont forget the sign!8. Now keep the red probe on Mg (s) and take measurements by moving the black probe around the circle in a clockwise manner. Record voltages and signs.9. Move the red probe to Cu (s) and continue around with the black probe. When you have finished switch the meter off. Record voltages and signs.10. Use the principles that you learned in Part II to calculate the standard reduction potentials for each half-cell, given that the Eo for Cu2+ + 2e- Cu (s) is 0.34 V.Note that Eo for a half-reaction is not dependent on the coefficients, provided of course that the reaction is balanced, i.e., Eo forAg+ + e- Ag (s) is the same as E o for Ag+ + 2e- 2 Ag (s)11. Arrange the standard reduction potentials and corresponding half-reactions in order, starting with the most negative at the top of the table and ending with the most positive at the bottom.

Data/ Observation:Refers to Procedures Part III Step 9

Data Table 1: Data Measurements in mV

Cu redPb redSn redZn redAl redMg red

Cu black0-319-384-904-720-1712

Pb black3350-70-586-380-1410

Sn black363380-514-360-1240

Zn black855580524020-790

Al black759508405-350-220

Mg black148011401220726166-480

* Pb results are most unreliable

Questions:Part I:A. What half-reactions (one a reduction, the other an oxidation) can be written for this redox reaction?

Cu2+ Cu (s) + 2 e- Zn (s) + 2e- Zn2+ B. How many electrons are being transferred from Zn (s) to Cu2+?

2 electrons are being transferred from Zn(s) to Cu2+.

C. Which is the reducing agent and which is the oxidizing agent in this reaction?

Zn is the reducing agent because it increases its oxidation number from 0 to +2. Cu2+ is the oxidizing agent because it decreases its oxidation number from +2 to 0.

D. Why does the Cu (s) produced in the redox reaction look very different from the copper metal piece?

Part II:A. Switch the multimeter on and touch the red probe (positive terminal) to the piece of copper metal and the black probe (negative terminal) to the piece of zinc metal. Read and record the voltage. Dont forget the sign! (Refers to Procedures Part II Step 7)-0.98 V

B. Reverse the probes. Record what happens to the sign and switch the meter off. (Refers toProcedures Part II Step 8)+0.98 V

C. Write the half-reaction occurring at the anode, Zn (s). (Refers to Procedures Part II Step 9)Zn(s) Zn2+(aq) + 2e-

D. Write the half-reaction occurring at the cathode, Cu (s). (Refers to Procedures Part II Step 10)Cu2+(aq) +2 e- Cu(s)

E. Calculate Eoox, the standard oxidation potential. (Refers to Procedures Part II Step 10)

F. Calculate the equilibrium constant for the spontaneous redox reaction. (Refers to Procedures Part II Step 11)

Cu2- Cu(s) 0.36VZn(s)zn2+0.76Eo cell =0.34+0.98=1.10logK= nEcell 0.0592K=1.45*1037

Part III:A. Which is the strongest oxidizing agent in the table?

Zn is the strongest oxidizing agent.

B. Which is the strongest reducing agent in the table?

Cu is the strongest reducing agent.

C. If you wanted to design a battery from relatively common materials that produced a voltage of about 1 V which cell reaction would you choose?

Oxidation reaction must be chosen.

D. Why were the measurements on Mg (s) and Al (s) so difficult? Because they both form oxidizing coating on their surface and it blocks the contacts with the probs. Students need to scratch the metals in such cases.

Analysis: The voltage that the student recorded is the electrochemical cell voltage of a cell which is made up of two half-cells joined electrically by wires and the salt bridge solution KN03. The probes are touched to the electrodes in the cell and the measured voltage will have a positive sign if the black probe is on the anode and the red probe is on the cathode. If the sign of the measured voltage is negative then the reverse of the last is true. The black probe is on the cathode and the red probe is on the anode. Conclusion:At the completion of this lab, students will have better knowledge of redox reaction with copper and zinc. Students will learn concepts of oxidation reduction.While doing experiment, students should be careful not to spill the aqueous solutions; thus one should not forget to place the plastic wrap on the surface before doing the experiment.Also, while taking measurements, it will help to sometimes scratch the metals with the probe to get the correct and accurate voltages.