edited by JAMES0.SCHRECK UNIVER~IN OF NORTHERN COLORADO GREELEY. CO 80639
Galvanic Cells and the Standard Reduction Potential
David 0. Tanis Coalition for Excellence
in Science and Math Education, Grand Valley State University, Allendale, MI 49401
This experiment is designed to introduce high school students M the standard reduction potential table'. It is helpful. nrior to~a~~ nerformine the exoeriment. that students have .~worked with nonspontaneous oxidation-reduction reactions (ex.. the electrolvsis of water. the electrolvsis of salt solutio& electroplating) and have had practice writing half reactions to represent oxidation and reduction processes. Rather than first examining the standard table and using class discussion time to cover the table's uses, students discover the table through experimentation, learn how to read the table to predict cell voltages,and toconfirm their predictions before formal class discussion of the standard reduction potential table. After completing this experiment, class discussion of the table goes more smoothly because students have grasped the concepts summarized by the table through exper~me&tion. Concentration dependence of cell voltages is not included in the experiment, although i t could easily be incorporated for advanced classes. This experiment provides practice writing half reactions for the ~rocessesoccurrine a t each electrode. Students measure the cell voltages of Gur half cells against a Pb/Pb(II) half cell in Part I. I n Part I1 the measurements in Part I are used to develop a reduction table. T h e Ph/Pb(II) half cell is arbitrarily assigned a value of 0 v a s the reference electrode for this table. rising this reduction table, cell voltages for the six other combinations of the available half cells can be predicted. In Part 111, the actual cell voltages of these six combinations are measured and compared with the voltages oredicted in Part 11. The nhvsical r. size of the half cells is chosen to use minimum amounts of metal ion solutions and yet to show clearly students how half cells can be connected in various combinations to produce voltages characteristic of galvanic cells. T h e voltages of the electrochemical cells are measured by connecting the two a -~-o r o.o r i a t ehalf cells to a high-resistance voltmeter; If minimum current is drawn by the voltmeter during the measuring process, stable readings for the galvanic cells are obtained. Each half cell is contained in one dropper assembly (see figure). Each dropper contains a metal electrode immersed in a solution of the metal ions. The half cells make electrical contact through the conducting agar in the bottom of the beaker. This conducting agar serves as a salt bridge and simultaneously prevents the metal ion solutions from mixing.
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Equipment Each lab group needs: 150-mL beaker 5 dropper pipets (withoutbulhs) 1cork to hold droppers in place 5 clip leads (Radio Shack Catalog number 278-1156, $3.99/10 leads) 5 metal electrodes about 3-4 cm Long; one each of Cu, Ph, Mg, Ag, and Zn. (Electrode material is availablefrom Aldrich Chemical Company, Inc., P.O. Box 355. Milwaukee, WI 53201.) 1 high-resistancevoltmeter that draws verylittle current. (Digital voltmeters greatly simplify matters for students who are not familiar with reading analog voltmeten. The Micronta LCD Digital Autoranging Pocket Multimeter, Radio Shack Catalog No. 22-171 at $24.95 works well.) super-finesteel wool to clean electrodes
Chemicals For the class, provide two dropper bottles each of: 0.1 M Cu(N03)~-2.96 g C U ( N O ~ 6H20 ) ~ . in 100 mL water in 100 mL water 0.1 M Pb(N03)~-3.31 g Ph(N03)~ 0.1 M Mg(NOd~2.56g Mg(NO&. 6Hz0 in 100 mL water 0.1 M AgN03-1.70 g AgNOl in 1W mL water 0.1 M Zn(N0a)z-2.97 g Zn(NOa)z 6Hz0 in 100 mL water conducting agar solution-2.0 g agar agar + 3.0 g NH4N03in 200 mL water; boil to melt agar; provide beaker clamp for pouring.
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Procedure Clean five droppers and the beaker with soap and water followed by rinsing with distilled water. Set up the half cells, holding the droppers in place with a cork, as shown in the figure. Carefullyadd hot agar solution to the beaker to a depth of about 1 em. This solution contains about 1%agar (so it will gel when it cools)
Cork lo .. hold
droppers In place
50 mL
beaker
Conducting
Electrode Metal Salt S~lutlon
Agar Agar Plug
' Dorin, H. Chemishy: The St* of Matter. 2nd ed.; Cebc~Allyn and Bacon: Newton. MA, p 559. Holtzclaw. H. F.; el a1 General Chemistry. 7th ed.; Heath: Lexington, MA; Appendix I, p A-14. 602
Journal of Chemical Education
Assernbled beakermtalnhg five half cells. Each elenrode is submerged In 19own salt solution. The conducting agar serves as the aan brldge.
and about 1.5% ammonium nitrate (an electrolyte, so the agar will conduct electricity). Allow the agar to cool undisturhed. While the agar is gelling, clean each of the electrodes (Ag, Cu, Mg, Ph, Zn) to brightness hy gently rubbing them with steel wool. After they have heen polished, rinse each electrode with distilled water. After the agar has gelled, fill each dropper about two-thirds full with the appropriate metal ion solution. Be careful to keep track of which solution is in which dropper. Put each polished electrode into the solution of its own metal ion. Bend over a small part of the electrode to keep it from falling below the lip of the dropper. Attach s clip lead to each of the metal electrodes. Be careful that the leads do not short out the cells. The other end of the clip leads can he arranged on a labeled paper so the various voltages can he measured more easily. Make all voltaee measurements with the function switch on the voltmeter in the & volts (DCV) oosition. When the meter is set this .way, s positive meter reading means that electrons are flowing into the mere, at the connector lahrkd "-". ( I f the meter registers a negative voltage, reverse the leads to the electrodes to obtain a positive value.) Observation of the polarity of the electrodes is as important as the voltage measurements in this experiment. The sign of the electrode reveals the direction of the half reaction at the electrode. When the meter reading is a positive value, the half reaction occurring at the electrode connected to the "-" meter connector is supplying electrons. This means oxidation is occurring at that electrode. For example, if a lead electrode is connected to the "-"meter connector, then the half reaction at that lead electrode must he: ~~~
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Note that electrons are a product of this reaction. The reaction is oxidation, and the electrode is called the anode. However, if the Lead electrode is connected to the meter at the connector marked "+", and the meter reading is positive, then the half reaction at the lead electrode must he consumingelectrons.The reaction must be: This is a reduction reaction and this electrode is called the cathode. Set up each of the cells described in the procedure, measure the cell voltages, and answer the questions. Data Part I. Measuring Cell Voltages; Writing Half Reactions A. The CuPh Cell. Measured cell voltage: The anode is (Cu or Ph?): The half reaction at the anode is: The cathode is (Cu or Ph71: The half reaction at the cathode is: The overall reaction for this cell is:
B. The MgPh Cell. Measured cell voltage: The anode is (Mg or Ph?): The. half at the anode is: - ~ -reaction -~~~~~ ~The cathode is (Mg or Ph?): The half reaction at the cathode is: The overall reaction for this cell is: ~~
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C. The AgPh Cell. Measured cell voltage: The anode is (Ag or Ph?): The half reaction at the anode is: The cathode is (Ag or Ph?): The half reaction at the cathode is: The overall reaction for this cell is: D.The ZnRh Cell. Measured cell voltage: The anode is (Zn or Ph?): The half reaction at the anode is: The cathode is (Zn or Ph?): The half reaction at the cathode is: The overall reaction for this cell is:
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Part II. Developlng a Reduction Table If the reaction Ph2+(aq) 2ec Ph(s) is assigned avalue of 0.00 V, the cell voltages measured in Part I of the procedure can he used to construct a set of reduction potentials for the other electrodes. Remember that if the reaction shown in this tshle occurred in the opposite direction during Part I, the sign of the voltage must he reversed. Reduction Potential vs. Pb/Pb(ll) Eiecbcde (V) Half Reactions
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Thevoltagesjust determinedcan be used to predict the potpntials of the fcdlowing electrochemiml ~ 1 1 s\l'rite . down the predicted voltages based on the values in the table.
Part Ill. Comparison of Predlctlons with Measured Values Measure the cell voltages for the following cells using the half cells: C u I A g ZnIAgMglAM g I Z n ZnICM g I C n How do these values compare with the predictions? Discard the metal ion solutions in accordance with local safety regulations. Rinse the metal electrodeswith distilled water, and return them to their proper place. Clean the droppers and the beakers by submerging them in boiling water. Rinse them thoroughly. A computer program is &iilable from the author for grading this laboratory experiment. This experiment has been used successfullv with~hieh school chemistw classes for fuur . ~ - ~ years. ~ d d i t i o n h i~t ,has been shared with over 600 high school chemistry teachers through the 1-week Dreyfus Chemistry Institutes a t 26 different sites across the United States. The exoeriment is based on suanestions made by Norm Craig (0Lerlin College) and avid- aster man &lackson Hole High School) during the 1983 Dreyfus Summer Institute for Chemistry Teachers at Princeton University and has been refined through numerous suggestions of the teachers and students who have done the experiment. ~~~~
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Volume 67
Number 7
July 1990
603
