Glenn Bacal and Dennis A. Lewis Rose-Hulman Institute of Technology Terre Haute, Indiana 47803
I Electrochemistry h a Nutshell
I
A general chemistry experiment
The study of electrochemical cells and their relationship to the important concepts of spontaneity, oxidation-reduction, the electromotive force series, and the derivation of electrical energy from chemical reactions justifiably occupies aprominent place in hoth the lecture and laboratory portions of most General Chemistry programs.2 However, the treatment which electrochemistry receives in the laboratory usually suffers for a number of reasons. First of all, the conventional construction of galvanic cells using beakers for electrode chambers and large U-tubes for salt bridges requires large amounts of hoth glassware and reagents. These requirements may lead to serious problems in courses with very large enrollments, especially in light of the continually rising cost of glassware and reagents. Secondly, the construction of galvanic cells in the above manner is cumbersome, tedious, and time-consuming. The accumulation of sufficient data to obtain quantitative relationships among a number of these cells (e.g., the generation of an electromotive force series of metals) almost certainly requires more than one laboratory period, especially if a numher of qualitative tests are also planned. In order to economize on the amounts of reagents and glassware used, and to minimize the time spent assembling equipment, we have formulated an electrochemistry experiment which utilizes the 9-Chamber Plexiglas Unit3 shown in Figure 1. The center of each chamber contains two small Plexiglas support uprights for the insertion of metal strips (electrodes). The nine chamhers are fded with the appropriate ionic solutions, and small U-tubes filled with agar-agar4 are used for salt bridges. The nine adjacent half-cells allow for the rapid "coustruction" of a number of galvanic cells and permit their cellvoltages to he measured in a very short time. The construction of an electrochemical cell is accomplished by connecting any two half-cells by means of a U-tube and measuring the cell voltage. In our experiment we have found that a pair of students can construct and obtain the cell voltages of two precipitation cells, three concentration cells, and six redox cells in 30-40 min. An electromotive force series containing seven halfreactions can then be obtained from this data. Materials Equipment Required
Digital voltmeter (leads included) 9-Chamber Plexiglas unit Two 8-mm Pyrex U-Tuhes (salt bridges) One set of alligator clips Small funnel Eye dropper Three 150-ml beakers Emery cloth (fine) Reagents Required
Three sheets of copper metal Three sheets of lead metal One sheet of zinc metal One sheet of silver metal One leadllead dioxide sheet (obtainable from an old automobile battery) 1.00 M solutions of: lead nitrate, zinc nitrate, silver nitrate, copper sulfate, potassium iodide, and sulfuric acid 0.10 M copper sulfate solution 0.010 M capper sulfate solution 804 1 Journal of Chemical Education
Figure 1. Nine-chamber Plexiglas Unit.
Figure 2. Arrangement of metal sheets on the paper towel. Experimental
To facilitatethe construction of the nine half-ceh, the three sheets of copper metal, the three sheets of lead metal, and the single sheets of zinc and silver metals are placed on a dry paper towel in the arrangement shown in Figure 2. Theleadfieaddioxide sheet should be kept in 1.0 M H&O1 solution until needed. The eight sheets of metal are cleaned thorowhlv with pieces of emerv cloth, then dated in their respecitve positions in the 9-Chamber ~lkxiglasunit. By means of a funnel, 1.00 M lead nitrate solution is carefully added 'Taken in part from Hill, R. D., Baca, G., Lewis, D. A,, and Sakano, T. K., "An Introduction to Experimental Chemistry," Rose-Hulman Institute of Technology, 1976, pp. 57-65. For excellent discussions of electrochemistry,see Masterton,W. L., and Slowinski, E. J., "Chemical Principles," 4th Ed., W. B. Saunders Company, Philadelphia, 1977,pp. 524-5713. or Pimentel G. C., and Spratley, R. D., "Understanding Chemistry," Holden-Day, Inc., San Francisco, 1971, pp. 184-224. The Plexiglas units measure 7 X 7 X 1%in. overall; each of the nine half-cells measures 2 X 2 X 1in. Each metal electrode is Illz in. long and 1in. wide. 'The agar-agar solution is prepared by adding 3.0 g of agar-agar to 600 ml of boiling 1 M potassium nitrate solution. The mixture is stirred until the solid dissolves. The U-tubes, which are 80mm pieces of 8-mm tubing, are filled with the hot solution and sllawed to cool.
batteries, in this case two galvanic cells in series. In the first ease, the voltages of the Zn-Ag and Ph-Cu cells add to one another, Zn and Pb actingas anodes, and Agand Cufunctioning as cathodes..In the seeond case, reversal of the Zn and Ag electrodes forces Cu to function as an anode and Ph as a cathode, causing the voltage of the Pb-Cu cell to subtract from the overall cell voltage. At the conclusion of the exoeriment the silver nitrate is recovered usmg nn eychpper. The meial shwrlarp remowd and rinsed wilh water only and dried. The metal shrrts arc placed on n dg. paper twel i n thr same nrmngrment that war u r d at the heymnlnr of the e x periment. Figure 3. Arrangement of metal electrodes and solutions in the nine-chamber Plexiglas unit.
Calculations T h e following half-reaction has a potential of -0.126 V Pb2+i,, 1 . ~ +~2e-1 - P ~ I Using t h e experimental voltages of t h e four redox cells a n d t h e two precipitation cells, t h e voltages of t h e following six half-reactions are determined
Figure 4. Construction of batteries
to the center chamber until the chamber is two-thirds full. If any soillaee . .. occurs at this mint. . . the solution is discarded. the entire unit 1s rlmned, nnd the pnrcdurr is repeatrd. Onrr thr l'h;W ,, solutim hns heen added tu the center rhamhcr, theuther suluri~rns~an br i l l tmducd uithout a funnel. The proper arrsngelnent oi metal elrctrades and solutions is shown in Figure 3. Concentration Cells A concentration cell is constructed as follows: A salt hridge is rinsed i n a beaker of distilled water, then placed between the chamber cantaining the 0.100 M CuSOa and the chamber containing the 1.00 M CuS04. The pasitiue terminal of the voltmeter is connected to the cnpper sheet immersed in the LOOM CuS04and the negative terminal to the copper sheet immersed in the 0.100 M CuS04. The voltage of the following electrochemical cell is now measured C U ( ~/ C ] U ~ +0.lW ( ~ ~MI, / / C U ' + ( ~1.W ~ , MI 1C U I ~ The following concentration cell is then constructed and its voltage measured: ( ~MI ~ ,I C U I ~ I Cue] / Cu2+(,, 0.010MI I / C U ~ + 1.M Precipitation Cells A snlt hridge is rinsed in dirtilled water and placed lwtwrra the chamber nmllininy the i.(nlAl PI,(SO,)?wlutim and the 1 . ~ 1 .1 f K I solution and the wltazp uf the fdlwing cell is measured
Once t h e half-cell potentials have been determined, t h e seven half-reactions are arranged in order of decreasing activity (largest negative numher first). Tahle 1 summarizes typical cell voltage d a t a obtained by t h e students. Table 2 shows t h e resulting Activity Series, or Electromotive Force Series. Conclusions From t h e d a t a in Tables 1 a n d 2, i t is evident t h a t t h e measured half-cell potentials for metals below hydrogen i n t h e electromotive series very closely approach t h e actual literature values, whereas those above hydrogen are significantly lower than t h e theoretical values. Our students are required t o write a formal laboratory report on this experiment a n d m u s t include i n their discussion a reasonable explanation for their inability t o obtain voltage readings close t o the theoretical values for metals above hydrogen in the electromotive series.
A salt hrid~eis rinsed in distilled water and rhrn placrd hercrn t h r chsmlwr containing the 1.00 31 Pl,tNOm +olotion and the 1 00 hi HISO, i d u t i m tt, mwiure the \.cdtugr of the lollouing crll
Quantitotiue Reactions: The voltage of each of the following standard cells is measured 1) Lead-LeadlLead Dioxide in 1.03 M H ~ S O I 2) Lead-Zinc 3) Lead-Silver 4) Lead-Copper In eaeh ease the positive terminal is connected to the metal sheet which will result in a oositive voltaee " for the cell. This assures that reduction is occurring a t this electrode (positive terminal). Voltage Additiuity: The voltage of eaeh of the cells (Fig. 4) is measured. This portion of the experiment demonstrates the construction of Weast, R. C. (Editor), "Handbook of Chemistry and Physics," 56th Ed., CRC Press, Cleveland, Ohio, 1975,pp. D141-143.
Table 2.
Electromotive Force Series
Half-Reaction
--+ -
+ 28 Znt., + 2e Pbgi + 21-t,,1 PbS04isl+ 2e PbcI + S02-r,,, Pb2tlosi + 28 PblSi Zn2+laql Pblas1
Cu2+taql 2 s
Culs1 Aggi 4HLeql S042-4iad 2 e
.%+I-] + e PbOsgl
+
+ H201,) b
+
+
+
Calculated ~ov.4uesas~uming:Pb2+,,, Literature go values in footnote 5
-
PbS04(=l
+ 2e-
Pb,.);
to.
eoh
-0.572 -0.285 -0.330 -0.126 +0.346 t0.796 +1.659
-0.763 -0.358 -0.356 -0.128 +0.340 t0.800 t1.685
to
= -0.128 V.
Volume 55,Number 12, December 1978 / 805
In the Voltage Additivitv portion of the experiment, the sudmt.< are asked to conipa~e~the cell voltngw (if the batteries they construct with the experimentally drternlined half-cell reactions. The observed cell voltage f i r the first battery was
806 1 Journal of Chemical Education
1.828 V; the voltage calculated from the experimental half-cell reactions is 1 . 8 1 ~l'he ~ . ubserved cell voltage for the second battery was 0.891 V, compared to a calculatrd value of OX96 V
