PACIFIC SOUTHWEST ASSOCIATION OF CHEMISTRY TEACHERS
FRESHMEN' MEL GORMAN University of San Francisco, San Francisco, California
IT
IS very instructive to read in the literature of chemical education of the many unusual laboratory projects which instructors of freshman chemistry employ to arouse student enthusiasm. Such procedures prevent students from development of a "cook book" attitude towards their experimental work. It is almost an axiom for teachers of freshman chemistry that as many experiments as possible should be quantitative within the limits of student experimental skill and the apparatus used. Many teachers insist that the students benefit more if an experiment is of the "unknown" type. The purpose of this discussion is to present an exercise for freshman laboratory work involving electrochemical unknowns and special electrode potential projects not usually studied in the first-year course. The customary way of teaching electrochemistry in the laboratory is to have the student construct a number of cells from such metals as copper, zinc, and lead and one-molal solutions of their salts, the half-cells being separated by a porous cup or a salt bridge. The measured cell voltage is then compared with that computed from a table of standard potentials of halfreactions. The effect of concentration on voltage, together with a correlation of results in terms of strengths of the oxidizing and reducing agents involved, completes the study.2 In our course the zinc-copper, zincbromine, and copper-bromine cells are so studied. student,^ work in groups of two or three. The cells and external circuit are completely disassembled; a minimum of directions is supplied. With the experience thus acquired, the students are in a position to undertake the experiments which are to be described. I t is to be understood that not all of the experiments are performed by the whole class; rather different groups are a t work on different.projects. This develops a sense of curiosity which adds to the general interest.
Presented before the Division of Chemied Education a t the 131st Meeting of the American Chemical Society, Miami, April, 1957. $BRAY,W. C., W. M. LATIMER, AND R. E. POWELL,"A Course in General Chemistry," Maemillan Company, New York, 1950, pp. 107-11; H. W. FRANTZ, "A Labor~toryStudy of Chemical Principles," W. H. Freeman and Company, San Francisco, 1956, pp. 129-34.
VOLUME 34, NO. 8, AUGUST, 1957
These experiments are not intended as a conlplete study of the phenomena and calculations involved. 9 full explanation of the operation of these cells would include a consideration of polarization and liquid junction potential, and a strictly quantitative treatment of results would require the use of activities instead of concentrations. All of these topics may be studied more advantageously in advanced courses. For the purposes of the experiments mentioned in this paper the "standard" potential is defined as the potential of an electrode in which all concentrations are one molal. Any errors due to polarization and liquid junction are assumed to be negligihle. ELECTROCHEMICAL UNKNOWNS
Two types of unknowns are used for the purpose of enhancing student interest. One of these consists of a metal such as cadmium and its one-molal sulfate. To freshmen, cadmium looks like several other metals, so it cannot be identified by mere visual observation. The group simply is told that the solution is a onemolal solution of a salt of the unknown electrode. The students are asked to construct two cells using the unknown couple and copper and zinc couples, respectively, and to report the potential of the unknown metal. Nothing new is involved in the mechanical construction of the cells, but the students have no way of knowing which electrode will function as the reducing agent, so they have to experiment with the voltmeter connections. Also, since they have no way of checking the accurary of the cell potential, they are advised by the instructor to take readings a t intervals until they are convinced that as constant a potential as possible has been established. The results must be handled differently than those from the preliminary cells. Instead of adding the potentials of two known halfreactions, there is only one known half-reaction and the observed cell voltage. From these the students calculate the "standard" potential of the unknown couple. The results obtained by eleven freshmen students are given in the accompanying table. The "standard" potential of the zinc couple is taken as 0.76 volt and that of copper as -0.34 volt.
Typical Results for Evaluation of Cadmium Electrode Potential 1
B
Cd-Zn cell
E c d . ealc. f m col. 1
&A.
S Cd-Cu cell &&.
4 &cd calc. from col. S
6 Au. of col. B and L
It is obvious that some of the best results are fortuitously close to the true standard potential of cadmium, +0.40 volt. In previous years some students have obtained somewhat lower averages, and have reported that the unkuowu metal is either indium (standard potential, 0.34 volt) or cadmium. Likewise, those who obtain higher averages are apt to report that the metal is iron (standard potential of the iron-ferrous couple, 0.44 volt) or cadmium. Another type of unknown is a nonmetallic couple such as ferrocyaoide-fcrricyanide. Students are given equimolal solutions of potassium ferrocyanide and ferricyanide, identified only as A and B, respectively. They are told merely that the valence of a certain element in B is higher than the same chemical form of the element in A . (The class has not studied iron and therefore cannot identify the solutions by color.) This presents a more difficult unknown than the first t.ype. Student questions indicate that they wonder about such things as the necessity of using two cells, one of each solution, or whether they really need both solutions, and what kind of an electrode to use. It is hoped that the experience in using the bromine-aqueous sodium bromide couple will serve to indicate that the solutions must be mixed and a carbon rod used as an electrode. Some students can see this, others need assistance. However, once the couple is combined rrith a known couple, the cell voltage is measured without difficulty. Some students will have a little trouble with the sign of the "standard" potential (-0.36 volt), but results obtained are comparable to those of the unknown metallic couples. Other choices for use as unknowns are the ferrous-ferric and the cerousceric couples.
FRESHMAN RESEARCH
Special projects in the study of cells can be assigned which will give the freshman an introduction to the research activity of a chemist. Many cells, no matter how carefully constructed, will not necessarily work t,o give their calculated potential, or even work at all. Consequently, the instructor can propose to the students as a genuine problem a cell whose electrodes are more complicated than those of the preceding unknowns. For the purpose of challenging the students the best cells are those which present problems which affect the voltage. Very well suited for this purpose are couples such as copper-cuprous chloride-aqueous chloride. Iodides, bromides, and chlorides of copper and silver are used. The electrodes are prepared easily; for example, a strip of copper can be dipped into a solution of cupric chloride to form a film of cuprous chloride, and a strip of silver can be immersed in bromine water to form a coating of silver bromide. A typical cell would be Zn, 1M ZnBr*, CuBr, Cu. Such a cell gives erratic voltmeter readings, varying by *0.2 volt. Probably most of this behavior is traceable to the cuprous bromide-its method of preparation. action of dissolved oxygen, and complexing with bromide ion. In the past, students have worked on thew difficulties under the instructor's direction and have gained va.luable experience in tackling problems whosr answers cannot be found in textbooks. It should be emphasized that the purpose of this approach is not to increase the student's knowledge of technical information nor to claim a superior way of teaching electrochemistry, but rather to show the beginner how a typical subject in chemistry present,? interesting and challenging problems. There is a spirit, of zestful animation while the students work on the unknowns and special problems ~rhichis not apparent when the class is doing any run-of-the-mill experiment. Many more students stay after the end of the laboratory period to continue these experiments, and many request permission to use the laboratory when they are not scheduled to do so. On occasion as much as 25% of the class has volunteered to work a whole afternoon on some of the electrode problems. Students have also requested materials to work with in their home laboratories. From this, one concludes that the idea seems to he succcessfnl for generating student enthusiasm.
JOURNAL O F CHEMICAL EDUCATION
