edited bv GEORGE L. GILBERT Denison University Granville. Ohio 43023
The Construction and Use of Commercial Voltaic Cell Displays in Freshman Chemistry SUSUlrnD BY
Edmund C. Shearer
Fort Hays State Universly
Hays, KS 67601 CHECKEDBY
khdmistry instructors are always looking for simple illustrations and demonstrations that, among other things, dramatically increase student interest, show practical applications of chemistrv, and illustrate how science and technology work together t i produce a better standard of living. his contribution reports two displays in electrochemistry that serve this purpose. While the displays may he used in elementary through university levels, they are targeted for the freshman chemistrv course. There seems to he a curiosity in most people to see what is inside objects. Electrochemistry chapters in textbooks heighten this curiosity by presenting diagrams of commercial cells and hatteries used hv almost everv household. Do manufactured cells actually look like the illustrations? T o find out, a hacksaw was used to cut open several hatteries and cells including carbon-zinc cells, alkaline cells, nickel-cadmium cells, mercury cells, lithium cells, silver oxide cells, 9-V carbon-zinc and alkaline batteries, and 11.2-V mercury batteries. The results are intriguing, but, because of chemical hazards and because ooened cells were rather messy, it was not prudent to allow students to handle them. Toovercome these ~rohlemssome of the bisected cells and batteries were encased in casting plastic. The product ohtained is pictured in Figures 1and 2, with Figure 2 showing the back side of the casting in Figure 1. The internal features of all the cells and hatteries are clearly visible to the observer except for the nickel-cadmium cell. Some mercury cells display better than others. While the features of the alkaline cells can he seen distinctly, they were difficult to prepare because the electrolyte material was verv sounv and the internal metallic Darts were thin and sort. ~ e r h a p i i h i is s why 9-Valkaline baireries are made by assembline six individual cells. while nine 9-V carbon-zinc hatteries are manufactured by layering six cells without encasement inside the battery case. Preparation of this display required some practice in techniques. In order to cut cells in half acceptably, a grooved block of wood was prepared to hold the cells in position and to a i d e the hacksaw blade. A hand saw was unsuccessfully used except for large batreries. Nine-volt batteries may also be prepared by prying apart the case. In all opening procedures. caution i s advised. he directions for using the casting plastic, obtained from a local craft store. were modified considerablv. About onethird the recommended amount of hardene; was used. A layer of freshly mixed plastic was poured into the bottom of the mold, a Pyrex bread dish whose inside surface had been coated and polished with automobile wax. At the same time alittle plastic was poured over the exposed inside surfaces of the cells and hatteries to prevent trapped air bubbles from coming to the surface after assembly. When the plastic be-
-
Presented in part at the American Chemical Society National Meeting. September 12. 1985: CHED-079. 158
Journal of Chemical Education
Figure 1. Photograph ofacasting showingtheIdentity of theencased bslteries. Top row cells ere a nickel-cadmium ceii (ien)and two mercury cells. Bottom row batteries from len to right are a 9-V carbon-zinc battery, a 9 4 alkaline cell, e carbon-zinc C ceii, an alkaline C call, and an 11.2-V mercury battery.
Figure 2. Photograph of the back side of the casting pictured in Figure 1 came tacky, the cells were properly positioned and a new batch of plastic was poured over the cells to an appropriate level. The displavs took several hours to prepare, but thev will Since the first be contin"oudy available for severa~-~e&s. casting was made early in 1983, only one cell in one casting has changed significantly in appearance. The electrolyte region of an alkaline cell has developed a deep rust-red color. While the castings are being passed among students, a number of interesting points about the chemistry and uses of the cells are presented. A survey of freshman chemistry texts reveals that while the basic components of the oxidationreduction reactions takine d a c e are agreed uuon, the precise chemical species are not. I t is ~ o i n t l dout that the knergy densities of the various cell types vary substantially and that the discharge curves are quite different for different cell types of similar size. It is emphasized and demonstrated that the voltaic cells traditionally assembled in chemistry teaching laboratories are not made to perform useful work. The roie of technology in c~nstructin~cells which perform useful work is discussed. Several unexpected discoveries were made while preparing the displays. One was the difference in construction of alkaline and dry-cell 9-V batteries. The lithium cell was the only one examined that used an organic electrolyte. The cell
edge, has not previously been reported: an ice cube of D 2 0 sinks when placed in ordinary water, figure. Materials Ordinary ice-cube tray; plastic is preferred for easy removal of ice cub& Tap water for making several ordinary ice cubes 25 mL deuterium oxide, D20 "heavy water" (Aldrich Chemical; 99.8%) Additional -75 mL deuterium oxide (optional; see Procedure beInml .....,
Frrcrer for freering hoth ordinary water and drucer~umwide 2%-ml. heakcr, opproximntelg haliiull oitap wter 150-mL haakrr \
