A High School Adventure Schuyler Cullen, Aaron Pardini, Yeng Vang, Erik Wasinger, Students, and Jenelle Ball, Teacher ~hico-seniorHigh School, Chico, CA95926 Ron Cooke Shasta Community College, Redding, CA 96049 Grover Willis California State University, Chico, CA 95929 During the Fall semester 1991 a t Chico High School, the four students in our chemistry enrichment course were assigned a n exercise fmm a current college general chemistry manual (1).The experiment involved the measurement of the solubility product of copper (11) carbonate, CuCOs(s), by means of a n electrochemical cell. We encouraged the students to delve further into the reactions and chemical species involved rather than merely to follow the given orocedure and calculations. This was a fortunate s u g e s iion. hccause they discovered not onl!, that the experiment was flawed but also that some texts and papers dating a s far hack a s 1924 contained the same error. Their discoveries caused t h e students a n u n u s u a l amount of excitement and interest in chemistry. They were quite proud to have competed successfully against professionals. I n fact, our reason for the presentation of this paper is to remind all of us of the value of approaching our teaching with a spirit of inquiry. The Routine Exercise The instructions in the manual were to make the electrochemical cell with a copper strip dipping in 1.0 M copper nitrate a s a reference cathode, a salt bridge o f 3 0 M potassium nitrate gelled with agar, and, as anode, another copper electrode dipping in 1.0 M sod~umcarbonate to which five dnms of 1.0 Y coooer nitrate had been added in order to prod&e a blue pre'cipitate. The manual stated that the precipitate was copper (11)carbonate. The cell representation is
and the net cell reaction may be written Cu2+(1.0M) + CuZ+(xM), where x is the molarity of the copper ion in equilibrium with the carbonate ion and the copper carbonate solid in the anode compartment. The directions suggested ignoring all activity coefficients (and made no reference to the liquid junction potential a t either end of the salt bridge). With this approximationx can be computed fmm the cell emf and the Nemst equation.
can be calculated. Then the K , of the CuC03,[C~Z+1[C03~l, Using this technique, the students obtained several preliminary values of about 1x 10-lo. This seemed to compare well with the value 1.4 x lo-'' found in several current texts (2,3) and a value of 2.5 x 10-lo dating hack to 1924 (4). However, the values were not very reproducible. The cell potentials varied quite a hit with each trial and also dropped off continuously during each run. This seemed to be connected with the fact that the electrodes became coated with a visible solid that could be seen in each run several minutes after immersing the copper electrode into the anode compartment. I n order to obtain the value of 1x
for the K,, i t was necessary to use the largest initial cell potentials obtained from several trials. Rather thanprocerd further with such lack of reproducibility, we suggested that the students look up the properties of CuCOds) and also look into the electrode coatings that had developed. Because they had access to only the typical high school library facility, Ron Cooke kindly provided us with several papers from the literature that he accessed with his computer expertise. The Adventure Patl To their surmise. the students found that CuCOs was not listed under propcrties of innrganic compounds in the CRC handbook' Yoreover a 1967 inorganic text r51stated that CuC03 "has not been o b t a i n e d n r ~ h only e carbonates of copper listed in the handbook were basic carbonates, C U C O ~ G I ( O H(malachite), )~ a green insoluble solid, and ~ C U C O ~ G I ( O (azurite), H)~ a blue insoluble solid. Copper(I1) hydroxide, Cu(OH)&), also was given a s a blue insoluble salt. At this point there seemed to be two possihilities for t h e blue precipitate, azurite a n d copper(I1) hydroxide. Continuing the Search I n order to decide between the two oossible blue compounds, the students again prepared t h i sohd a s described in the manual, immedlatelv washed and centrifured it five times, then dissolved i t in 6.0 M HCI. No ~ 0 ; g a swas evolved. They compared this result with the clear evolution of COz(gJobtained using a commercial sample of malachite. Besides eliminating the supposed CuC03, this also eliminated the blue solid azurite or other C03%ontaining species a s a possibility. Realizing that the carbonate ion could be acting as a base rather than directly a s a precipitating ion, blue Cu(OHI2 seemed to be the best guess. There is further evidence that the solid formed in this experiment cannot be CuC03. Copper carbonate actually was made in 1974 by Seidel e t al. (61, hut only in milligram amounts and only by using 500 'C and 20,000 atm. I t was a erav. crvstalline solid. Reiterer in 1981(7) calculated a v&efor orhe K,, of this gray, crystalline CuC03 from its solubilitv and other eauilibria involvinr comer wmdexes Conclusion ARer the preliminary laboratory work and the discovery that the compound forked was not copperill. wbonate and thnt the copper strip dipping in the basic solution dld not give stable ootentials. the work was dismntinued. It would have venfy that the blue solid was actually copbeen &eresting per(I1)hydroxide but, by this time, the semester was over and the students were preparingto graduate. Thc discoverie.s i h s t the students made led to nn exception;~lincrease in their enthusiasm for chemistry In fact, we heard from several of them later on after they were in
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Volume 70 Number 10 October 1993
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universities as chemistry majors, and they still rnenkd proudly on their accomplishment, We re.emphasize that it was the spirit of inquiry, even at this early stage in their studies that led to their success. Literature Cited 1. Roberts, J.: Hollenbelg, J.; Paatma, J. G e m 1 Chmiatry in t h Lobmtory, 2nd ed.: Freeman: New York, 1987.
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Journal of Chemical Education
2. MeQumie, D.;Rock,P. GmsmlChmistry.3rded; freeman: NewYor*, 1991. 3. Bmwl, T.: laMax H.: Bursten, B. Chmishy, T h Cenfml Science, Sthed.; PrentiaHall: Englewood Cliffs. 1991. 4, Haenel, O,JpMkt
10a,
5. Parkee. G. Ed. Mellork Modem Inorganic Chemistry; John W~ley:NewYork. 1967. 6. ~ ~ i d ~ ~ , v .K; :dohannes, ~ l ~ ~w.;~~ h~r th ~hr ~d Z.t ,, onerg. ~ . dig chm.1874, 410, 138. 7.Reiterer F.: Johannes, W.; Gamlrjager, H. MikraehimimAefo M a n 1 1981.1.63.
