Chemical Education Today
Letters More Experiments in the Penny Lab I read with great pleasure the very interesting and useful article “Introducing Scientific Reasoning with the Penny Lab” by R. F. Mauldin (J. Chem. Educ. 1997, 74, 952–955). In order to prove a new composition of “post-1992” pennies the author proposes either to heat a penny in a Bunsen burner flame (the coin would melt and molten zinc would fall out) or to cut a coin so that one could look at the inside of it and see the difference in color. I would like to propose another simple experiment that always makes an impression on students. Take a “post-1982” penny, scratch its edge slightly with a fine file, and put it in a diluted solution of hydrochloric acid. For several days (it depends on the acid concentration) bubbles of hydrogen are emerging from time to time from the edge of the coin, and eventually all zinc filling of the coin will be dissolved and only a hollow “envelope” with Lincoln’s profile will remain. After being rinsed with water this “hollow coin” could be added as the fourth sample to the collection of the ordinary, “silver” and “gold” pennies described earlier in this Journal (J. Chem. Educ. 1975, 52, 102; 1994, 71, 996) and in ChemCom textbook. From the weight of the copper shell one can calculate the percentage of each metal in the whole coin and the thickness of copper plating. Another unusual method to reveal the interior of a coin is to remove the outer copper plating. This is a difficult task because zinc is much more active than copper and could hardly be passivated. Many attempts were unsuccessful: treatment with diluted nitric acid, with chromic acid mixture, with ammonium persulfate dissolved in aqueous ammonia, with tincture of iodine saturated with potassium iodide (iodide ions form strong complexes with copper but not with zinc), anodic dissolution in potassium iodide solution with and without a salt bridge and so forth. Having spoiled a dozen of coins I succeeded at last in preparing one (“e pluribus unum”!) as follows. Take a coin with the forceps or tongs, dip it in concentrated nitric acid placed in a small vessel (under the hood!) and immediately rinse with water; repeat this several times; care should be taken not to damage the coin severely after dissolution of copper layer because the reaction proceeds autocatalitically and its rate increases rapidly. After slight polishing (an ordinary toothpaste is sufficient) the remaining “naked” zinc penny, although partly etched, is very much like a
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zinc-coated steel penny (issued in 1943) but one can easily distinguish between these two by using a magnet. Another interesting (although time-consuming) experiment could be accomplished as follows. Prepare a concentrated solution of potassium iodide and add about one-third (by volume) of concentrated solution of copper sulfate. Take a penny, clean it with a polish and a piece of soft cloth, place one drop of the solution prepared on the obverse of the penny and spread the liquid uniformly with the tip of the pipet. The surface immediately turns white (the color of copper(I) iodide) and the yellow-brown color of iodine quickly fades. Rinse the coin with water, brighten it with polish and cloth and begin the whole process again. Every time a few milligrams of copper are removed from the surface. After five or six cycles you will see that some protruding parts of the coin (Lincoln’s beard first) are no longer being covered with the white deposit: the copper has been completely removed from these parts. From this point on, do not use polish anymore, and, after rinsing, rub the coin with a piece of cloth only. Eventually the surface turns dark. Clean it thoroughly with polish and cloth again and you will see that the large area of the coin is shining zinc with only traces of zinc etching. It is better to stop the process here because further treatment will cause some damage to the zinc surface. Moreover, the very appearance of the coin (yellow copper rim and parts around inscriptions) reveals that it is made of zinc and plated with copper. In addition I wish to inform the readers that some countries are now minting steel coins plated with copper or copper containing alloys (e.g. pennies in England, pfennigs in Germany, crowns in Czech Republic and in Slovakia, santimes in Latvia). For years my students and I have been successfully removing the brass plating from Russian steel coins (1, 5 and 50 roubles issued in 1992/93) with ammonium persulfate dissolved in aqueous ammonia. A solution turns deep blue due to the formation of complex copper(II) ammine, and steel does not react with this solution. I. A. Leenson Department of Chemistry, Moscow State University 119899 Moscow Russia email:
[email protected] Letters continued on page 1385
Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu
Chemical Education Today
Circular Arguments or The Culture of Chemistry There must be an obvious obligation for chemical educators to refrain from tautologies and circular arguments when addressing the general public, their students, and their peers. Unfortunately, circular arguments used in chemical teaching and elsewhere are embarrassingly common. A typical example is the recent statement by DeMeo (J. Chem. Educ. 1997, 74, 844–846) “since copper is below hydrogen in the electrochemical series, copper is reluctant to displace hydrogen ions from electrolytes”. Being below hydrogen in the electrochemical series (or being a noble metal) is just the name chemists have chosen for the property of not displacing hydrogen ions from electrolytes. Thus, mentioning the self-chosen name of a phenomenon is by no means an explanation of this phenomenon. The scientific process consists (i) of the identification and naming of natural phenomena and (ii) of the interpretation of more complex natural phenomena as the consequence of other less complex natural phenomena, leading finally to a relatively small number of axioms. Confusion of these two parts of the scientific process is a mortal sin. Alexander Senning Department of Applied Chemistry Technical University of Denmark; Building 376 DK-2800 Lyngby Denmark email:
[email protected] The author replies: Senning’s response to my article is a nice example of a cultural aspect of chemistry. To answer why something behaves as it does, chemists do not always talk in terms of first principles (e.g., first law of thermodynamics, atomic theory, kinetic molecular theory). Instead, chemists often talk in terms of categories and lists. For example, when asked if CH3O– reacts with CH3I, chemists often say, “yes, because CH3O– is a good nucleophile”, or why HCl reacts with NaOH, we respond, “because it is an acid–base reaction”. Why do we do this? For one reason the actual explanations of why specific chemical systems behave as they do are often very complex and require lengthy arguments involving kinetic and thermodynamic concepts. Another reason is that chemists have accepted the first principles to such a degree that they are assumed when talking to other chemists and even students with some chemical training. Thirdly, chemists love lists. Used correctly, lists allow us to get a handle on the voluminous amount of chemical knowledge and help us make predictions of how systems will behave. We use lists and categories as a source of our explanations without explicitly invoking the accepted framework on which chemistry is based. Is this a “mortal sin”? In my opinion, I am more
interested in making transparent the culture of chemistry to students than judging if the manner in which chemists communicate with one another is “right” or “wrong”. Stephen DeMeo Department of Chemistry The City University of New York at York College Jamaica, NY 11451
Not an “Ideal” Reaction for Introductory Students While I do appreciate the work involved in the synthesis and characterization of SnI 2 and SnI 4 (1), I do not agree with the authors’ claims concerning the “ideal” nature of the laboratory activity: “The broad scope of the project makes it ideal for a multiweek project in the introductory course...” and again, “The SnI 2/SnI4 system is ideal for student work because the elements are easily accessible and relatively nontoxic, yet reactive.” I have spent quite a lot of time examining syntheses of binary compounds from the elements for pedagogical purposes (2), and have rejected the above syntheses on the grounds of safety and equipment considerations. From the labs I have visited and worked in both at high schools and on the collegiate level, the materials called for in the preparation of these compounds (hot-plates, cold-water condensers, vacuum desiccators capable of accepting an inert atmosphere, 3-neck flasks and a hardy supply of nitrogen) are not commonly available for the large number of students taking chemistry for the first time. To make them available would be prohibitively expensive for all but the most wealthy of departments. Additionally, I do not think it is prudent for first-year students to use toluene when other similar syntheses call for safer, less expensive substances such as water as a reacting medium or use heat to combine the elements (2). While toluene is not a carcinogen or mutagen, it cannot be disposed into sewers, it is very flammable, and “short-term exposure to high concentrations of toluene (e.g., 600 ppm) may produce fatigue, dizziness, headaches, loss of coordination, nausea, and stupor” (3). I might be alone in this, but I do not like to visualize twenty first-year chemistry students each heating and filtering hot 15–20 mL of toluene in the laboratory. I would feel more comfortable if students were using fume hoods (the authors do not mention this), and again, the matter of having enough fume hoods to accommodate twenty or so students is another practical limitation for most schools. The word “ideal” carries a very strong connotation and should be used sparingly and with good cause (the authors use this word a total of three times in their paper). Model, exemplary, or ideal experiments used in the introductory chemistry laboratory should involve readily available and inexpensive materials which are safe to use, be reliable and aesthetically pleasing, create as little waste as possible, be per-
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Letters formed in an appropriate period of time, and most importantly allow students to invent a chemical concept or principle during their time in the lab. While I do agree with the authors that this lab would be appropriate in upper-level courses such as inorganic chemistry where the number of students is smaller and student skills are more developed to handle noxious solvents, it is my opinion that there are more “user-friendly” and “ideal” experiments currently available for the average introductory laboratory course that demonstrate the direct synthesis of a binary compound from the elements.
posed (the same as that for preparing SnI2) without explaining why. Although the reaction works well and the product can be obtained without significant contamination with SnI2, students may ask why SnI2 does not form in these conditions. Again I find it difficult to give a good explanation for this behavior, and explanations that might be considered by students as “tailored” are pedagogically dangerous. So I suggest conducting the reaction with a molar ratio close to 2 (a slight excess of Sn is convenient to react with all of the iodine without extending too much the time of reaction).
Literature Cited
Literature Cited
1. Schaeffer, R. W.; Chan, B.; Molinaro, M.; Morissey, S.; Yoder, C. H.; Yoder, C. S.; Shenk, S. J. Chem. Educ. 1997, 74, 575–577. 2. DeMeo, S. J. Chem. Educ. 1995, 72, 836–839. 3. Prudent Practices in the Laboratory: Handling and Disposal of Chemicals; Committee on Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories, Board on Chemical Sciences and Technology, Commission on Physical Sciences, Mathematics, and Applications, National Research Council; National Academy Press: Washington, DC, 1995. Stephen DeMeo Department of Chemistry The City University of New York at York College Jamaica, NY 11451
SnI2 and SnI4 With regard to the article “Synthesis, Characterization, and Lewis Acidity of SnI2 and SnI 4” recently published (1), I wish to make some suggestions for those tempted to adopt it as a project for inorganic chemistry labs, since one of its main goals is the exploration of synthetic procedures. 1. The assertion in the fourth paragraph “Tin(II) iodide can be obtained by first preparing (or supplying) SnCl2. The SnCl 2 is easily obtained by procedures such as reaction of tin with HCl (5). This is then followed by treatment with I2” should be removed. Such a reaction scheme is contrary to the basic principles of redox properties of halogens and metals. I2 is unable to oxidize chloride; Sn(II) is easily oxidized to Sn(IV) and, of course, it does not work in the usual range of conditions. For a similar reason I also suggest removing the synthesis of Na2SnI6 unless a good supporting reference can be provided. 2. The experimental procedure to prepare SnI2 (Sn + I2 + aqueous HCl) also sounds more like an alchemist’s recipe than a rational synthesis because in the mentioned conditions a mixture of products is or can be easily obtained. I find it difficult to give students a satisfactory explanation for the choice of this procedure, taking into account that both HCl and I2 react with Sn powder, and both SnCl2 and SnI2 add oxidatively to I2. This difficulty can be overcome by preparing SnI2 by the conventional procedure of heating Sn powder with an excess of aqueous HI (2), which works in a more reliable manner. That is more important from a pedagogical point of view because it can be explained more rationally with regard to the low oxidizing power of H+ as compared to halogens and with regard to analogous reactions of Sn and metals such as Ti and Fe with hydrohalic acids. 3. The procedure to prepare SnI4 from the elements also deserves consideration, since a molar ratio I2/Sn = 1 is pro1386
1. Schaeffer, R. W.; Chan, B.; Molinaro, M.; Morissey, S.; Yoder, C. H.; Yoder, C. S.; Shenk, S. J. Chem. Educ. 1997, 74, 575. 2. Brauer, G. Handbuch der Präparativen Anorganischen Chemie; Ferdinand Enke, Ed.; Stuttgardt, 1954. Francisco J. Arnáiz Laboratorio de Química Inorgánica Universidad de Burgos, 09001 Burgos, Spain
The author replies: Francisco J. Arnáiz raises several points regarding our article entitled “Synthesis, Characterization, and Lewis Acidity of SnI2 and SnI4” (J. Chem. Educ. 1997, 74, 575). I will address the three numbered paragraphs in order. 1. Arnáiz objects to a literature method of preparing SnI2 because it is “contrary to basic principles of redox properties of halogens and metals…and, of course, it does not work in the usual range of conditions”. He goes on to say that for “similar reasons” and the lack of a “good” reference the synthesis of Na2SnI 6 ought to be removed. The preparation of SnI 2 is from the literature and was included in our paper for completeness—our approach is somewhat different (see objection #2). However, I strongly disagree with Arnáiz’s underlying presupposition: that a synthesis should be discarded, or at least not used in the teaching laboratory if it presents challenges to accepted principles. The synergy between experimental results and the current model is not something we should hide from our students; indeed, it should be showcased. The synthesis we described works and can lead to interesting and informative discussion about our understanding of the principles and mechanisms involved. I do not know what Arnáiz means by “the usual range of conditions”. The synthesis we described does not require any unusual temperatures, pressures, or other conditions. I would not regard the purging of a flask with nitrogen as an extraordinary condition. Arnáiz provides no reason to eliminate the preparation of the sodium hexaiodostannate beyond the lack of a “good reference”. 2. Beyond a reference to alchemy that was apparently meant as an insult, Arnáiz is critical of the method we describe for producing SnI2 because one might obtain a mixture of products. Based on melting points and powder X-ray diffraction results obtained by undergraduate students, the SnI 2 formed was fairly pure. But even if a messy mixture of products did form, most reactions do not yield a perfectly pure product and students need to learn how to deal with the process of isolation and purification.
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Chemical Education Today
3. Arnáiz points out that the amounts of reactants used in the synthesis of SnI4 are not stoichiometric. This is true precisely for the reason that he suggests—to illustrate the idea of a limiting reagent. However, we agree that the concept of limiting reagent could be illustrated with a reagent ratio closer to stoichiometric. Richard W. Schaeffer 22 Sunrise Ave. Lancaster, PA 17601
Articulation In her article on articulation from two- to four-year colleges (J. Chem. Educ. 1997, 74, 1156–1157), Tamar Y. Susskind makes a statement about articulation in California that needs clarification. She writes: “Other states, like California, have central offices to reinforce articulation agreements among institutions. However their statewide agreements are so weak that the only useful transfer agreements are those negotiated among sets of institutions.” Since three of those sets of institutions in the public sector, namely the California Community Colleges (CCC), the California State University (CSU), and the University of California (UC) have, respectively, 109, 22, and 9 campuses, the statewide agreements are not that weak. These agreements include the Intersegmental General Education Transfer Curriculum (IGETC) and the California Articulation Numbering system (CAN). Worked out among the academic senates of the cooperating public sector institutions, IGETC provides a core general education curriculum which, if completed at a CCC, will be accepted as fulfilling all lower-division general education requirements at any CSU or UC campus. CAN is a voluntary articulation scheme in which cooperating faculty groups from all segments agree to accept CAN qualified courses as meeting all requirements for their own campus’s curricula. Courses in chemistry qualifying for CAN to date include general chemistry for science majors, chemistry for allied health majors, and quantitative analysis. Harold Goldwhite Institute for Teaching and Learning California State University, Office of the Chancellor 400 Golden Shore Long Beach, CA 90802-4275
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