Chemical Education Today
Letters Concerning ”A Reaction That Takes Place in Beakers but Not in Conical Flasks” I find it distressing to encounter a lecture demonstration that seeks to make a valid scientific point through deception. I refer to the Tested Demonstration, “A Reaction That Takes Place in Beakers but Not in Conical Flasks: A Catalysis-Related Demonstration” (1). The surreptitious introduction of Fe(II) into the beakers but not the conical flask does indeed produce the indicated effects, as the title states. The take-home message, however, is not one of integrity. By the time matters are all sorted out, and the demonstrator confesses his trick, the innocent observer is not likely to retain much of a scientific message. Nor, perhaps, a highly favorable impression of chemistry. This is unnecessary. An honest, effective, and impressive demonstration makes the point correctly and directly. One should mix the Cr(VI), Fe(II), and KI–starch solutions in different orders. It will be perceived that only those in which KI–starch is the first or second component will yield the I2–starch complex upon mixing. Decolorization of the I2–starch with thiosulfate can also be incorporated if one desires. From there, the issue of induced reactions, specifically the trapping of the Cr(V) transient by iodide, will provide plentiful discussion.
The author replies: Although I respect the right of Espenson to object to my suggestion of a useful teaching aid, I fundamentally disagree with the argument put forward. A good teacher uses a variety of teaching styles and aids to stimulate and educate students, including, if appropriate, “trickery” or showmanship. I have used this demonstration many times with various groups of students over a number of years and it has always produced the desired results, that is, students enjoy this demonstration and—this is the key point—they do remember the take home message that substances are only catalysts if they bring about a reaction without being consumed in the process. Can Espenson guarantee the same about his suggested procedure for carrying out this demonstration? I think not, especially since adding the iodide last still produces, albeit more slowly, the black I2–starch coloration! Colin White Department of Chemistry The University Sheffield S3 7HF, England
[email protected] Literature Cited 1. White, C. J. Chem. Educ. 2004, 81, 364. James Espenson Department of Chemistry Iowa State University Ames, IA 50011
[email protected] www.JCE.DivCHED.org
•
Vol. 82 No. 4 April 2005
•
Journal of Chemical Education
527
