edited by DONNA BOGNER Wichita State University Wichita, KS 67208
Tools of the Trade Mlchael J. Bruno Cherokee High School Mariton. NJ 08053 Ad men and bridge players are not the only people who use unconventional means to persuade (a consumer) or to inform (a partner). High school chemistry teachers also use them and we can never have too many audio-visual aids in our pedagogic tool boxes. Since most of the techniques in my own repertoire have been acquired through the painstaking process of stumbling upon ideas by accident, one could suhtitle my teaching experience as "The Acquisition and Strategic Use of Chemical Imagery". After all, our ultimate goal as chemistry teachers is to persuade our students that chemistry is fascinating and relevant by presenting the suhstance of chemistry in a stimulating way. The technique that is descrihed here is one in which the relative size of the chemical symbols is used in conjunction with classroom demonstrations to convey several kinds of information about equilihrium systems and about the behavior of such systems when the concentration of a reactant is changed. This is achieved by incorporating the type of symbols that Peters' utilized to illustrate gaseous equilihrium into demonstrations similar to those descrihed by O'Connor et aL2,involving .the chromate ion-dichromate ion equilihrium. Aaueous chromate solutions can he represented by the net equation 2Cr02-
+ 2Ht
-
CrzO$-
+ H20
(1)
Le Chatelier's principle would suggest that the anionic concentrations are sensitive to changes in pH, with the concen, tration of the chromate ion, Cr04-2, predominant a t high pH values and that of the dichromate ion, Cr20,-2, predominant at low nH values. The stoichiometric relationship amona the concentrations of the ionic species ran he expressed in terms of the following equilibrium condition: ~~
and 6 M NaOH. Prior to the demonstration, write eq 1on an overhead transnarencv to introduce the svstem under investigation. ~ r o n i u c ethe names of the compounds as you write the formulas. Have a large sample of reagent-grade potassium chromate to show the students and explhin that the yellow color is due to the presence of the chromate ions. Do the same thing with potassium dichromate. Show the following equation in the same manner. chromate dichromate 2Cr0;- + 2Ht Crz0,2- + H,O (2) yellow orange Set a beaker half filled with water on the stage of the overhead projector and add 20 g (0.1 mol) of K2CrO4. Delay stirring for 30-60 s to allow for ohservation. When the "oohs" and "aahs" subside, stir the mixture until all the solute has dissolved. Ask the students to describe their observation that "the solution hecomes yellow" and then to interpret it in light of the foregoing presentations and equations. A few students
-
Figure 1. The chromate iondichromate ion equilibrium before H I ion has been added. Symbol size is directly related to amount of substance present.
Figure 2. The immediate elfects ofadding 1 M HCi mthe system;the amount of Ht ion increases as does the rate of the fwward reaction.
~
Figure 3. Equilibrium bas converted lo Cr2072- ion.
been
reestablished. Some Cr0,2- ion
has
been
where the parentheses signify molar concentration. The value of the equilibrium constant, K, is 3 X 1014a t 25 DC.3Since this value is unchanged a t constant temperature, any variation in the concentration of an ionic species will lead to concomitant changes in the concentrations of the other two ionic species. (The concentration of water is incorporated into the constant, K, since i t is essentially unchanged due t o its presence in large excess and because it dissociated to an extremely small degree.) The equipment needed is an overhead projector, a 400-mL beaker, reagent hottles of potassium chromate, KzCrOa, and potassium dichromate, K2Crz04, and 20 mL each of 6 M HCI I Peters. E. I. lntroductlon to Chemical Prlnclples; Saunders: Philadelphia, 1982; p. 478. O'Connor, P. R.; Davis, J. E.; McNab, W. K.; McClellan, A. L. Laboratow Manual for Chemlstw: Experiments and Principles; Heam: ~ e x i n ~ t oMA. k 1982; pp 74-75. O'Connor et al., p 178.
898
Journal of Chemical Education
Figure 4. A summary of the effects of adding 1 M NaOH to the mixture represented in Figure 3. (a) Before OH- ion is added. (b) immediate elfensof adding 1 M NaOH. (c)Equilibrium has been reestablished.
will shortly conclude that the solution contains many chromate ions and very little, if any, dichromate ions. Ask them to explain the evident scarcity of the latter anion, and wait for the reply that water contains only miniscule quantities of the hvdroeen . .. ion. Illustrate the situation using enlarged and small symbols on the chalkboard or on an overhead transpancv (see Fia. 1). It is almost unnecessary to explain the cokela& between the size of the symbols and the relative amounts of substances. Before you add the 6 M HCI ask your students to predict any "immediate" consequences of adding acid to the equilibrium. When you have consensus that the amount of hydrogen ions will increase, present the next illustration (see Fig. 2) and use the natural curiosity of the students to advantage to elicit the significant of the longer arrow (increased forward reactions rate) and the reason for it (increased collison frequency).
Now add 20 mL of 6 M HCI slowly and, as before, ask the students to describe and interpret their observations. It is usually necessary to point out that the cessation of color change signals a return to equilibrium, albeit the position of the equilihrium has shifted. By this time many students will have anticipated the next illustration (see Fig. 3). As a follow-up exercise ask students to describe the situation when one starts with an aqueous solution of potassium dichromate and to interpret the effect8 of adding 6 M NaOH to the solution by drawing their own illustrations. Explain the effect of the sodium hydroxide on the concentration of hydrogen ions prior to the addition of the base (see Fig. 4). This graphical use of chemical symbols has the desired effect of elucidating a number of chemical concepts in a manner that is clear, interesting, and likely to be retained. In addition, the demonstration can serve to illustrate Le Chatelier's principle and to represent the stoichiometric relationships implicit in the chemical equation.
Volume 65 Number 8 August 1988
899
