A Suggested Modification of the NitrltWNitro lsomerization Kinetic Experiment
To the Editor: The experiment described in: "Base-Catalyzed Linksee k 1981, lsomeriz&ionn [Jackson, W. G. et al. J. ~ h e Educ. 58, 7341 has been successfullv performed in the advanced inorganic laboratory during t h i past six years. Some inaccuracies, however, are present in the theoretical introduction to the experiment. These may be corrected or modified as follows: 1. It is argued in the above paper that the uncatalyzed reaction is too slow for comnletion at 25 "Cdurine a reeular .. rtudentr'lnboratory pcnod, and, therefore, the hnsr.rnfa. lyzed path is preferred. It hnr hecn dernonstrawd in this labnrntory that increasing the temperature to 50 'C allows the complet~onof the uncatalymd isomerira~ionreaction w i t h i n l e s ~than an hour. Despite the faster reaction raw, it was still possible to obtain a rehable set of data for further mmpu&tians. The slow rate of reaction is. however. not the orimarv reason for adontine .. the basecatalyzrd path; rather, the experimcnr serves t o demonstrate a ligand submtutinn mechanism in transition metal cnmplexes and, therefore,should be performed in the way described in the above cited paper.
..
.
2. It is also indicated in the above paper that the nitritonitro isomerization,both in the solid state and in aqueous solutions, is accelerated appreciably by sunlight. Balzani, V. et al. .IZnore. on the mntrsrv. .. Chem. 1986.7.13981. . . ".suegested that hght acceleratrs thr hack reaction A simple demonstration cleared up the dlsafireement kcween the scientists: A powdered nitro isomer (yellow in color) was exposed to sunlight radiation for a week. The top layer of the powder turned orange, characteristic of the unstable nitrito isomer. The bulk powder, which was not exposed to sunlight, remained yellow. The powder was than mixed and exposed again to sunlight. After another week, the same layering was observed. An aquwus solution of the nitro isomer also turned orange under the same eanditions. It is thus concluded that sunlight radiation accelerated the hack reaction.
-
I t is therefore recommended to add this demonstration to the experimental procedure in order to introduce the subject ofphotochemical reactions of transition metal complexes. If enough time is available we suggest students also perform a n uncatalyzed catalytic study a t 50 'C and evaluate the data obtained.
Here, N is the number of species in the system, P, the num"ber of phases, and R, the number of relations that connect the species. This relationship is usually taken one step further by noting that N-R=C Thus, C, the number of components, and N, the number of species (often not the same) are related in a well-defined way. The real brain teaser arises in deciding on the value of R, because that depends on the history of the systemhow it was prepared, whether it was at true equilibrium, and so on. The quantity R has a more general meaning than this special application to stoichiometry. It is the number of relationships not usually incorporated into the kinds of heuristic arguments that lead to expressions that look like the phase ~ l e These . arguments &ally contain statements about the temperature, pressure, and chemical potentials of the components of the various phases. One should really write F=C-P+2-R where Ft is the number of added constraints other than the familiar stoichiometric ones. Usually % = 0, but occasiond v", . not. At a critical ooint comine from the one ohase regiou, there are two special constraints, ones nearly every chemistm student must have seen aonlied to the van der Waals equation to fmd the critical co&tions,
-
To.the -Editor:
~~~
~
With C = 1, P = 1, and Ft = 2, the number of degrees of freedom will be zero. Thus the critical point problem can be viewed in two ways, one starting with two phases, as suggested by Battino, (having Ft = 1) and the other starting with one phase (having R = 2). I have little doubt that there are yet others. Every now and again, special phase cases arise, ones that don't appear to fit into the phase rule. When such situations arise, it should be a warning that there is something else importantadditional constraints and special symmetries, to name two examples-that one must know to define the system. Graham Morrlson Research Chemist National Institute of Standards and Technoloav U S . Department of commerce Gaithersburg, M D 20899
Smadar Goren Marcos Zayat Carla Heitner-Wirguin The Hebrew University of Jerusalem Jerusalem 91904, Israel
Determining Degrees of Freedom
.
Explaining the "Oldie but Goodie" To the Editor:
~
There was a short article in the April 1991, edition of the Journal of Chemical Education by Rubin Battino that addressed the subject "The Critical Point and the Number of Degrees of keedom." How well I remember as an undergraduate being confronted with problems like this ! I am writing to elaborate on what the author did and to present another view of the critical point problem. The author gave the solution to what appeared to be a conundrum by introducing an additional constraint. Students will have seen the matter of added constraints and the Phase Rule in multicomponent systems with chemical reactions. In such systems the number of degrees of freedom is often expressed as follows:
606
Journal of Chemical Education
The note, an "Oldie but Goodie Test for Carbon Dioxide", submitted Al Hazari of "ole in the September 1991 issuecaught my [J. them. ~ d1991,68, ~ ~
7781, I question his explanation of the decolorization of the phenolphthalein, I submit that the carbon diodde reacts with the OH- ions from the Ba(OH)2,forming BaC03, This removes OH- ions from solution, decreasing the pH below 8,which is the p~ of phenolphthalein, thus causing the loss of color. Robert E. Schaffrath C. W. Post Campus Long Island University Brookvllle, NY 11548
.
