edited by GEORGE
L. GILBERT
Denison University Granville. Ohio 43023
Fast and Slow Reactions of Chromium Compounds Kerm Knox Rlncslon Unbemly Prlneeton. NJ 08544
It is difficult to demonstrate inertness all by itself because the oerformance of a demonstration in which nothine haw pens is somewhat dull. If, however, that lack of actionis ~recededbv some stem in which something does happen, .. . A d if a comparison experiment accompani;?~the no-reaction step, the result of inertness can be made interesting and apparent. In this demonstration the inertness of ligand substitution by chromium(II1) ions is compared with other reactions that do proceed at reasonably fast rates, and an outcome is obtained in which two solutions of identical composition contain different chromium species of different colors (one is, of course, thermodynamically metastable). The following solutions are made up in pairs: A. 2 g of K2Cr201 in 1000 mL of distilled water in 2-L beakers B. 20 mL of 1M NaOH C. 25 mL of concentrated HzSO1 in 100 mL of water D. 400 mL of concentrated HCl E. 250 mL of 30%Hz02
The ootassium dichromate solutions are olaced on maenetic siirrers side by side in front of a ligh&d white hacimound and vieorouslv stirred. The sodium hvdroxide soluLions are addld to eich. The color change &om orange to yellow is seen to take place essentially as fast as the solutions are mixed. It can be noted at this stage that the colors of these transition-metal com~oundsare not due to the usual factor of a partially filled set of d orbitals, since the chromium has lost all of its valence electrons in coing . - to the 6+ oxidation state. Rather the colors arise from charge-transfer bands, and the color change is a result of a subtle structural change. The chromium is tetrahedrally surrounded by oxygens both in the monomeric, yellow Cr0;- and the orange, dimeric CrnO??. The so~utidnsare now brought back to the acid side by the addition of the sulfuric acid: the color chanee wain occurs rapidly. Then to one of the solutions is added
