Characterization of the Product from the Thermal Decomposition of Chromium(lll) Nitrate and Urea An Undergraduate Project Involving an Unknown Metal Complex Donald A ~ o u s e ' University of Canterbury, Christchurch, New Zealand Graeme H searle2 University of Adelaide, Adelaide, S. Australia, Australia As part of our series of project experiments for undergraduate students involving the complete characterization of unknown metal complexes (1-4) we describe here the product resulting from the thermal decomposition of urea in the presence of chromium(II1) nitrate (5). To be successful, such unknowns should he unpredictable in terms of the student's chemical knowledge, and yet the structure should be deducible using the same knowledge resources. The project is part of a n optional program for our finalyear BSc. students. With students working in pairs, about 8 h of laboratory time is required, after the initial synthesis. The unknown complex also has a desirable component insofar as many textbooks of chemistry have some historical comment on th_e "vital force" associated with the living cell and the production of "organic compounds". The ohservations t h a t led to t h e abandonment of this doctrine (Wohler (1828)) are intimately involved in the formation of the "unknown" species. Preparation of the Complex Caution: The oven should have good vents and be placed in a well-ventilated fume hood because gaseous ammonia is a major product. Grind together in a mortar 10 g each of Cr(N03)3.9Hz0 and urea, and transfer the rather hygroscopic mixture into a 250-mL beaker. Cover the beaker with a loose lid of aluminum foil in the shape of a n inverted filter funnel, and heat the mixture in a n oven a t 130-140 "C for 48 h. As the reaction proceeds, the mixture first melts to a green, fluid liquid that gradually becomes more viscous and finally solid. Gas evolution during the final stages of solidification can cause splattering of small amounts of the mixture on the walls of the beaker. After 48 h the cooled mixture is a brittle green solid with a brownish reflection. Microscopic examination shows orange crystals of the product embedded in a greenish matrix of nnreacted starting material. Schmidtke and Schonherr (5) describe the preparation by heating a t 170 'C. We were unable to control the reaction a t this temperature, and decomposition to Crz03was the usual result. Even under the conditions described for our preparation there are occasional samples where irreversible Cr203formation occurs. Consequently, there is a more than usual expectation of irreproducibility associated with this synthesis. Extract the cooled solid with a mixture of 50 mL of ethanol and 50 mL of 0.5 M HN03 with gentle warming (
