In the Classroom edited by Tested Demonstrations Ed Vitz Kutztown University Kutztown, PA 19530 Catalytic Oxidation of Ammonia: A Sparkling Experiment submitted by: Vladimir A. Volkovich and Trevor R. Griffiths* School of Chemistry, University of Leeds, Leeds LS2 9JT, UK; *[email protected] checked by: Paul E. Smith Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393 The usual lecture demonstrations of the catalytic oxida- Particles of the catalyst remain suspended in the flask, tion of ammonia involve the use of preheated platinum or glowing red and yellow, some staying hot for up to a minute. copper metal coils (1, 2). Several transition metal oxides can The addition of the catalyst and the reaction can be repeated be used as catalysts for oxidizing ammonia, but chromium(III) several times. At the end of several additions the flask is oxide, Cr2O3, allows this reaction to be demonstrated in a seen filled with a white cloud of water vapor. Ideally, the really spectacular way, generating sparks in a flask of ammonia whole demonstration ought to be performed in a hood or gas. Several lecture demonstrations on Cr2O3 catalysis have next to a source of down-draft ventilation. However, if the previously appeared in this Journal (3), including that on the chromium(III) oxide is prepared in advance, then there is very oxidation of ammonia, but we have now expanded it with a little dust from Cr2O3 particles in the demonstration itself and truly vivid effect. The experiment described below is relatively the main source of irritation is just the smell of ammonia. simple, but surprisingly, we could not find any mention of it The mechanism of this reaction is quite complicated. in the published literature. Most of the ammonia is oxidized to nitrogen: Experimental Procedure 4NH3 + 2O2 = 2N2 + 6H2O(1) A suitable form of chromium(III) oxide is best prepared In principle some ammonia may oxidize further, forming by thermal decomposition of ammonium dichromate, by the nitrogen(II) oxide, ultimately producing ammonium nitrate: well-known “volcano” experiment (4, 5). This yields an active 4NH3 + 5O2 = 4NO + 6H2O(2) catalyst with light particles. Commercially available Cr2O3 4NO + 4NH + 3O + 2H O = 4NH NO (3) reagent is generally not suitable for this particular experiment. 3 2 2 4 3 Once prepared from the dichromate, the catalyst can be stored However, we tested solutions after several experiments and for a long time without loss of activity. were unable to detect the presence of nitrate ions. Therefore, if any nitrate is formed its concentration is very small. The CAUTION: Chromium(VI) compounds and chromium(III) absence of nitrate may be due to the reaction between NH and oxide are irritants for the skin and eyes and especially the 3 NO to form N2, N2O, and H2O, catalyzed by chromium(III) respiratory tract if inhaled (6). In addition, some chemical oxide (7, 8). A detailed investigation of this apparently simple suppliers note that chromium(VI) compounds, including reaction can thus be a topic for a student project. (NH4)Cr2O7, are suspected carcinogens, the risk being re- We have used this experiment successfully in several lated to the length of exposure. Gloves and safety goggles public chemical “magic shows”. It works especially well when should be worn when handling (NH4)2Cr2O7 and Cr2O3. a sequence of reactions is undertaken. First, the catalyst is The “volcano” experiment should be performed in a well- prepared by the “volcano” experiment, and then it is used to ventilated area. If performed on an open bench, this reac- oxidize ammonia (“sparks in the flask”). Finally, the rest of tion can be carried out in large beaker covered with several the prepared chromium(III) oxide can be used to demon- layers of fine wire gauze as suggested in ref 4. strate the thermite reaction (5). The demonstration of the catalytic oxidation of NH3 is best performed in a darkened room. The effect is clearly vis- Literature Cited ible even in a large room, but a video projector (if available) 1. Fowles, G. Lecture Demonstrations in Chemistry, 3rd ed.; G. Bell can be used. The capacity and shape of the vessel in which & Sons: London, 1947; pp 175–180. the reaction is demonstrated does not matter much, for the 2. Tested Demonstrations in Chemistry, Vol. 1; Gilbert, G. L.; Alyea, experiment works well in a 1-L conical flask or in a 20-L bottle. H. N.; Dutton, F. B.; Dreisbach, D., Eds.; Journal of Chemical For our demonstrations we used a 10-L round-bottom flask. Education and Division of Chemical Education, Inc.; American Chemical Society: Granville, OH, 1994; p I-35. Approximately 15–20 mL of concentrated ammonia solution 3. Briggs, T. S. J. Chem. Educ. 1970, 47, A206. ᎑3 (35% [0.880 g cm ] or higher) is placed in the flask, which 4. Shakhashiri, B. Z. Chemical Demonstrations, a Handbook for is then stoppered and swirled to mix the ammonia with the Teachers of Chemistry, Vol. 1; The University of Wisconsin Press: air in the flask. Chromium oxide is heated in a deflagrating Madison, 1986; pp 81–82. spoon in a Bunsen flame until it begins to glow red, for ca. 5. Tested Demonstrations in Chemistry, Vol. 1; Op. cit., p H-9. 6. Hazards in the Chemical Laboratory; Muir, G. D., Ed.; The Chemi- 5–10 s. The stopper is then removed and the catalyst is gently cal Society: London, 1997; p 199. tipped from the spoon into the flask. A fountain of red sparks 7. Niiyama, H.; Murata, K.; Can, H. V.; Echigoya, E. J. Catal. 1980, is immediately produced, momentarily coming out of the 63, 1–10. neck of the flask (CAUTION!). It is wise at this time to replace 8. Curry-Hyde, H. E.; Musch, H.; Baiker, A.; Schraml-Marth, M.; the stopper to trap the hot particles and the smell of ammonia. Wokaun, A. J. Catal. 1992, 133, 397–414. JChemEd.chem.wisc.edu • Vol. 77 No. 2 February 2000 • Journal of Chemical Education 177.