In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
Microscale Thermite Reactions Francisco J. Arnáiz, Rafael Aguado, and Susana Arnáiz Laboratorio de Química Inorgánica, Universidad de Burgos, 09001, Burgos, Spain
The reaction of aluminum with the oxides of a variety of elements—aluminothermy, or the Goldschmidt process— illustrates exothermic reactions that require a high activation energy. It is also an appropriate experiment for introducing Ellingham diagrams. Because of its spectacular nature, this reaction is a favorite for demonstrations, and a number of articles on ways to initiate and conduct the thermite reaction have been published in this Journal (1–11). When a spectacular show is not the main objective, these reactions may be conducted on a microscale level, as described below, with numerous advantages over the usual scale (20– 100 g of Al–MOx mixture). These include: 1. The hazards are insignificant so that the usual protective shields are unnecessary. The reaction can be observed at short distance and conducted simultaneously by several students without special oversight. 2. Sparks and fumes are easily controlled so that hoods, though convenient, are not required. 3. Crucibles used to contain the mixture can be reused. 4. Work-up is facilitated and substantial time is saved, especially in handling the reaction products and cleaning or protecting the installations. The entire procedure is completed in less than 1 hour so that instruction and discussion before or after lab are possible in a normal 3-hour session.
2. Kitchen aluminum foil can be used instead of paper. However, it is prone to irreversible deformation which makes it more difficult to handle. 3. Granulated alumina can be replaced by silica or white sand to facilitate the identification of the products. Obviously, alumina is more advisable to avoid possible contamination of the product. Finely powdered materials are less appropriate than those in the range 50 to 70 mesh because they are partially blown out of the crucible during the reaction. 4. A good gas lighter is sufficient, although heating the magnesium ribbon for 5–10 seconds is usually required. We have failed to ignite the mixture with match heads, but the couple KMnO4/glycerine works satisfactorily. Allowing the students to prepare the starting mixture KClO3/sugar is inadvisable because of the risk of explosion due to shock sensitivity. 5. This facilitates observation of Mg ribbon burning, which should never be undertaken without eye protection. 6. In this manner sparks and fumes remain confined in the inverted vessel. 7. Usually a 100–150-mg iron ball is obtained. It is convenient to transfer the content from the crucible to a Petri dish because the dense regulum frequently appears at the bottom of the crucible. Unburned paper pieces and ashes remaining in the crucible are easily separated because of their low density (blowing carefully is sufficient). 8. At this point, before dissolution of iron, comparing differences in mechanical properties (e.g. deformation by pressing with pliers) with those of a small steel ball is pertinent. 9. In case the metal is completely dissolved, some Fe(III) can be formed by aerial oxidation during the handling of the solution.
Procedure
Further Suggestions
An intimate mixture of 0.9 g of dry Fe2O3 and 0.3 g of dry, finely powdered Al is placed in a small cone1 of common paper.2 The cone is placed in the center of a crucible 4–6 cm in diameter, which is filled to approximately 1 cm from the top with dry granulated alumina3 so that the level of the mixture in the cone is approximately that of the alumina in the crucible. A 4–5-cm piece of magnesium ribbon is introduced vertically in the center of the mixture and ignited with a burner.4 The crucible is immediately covered with a 1-L amber5 glass vessel.6 When the reaction is over, the vessel is removed and the iron regulum7 is recovered. Identification can be made by treating it in a small test tube or vial with some drops of 4 M NaOH to test for the absence of (and eventually to remove) any unreacted aluminum. The iron remains unaltered. Then, the regulum is washed with water8 and treated with warm 6 M HCl to obtain a faintly green FeCl2 solution in which Fe(II) can be tested by conventional procedures.9
Probably many other metallothermy reactions can be satisfactorily conducted under similar conditions. We have tested the following:
Notes 1. A cone angle between 45 and 90° is recommended because we usually failed to ignite the mixture, at this scale, when it was contained in a very acute cone. The same occurred when using homemade clay microcrucibles and glass containers approximately 5 mm i.d.
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Mn 3O4/Al, 3:1 by weight, for manganese. The oxygenrich MnO2 reacts too violently, making it difficult to recover the metal. Mn3O 4 can be obtained by heating MnO2 in a furnace at about 1000 °C for 4 hours. Cr2O3/K 2Cr2O 7/Al, 6:2:3 by weight, for chromium. (CAUTION : Cr(VI) compounds are mutagenic; use gloves and especially avoid dust inhalation.) A high amount of K2 Cr2O 7 is advisable to ensure the ignition. If Cr2O 3 is prepared from (NH 4)2Cr2O 7 we suggest placing the salt into a half-filled ceramic crucible standing on a white tile, igniting the salt with the magnesium ribbon, and covering with the inverted vessel. This facilitates the recovery of spilled Cr2O3 ashes. SiO2/Mg powder, 6:2.5 by weight, for spongy silicon. Wasted silica from chromatography experiments, finely powdered, and heated at 1000 °C for 4 hours, is an excellent starting material.
Acknowledgments To José A. Olivares and to the editor for critical review of the manuscript, and to Dirección General de Enseñanza
Journal of Chemical Education • Vol. 75 No. 12 December 1998 • JChemEd.chem.wisc.edu
In the Laboratory
Superior (PB95-0832) and Junta de Castilla y León (Bu04/ 95) for financial support. Literature Cited 1. Klein, O. C. J. Chem. Educ. 1937, 14, 320. 2. Brockett, C. P. J. Chem. Educ. 1952, 29, 525. 3. Chakoumakos, C. J. Chem. Educ. 1959, 36, A219.
4. 5. 6. 7. 8. 9. 10. 11.
Kindler, L. I. J. Chem. Educ. 1965, 42, A607. Espelund, A. W. J. Chem. Educ. 1975, 52, 400. Bozzelli, J. W.; Barat, R. B. J. Chem. Educ. 1979, 56, 675. Kauffman, G. B. J. Chem. Educ. 1981, 58, 802. Eastland, G. W., Jr. J. Chem. Educ. 1984, 61, 723. Trogler, W. C.; Watkins, K. W. J. Chem. Educ. 1984, 61, 908. Moss, A. J. Chem. Educ. 1987, 64, 257. Foseid, T. L. J. Chem. Educ. 1994, 71, 327.
JChemEd.chem.wisc.edu • Vol. 75 No. 12 December 1998 • Journal of Chemical Education
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