Demonstration 13
Volcano (Pyrotechnics ‘Vesuvius fire’)
This reaction is one of the classics of chemistry demonstrations and can now be found in essentially every book on this subject [1-5]. The orange crystals of ammonium dichromate, if heated to a sufficient temperature, start to decompose producing voluminous green chromium(III) oxide. Once started the reaction is self-supporting because it does not require any external oxidising or reducing agents. In fact, both the oxidant (Cr+6) and the reductant (N-3) are present in the same molecule,
(NH4)2Cr2O7==> Cr2O3 + 4 H2O + N2 (13.1)
Normally this demonstration is performed as the classical Volcano experiment, so called because ash produced occupies a far greater volume than that of the original dichromate and so builds a ‘mountain’ of dark green ash with sparks and read heat emanating from a ‘Caldera’ at its summit. Small packets of ammonium dichromate were once sold, or sometimes popped out of Christmas crackers, for a tabletop pyrotechnic display along with the instruction to place it on a saucer. Because the chromium(VI) and (III) compounds, like so many other things, are now suspected carcinogens this demonstration should no longer be carried out on an open (sand) tray as we have done here. We had the benefit of bench extraction and you can see the oxide particles being drawn into the vent. When extraction is not available we place the dichromate in the bottom of a 5-litre round-bottom flask with a long neck and heat the outside; decomposition begins at about 180 oC and is self-sustaining at ca. 225 oC. A porcelain filtering funnel in the top prevents most of the particles from escaping.
Preparation. No specific preparation is required. Approximately 20 g of ammonium dichromate
Demonstration. Place a pile of ammonium dichromate on a sand tray (or a ceramic tile) near a source of extraction ventilation. Use a gas burner to heat the tip of the ‘mountain’ until the reaction begins. The experiment should be shown in both lighted and darkened room.
An alternative way to start the ‘volcano’ is to wet the tip of the cone of ammonium dichromate with a combustible liquid (i.e., ethanol, acetone) and to set light to it by a gas burner (or a match).
Safety. This experiment should be carried out on an open tray only in a well-ventilated area. Chromium(VI) compounds and chromium(III) oxide are irritants for the skin and eyes and especially the respiratory tract if inhaled [6]. In addition, some chemical suppliers note that chromium(VI) compounds, including ammonium dichromate, are suspected carcinogens, the risk being related to the length of exposure. Gloves and safety goggles must be worn when handling (NH4)2Cr2O7.
References.
1. M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, 2-nd ed., Moscow, Vyschaya Shkola, 1984, p. 38 (in Russian).
2. B.Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1, Madison, The University of Wisconsin Press, 1986, pp. 81-82.
3. Tested Demonstrations in Chemistry, ed. G.L. Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. A-2.
4. Ref. 2, p. H-9.
5. H.W. Roesky and K. Möckel, Chemical Curiosities, trans. T.N. Mitchel and W.E. Russey, New York, VCH Publishers, Inc., 1996, p. 78.
6. G.D. Muir (ed.), Hazards in the Chemical Laboratory, London, the Chemical Society, 1997, p. 199.