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    Thermite - A Solid Reaction

    Kids, can you imagine doing chemistry with just aluminum foil and some rust?   By using these compounds in just the right way, you can perform a simple yet rather spectacular process that is one example of the so-called "thermite" reactions.   A full-scale thermite reaction is much too dangerous for classroom demonstration, but you can get the idea by doing a scaled-down version with an adult partner.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    First, find two large, rusted iron spheres such as old track-and-field shotputs or 2-3" diameter ball bearings - the rustier the better.   Cover one with regular aluminum foil.   While you are both wearing goggles and taking much care not to smash fingers, have your adult partner strike the spheres together with a glancing blow.   Simply banging them together head-on will not work; they must be scraped and slid across each other like striking a match.   When they hit just right, you will see and hear a mini-thermite reaction at the point of contact.   The resulting sparks and noise are like firing a toy cap gun.

    What's going on here?   This thermite reaction is a single replacement reaction between iron(III) oxide (rust) and aluminum metal to make aluminum oxide and iron metal.   Chemists write it as: Fe2O3 + 2 Al(0) " Al2O3 + 2 Fe(0) + heat.   Because heat is released it is one example of an exothermic reaction.   The heat release is so high in fact that temperatures reach 2,200°C!   This is actually hot enough to melt iron, which has a melting point of 1,530°C.   This property is put to good use for producing molten iron that welds train track rails together out in the field (see references below for more information on this).

    One very interesting aspect of the thermite reaction is that it takes place in the solid state.   Just a few reactions in nature occur between solids, and these few tend to go so slowly that they are rarely even noticed.   Fires derived from thermite energy are quite difficult to extinguish.   Unlike combustion, thermite fires do not need oxygen from the air to start, so the typical extinguishers using CO2 or other gases to smother flames are not very effective.   And using water can be downright dangerous.   The high temperatures decompose water into hydrogen and oxygen, causing even more explosions.  So the recommended method is to cover the reaction with sand.

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    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    kcarrado@anl.gov
    September 2003

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    References:   Michael Tinnesand, "Mighty Thermite", in ChemMatters, ACS, Feb 2002, pp. 14; A. L. Feliu, "Thermite Welding Gets High School Chemistry Class on Track", J. Chem. Ed., Jan 2001, pp 15.