Thermite Reaction

Equipment

Two clay flower pots whose tops are 2.5 inches in diameter and which have approximately 1 cm diameter holes in their bottoms, durable, non-combustible container filled to a depth of approximately 2.5 cm of sand. The diameter should be large enough to catch the molten iron which will flow from the flower pots. A large ringstand with a ring that will support the flower pots. A safety shield, barbecue lighter, and tongs.

Reagents

40-50 grams of thermite mixture. The thermite mixture is a combination of finely powdered aluminum and iron(III) oxide (also called ferric oxide) in a mass ratio of 1:3 respectively, a 5-7.5 cm length of fireworks sparkler.

Presentation

1. Plug the hole in the bottom of one of the flower pots with a piece of paper or tape.
2. Fill this flower pot approximately 2/3 full with the thermite mixture. This will take 40-50 grams.
3. Push the sparkler down into the thermite mixture in the center of the pot. Push it into a depth such that it is firmly held in place, somewhere between 1/4 and 1/2 of its length is sufficient. Gently tap the bottom of the pot onto a hard surface to insure that everything is well packed.
4. Nest the filled flower pot into the empty flower pot. The inner flower pot inevitably cracks and the outer flower pot contains it.
5. Place the flower pots into the ring and adjust the ring height, so that the pots are clearly visible to the audience.
6. Make sure the sand container is correctly positioned to catch the molten iron that will flow from the pots.
7. Place the safety shield between the audience and the pots.
8. Light the top of the sparkler with the barbecue lighter and step back

Hazards

This reaction produces a large amount of heat flying sparks and molten iron. All combustible materials should be removed from the vicinity of the demonstration and a fire extinguisher should be readily available. Water should not be used to put out any fires or to cool the molten iron. The molten iron can decompose water into hydrogen and oxygen which can be an explosive mixture. Molten iron dropped into water can shatter with a grenade like effect. The molten iron can cause very severe burns and should only be handled with tongs after it has solidified.

Discussion

The chemical reaction that is occurring in this demonstration is as follows:

Fe2O 3 (s)  +  2 Al (s) ®  Al2O 3 (s)  +  2 Fe (s)

The enthalpy change for this reaction is -849 kJ/mol of iron(III) oxide. To give some idea of what this released heat is doing, keep in mind that iron melts at 1,530 ^oC. The amount of themite used in this reaction is suitable for a large lecture hall. The amount may be scaled down for smaller rooms while still providing a spectacular demonstration. After the pots have cooled, separate them and throw the inner pot away. The outer pot should be suitable to be used as the inner pot in the next demonstration. There are numerous methods listed in the literature for setting off this reaction, magnesium ribbon, sulfuric acid and potassium chlorate and sugar, potassium permanganate and glycerine. None of these methods is as convenient, safe, or reliable as the method described herein.

References

1. B. Z. Shakhashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, Wisconsin, 1989, Vol.1, p.85-89