Thermite

In this spectacular reaction iron oxide and aluminum undergo a single replacement reaction producing great quantities of heat and a stream of molten iron and aluminum oxide which pours out of a hole in the bottom of the pot. 

Curriculum Notes 

A lot of instructors use this demo in the first week to start their course with a bang, but it can also be used to demonstrate an exothermic, single replacement, or redox reaction. By emphasizing very high temperature to which the mixture must be raised to initiate this reaction, you can use it to illustrate activation energy. This demo may only be performed by the lecture demonstrator. Allow about 10 minutes to perform this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

The equation for this reaction is: 2 Al(s) + Fe2O3(s) --> 2Fe (l) + Al2O3 (l). The large decrease in the energy of the system is a consequence of the high charge concentration on the aluminum ion due to its small size. The oxide ions can approach the aluminum ion more closely than than they can approach the ferric ion. Thus coulombic forces stabilize the aluminum oxide relative to the ferric oxide. The enthalpy change of this reaction is about -850 kJ/mol! The heat generated melts the iron and the aluminum oxide which pour out of the hole in the bottom of the pot. While they are liquid, they will usually separate out with the denser iron sinking to the bottom of the"puddle." After the liquid has solidified and cooled enough to handle, the aluminum oxide "slag" can usually be broken off of the iron, sometimes by simply dropping it on the floor a few times. 

Materials 
  • thermite mixture (finely powdered iron (III) oxide and aluminum mixed in equimolar quantities)
  • thermite starter
  • magnesium strip about 15 cm long
  • a piece of filter paper 3 or cm in diameter
  • propane torch and striker
  • ring stand
  • 1 small ring
  • 1 larger ring
  • steel secondary containment unit (or another flower pot)
  • small flower pot
  • 5 gal. steel bucket filled with dry sand
  • blast shield
  • insulated gloves
Procedure 

Due to the extreme heat generated in this demonstration, particular care must be taken to follow all instructions carefully, particularly safety instructions. The setup consists of a small clay flowerpot in secondary containment (either the custom steel unit that we use or another flower pot) suspended by a ring over a steel bucket with several kg of dry sand in it. Another flower pot is embedded in the sand in the bucket to catch the molten iron and slag. A larger ring is used to keep the flower pot containing the reaction mixture from spilling if the ring clamp loosens. Inside to the flowerpot is a mixture of powdered aluminum and ferric oxide. In a small depression in the center of the thermite mixture is a small amount of "thermite starter" powder. I'm not sure what is in the powder. Inserted through the thermite starter powder into the thermite mixture is a strip of magnesium ribbon. To initiate the reaction, the lecture demonstrator ignites the magnesium ribbon with the propane torch. The magnesium ribbon ignites the thermite starter mixture, which in turn ignites the thermite mixture. In an extremely exothermic reaction, the aluminum reduces the iron in the ferric oxide. The heat generated is sufficient to melt the iron and the aluminum oxide. A stream of molten iron and aluminum oxide pours out of the hole in the bottom of the flowerpot into another flower pot in the sand in the bucket. 

Safety Precautions 
  • Only the lecture demonstrator may perform this demo.
  • The demonstrator should wear safety goggles.
  • Keep a fire extinguisher handy to extinguish any fires started by the molten iron and slag. But do not try to stop the thermite reaction once it has been initiated. You can't!
  • Be prepared to contain the resulting fire if some molten metal escapes your containment system.
  • Under no circumstances should the molten iron and slag be allowed to come into contact with water. A steam and/or hydrogen explosion could result.
  • Make sure that there is a blast shield directly in front of the demonstration apparatus.
  • Be sure that the flower pot in the sand in the bucket is directly beneath the hole in the bottom of the flowerpot.
  • Be sure that there is at least five centimeters of sand under the flower pot in the bucket of sand to provide adequate insulation between the molten iron and the tabletop. It is a good idea to move the tray as soon as it is cool enough to approach to avoid scorching the top of the table. You may have to move the tray several times as it cools.
  • Burning magnesium emits strongly in the UV region of the spectrum. If the blast shield is made out of polycarbonate (most are) it should protect the students by filtering out the harmful UV light. If the demonstrator is not looking at the demo through the shield, she should avoid looking directly at the burning magnesium.
  • If the thermite mixture does not ignite, the lecture demonstrator should wait a full minute before approaching the apparatus. Then she should place a few cubic centimeters of the starter powder in a small depression in the center of the thermite mixture, insert a 15 cm piece of magnesium ribbon about half way into the mixture, and ignite the magnesium ribbon with the propane torch.
  • This reaction spatters small pieces of hot slag. Make sure that there are no easily combustible materials within 3 m of the demonstration.
  • The table on which the demonstration is performed should be at least 5 m from the front row of the class.
  • The demonstrator should immediately move 4 to 5 m away from the apparatus after igniting it.
  • This demo emits fumes with the distinctive odor of very hot metal. Only perform this demo in a large room with good ventilation.
Prep. Notes 
  • The iron oxide and aluminum must be very finely divide for this demo to work.
  • Tamp the thermite mixture solidly into the bottom of the pot with the bottom of a small beaker.
  • Try to let this demo cool for at least 10 minutes before attempting to break it down.
  • The filter paper is placed in the bottom of the flower pot to keep the reaction mixture from spilling out of the hole in the bottom before the reaction is initiated.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group