THERMITE

CHEMICAL CONCEPTS:

single replacement reactions
exothermic reactions
chemical reaction driving forces (enthalpy, entropy)
oxidation/reduction reactions

BACKGROUND:

A balanced equation for the reaction is:

2Al + Fe₂O₃ à Al₂O₃ + 2Fe

In this reaction iron (III) oxide is reduced to metallic iron by aluminum. It is reported that the reaction will reach a temperature of about 3000�C, meaning both reaction products are molten (mp Fe = 1530�C; mp Al₂O₃ = 2030�C).

MATERIALS NEEDED:

Aluminum metal, fine granular
Ferric (iron III) Oxide powder (Fe₂O₃)
Magnesium ribbon
two 4" clay flower pots (with drainage holes in bottom)
dry sand
trash can or large metal bucket
sturdy metal support stand
heat-protective gloves
tongs

CAUTION:

This activity has many potential hazards which need to be evaluated. The reaction produces smoke, and sparks may be thrown a great distance. I have always performed this demo outside! If you have never performed this demonstration, I would recommend that you try it outdoors first. I would also recommend using a smaller quantity of powder than described below, if you are performing this indoors.

Think twice before jumping into this activity. Moreover, when performing this or any experiment, use common sense and wear protective safety gear!

PREPARATION:

The thermite mixture should be composed of 25.3% Al and 74.7% Fe₂O₃ (by weight) to provide the proper molar quantities. I mix the two reactants thoroughly, using anywhere from 30 - 120 g of powder for each demonstration.

I have found that using one flower pot will provide less than optimum results. The pot always shatters and drops into the sand where the reaction finishes out of sight. To remedy this I put one flower pot into a second flower pot. This reinforces the internal flower pot so that when it breaks under the heat stress, it will remain in place and allow the molten products to flow freely out the hole in the bottom and onto the sand below.

To prevent the powder from falling through the hole when loading the flower pot, I cover the hole with a small piece of loose-leaf paper. When the contents have been emptied into the pot, I add the fuse. Despite all the elaborate fuses I have read about, I have found that a simple piece of magnesium ribbon works fine (the burning temperature of magnesium is about 315�C). I place a piece several inches long down into the center of the powder, making sure to allow enough fuse to protrude so that I have enough time to back up several feet after lighting it, before the reaction begins.

My setup consists of a 2 inch thick slab of metal as a base (optional). Upon this rests a typical wastecan filled � full with dry sand. On either side of the wastecan, two metal stands (or folding chairs) support a metal shelf with a hole cut into the center of it. The flower pots rest in this hole.

SAFETY PRECAUTIONS:

This reaction is highly exothermic and produces molten metal. Remove all flammable material from the demonstration area. Water should not be used to extinguish the reaction, since addition of water to hot iron produces potentially explosive hydrogen gas.

Since the reaction products will remain red hot for some time (see video below), they will not be able to be moved until they have cooled some. Wear heat-protective gloves and use tongs to remove the products.

DISCUSSION:

I begin this demonstration by handing out a worksheet to my students which gives them some background on this reaction and asks several review questions that they should know. We then go outside to see the demonstration - before I begin I make sure that all students see the reactant mixture. The students are asked to make several observations about the reaction and we then go back inside to finish the discussion. So many things can be tied into this demonstration, such as states of matter, energy/activation energy, etc. The next day I pass around the vitrified products whole and broken apart so they can see the actual iron as well as the glass made from the melted sand.

An interesting side to this experiment involves the magnetic properties of iron and the aid of a small magnet. Before performing the demonstration, place a small magnet near the iron oxide to demonstrate its lack of magnetic properties. When the reaction has finished and the products cooled, place that same magnet near the iron to demonstrate the change in magnetic properties that have occured.

Video Clips (QuickTime)

Video Clips (AVI - Windows only)

thermite reaction (484 KB)
thermite reaction products (381 KB)

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OTHER REFERENCES (sites will open in a separate browser window):

Microscale Thermite Reactions
Thermite Reaction
The Thermite Reaction
The Thermite Reaction
Thermite Reaction