Microscale Thermite Reaction | Harvard Natural Sciences Lecture Demonstrations

When two rusty iron balls—one wrapped in aluminum foil—are struck together, sparks and light are given off in this small scale thermite reaction.

Video of Microscope Thermite Reaction can be found at https://www.youtube.com/watch?v=V5VXt0hZOc8

What it Shows

This is a much safer, microscale version of the thermite reaction—very fun and easy to perform! The classic thermite reaction is a spectacular oxidation-reduction reaction that takes place when aluminum metal is reacted with rust, Fe₂O₃.

2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(s) + heat

In this version, two rusty iron balls, approximately 2 kg each and 7 cm in diameter, are struck together with a glancing blow.

The mechanical energy of the balls colliding provides enough activation energy to allow the reaction to occur, resulting in the formation of the products and large amounts of heat. There is a loud noise and flying sparks that occur due to the large amount of thermal energy released in the reaction.

How it Works

The classic thermite reaction is an energetic oxidation-reduction reaction where aluminum is oxidized and iron is reduced, according to the equation above. This highly exothermic single replacement reaction is so energetic that it is used to weld railroad tracks together. Variations of the reaction are also used in underwater welding. Once the reaction starts, it is self-sustaining and does not depend on an external supply of oxygen. A video of the large scale version can be viewed here.

The heat of reaction is ΔH° = -849 kJ/mol, resulting in a reaction temperature of approximately 2200 °C. This is hot enough to melt the iron produced, which has a melting point of 1530 °C. The bright yellow sparks result from microscopic amounts of extremely hot molten iron being thrown out into the air. The molten iron immediately oxidizes with the oxygen in the air, producing additional energy. The yellow color of the sparks is caused by the emission of light following the excitation of electrons in iron atoms. There is also a loud cracking sound when the reaction takes place that is not just the sound of the impact.

Setting it Up

Safety

Wear safety goggles. A lab coat is recommended, as the sparks can travel 2 – 3 feet. Gloves are optional, but can be worn to prevent getting rust on your hands. Practice striking the balls, as it requires a hard glancing blow. Hold the balls tightly and make sure to keep your fingers out of the way when striking the balls together. It is recommended that only the instructor perform this demonstration. Repeated striking can cause arm and shoulder pain!

Demonstrating it

1. Check the aluminum foil. Replace it if there are too many holes. Use a single layer of regular thickness aluminum foil.
2. Place the rusty ball in one hand and the foil covered ball in the other. Usually the rusty ball is held in the dominant hand.
3. Keeping your fingers out of the way, strike down on the foil covered ball with the rusty ball. Use a hard glancing blow. When the balls are struck with enough force, there will be a loud crack and sparks that fly out.
4. Rotate the rusted ball and the foil covered ball to expose fresh surfaces and repeat the procedure. With practice, you will be able to repeat some nice big sparks.

Cleanup and Disposal

The iron balls can be repeatedly reused. If you ever get tired of them, they can be thrown in the trash.

Comments

Any sort of steel/iron ball can be rusted for this demo. Depending on the size of the balls, it may be easier or harder to handle them. If the rust is all removed from the reaction, place the balls in salt water several days and let air dry to restore the rust. If an oxide coating is present, use emery cloth or a scouring pad to remove the oxide. One extension is to place a piece of paper between the ball and hit the balls together to burn holes in the paper. A dark spot and hole will appear.

References

This reaction was first performed by Larry Peck from Texas A&M University, at the 16th BCCE in 2000.

Slater, A. (2016) Smashing Thermit Reaction. Flinn Scientific ChemFax. https://www.flinnsci.com/api/library/Download/e1dc013804de4cd98379eadeea372db2

University of Nottingham, School of Chemistry, Periodic Videos. (2010, April 10). Iron – Periodic Table of Videos [Video]. YouTube. https://youtu.be/euQUgp5AY-Y?si=XueWUV2EgvGoDr8M