Thermit

Introduction

Flames, sparks and molten iron are produced by this spectacular reaction.

Concepts

• Single replacement reaction
• Oxidation–reduction
• Exothermic reaction
• Activation energy

Background

Thermit is a generic name given to very high-temperature reactions between a metal oxide and aluminum. The thermit reaction produces a metal (iron in this case), aluminum oxide and a large amount of heat. An exterior source of very high temperature must be provided to start the reaction. Once the activation temperature is reached, the entire reaction proceeds very quickly and is over in 10 to 20 seconds.

Thermit is a product that has a wide variety of industrial applications. Thermit is most commonly used for fusion welding. Two metal parts are fusion welded by placing them in juxtaposition, inserting thermit between the parts and then igniting the thermit. A common application is to weld railroad track together.

In school science laboratories, thermit is used to demonstrate an exothermic reaction. However, this reaction must be a science teacher demonstration only!

A temperature of almost 1,200 °C is required to start the thermit reaction. A special starting material, a thermit igniting stick, is required. It is possible to use other materials to start the reaction (like glycerin and potassium permanganate); however, thermit igniting sticks are the most effective, safe and controllable means of igniting the thermit mixture.

Materials

Safety Precautions

The thermit reaction produces a temperature of about 2200 °C which will soften steel. This reaction must be performed in a vessel that can handle the temperature. Thermit igniting sticks are highly flammable and potentially explosive. Keep them dry and away from sparks. Wear a face shield, heat-protective gloves and a chemical-resistant apron. If dark safety glasses or welding glasses are available, they would be a good choice. Place a transparent safety shield between the audience and the reaction vessel. Make sure all flammables and people are at least 10 feet vertically and 15 feet horizontally away from the reaction vessel. Do not use larger quantities of thermit than the amount specified in this procedure. Do this demonstration outdoors. Once the thermit has ignited, do not handle the reaction mixture until all signs of the reaction have ceased. The molten metal and the pan will still be very hot—handle with care.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. After cooling, the aluminum oxide and iron produced may be disposed of in the trash according to Flinn Suggested Disposal Method #26a.. After cooling, the aluminum oxide and iron produced may be disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Procedure

1. When preparing for and performing this demonstration, all safety precautions should be taken. Students should not be allowed to participate. This demonstration should be performed outdoors and should be a science teacher demonstration only.
2. Put a transparent safety shield between the demonstration and the students. Sparks may be thrown 6 feet vertically and 15 feet horizontally; therefore, make sure all flammables and people are at least 10 feet vertically and 15 feet horizontally away from the reaction vessel.
3. Wear heat-protective gloves, a chemical-resistant apron, and a face shield. All students should also wear protective eyewear.
4. A supply of clean, dry sand or a bottle of Class D fire extinguishing material should be immediately available.
5. Place a cast iron skillet or similar vessel on a heat-resistant pad (30 cm x 30 cm) behind the transparent safety shield. Remember: temperatures approaching 2200 °C will be generated. Adequate protection must be provided in the form of a very heavy iron vessel. Do not perform this demonstration on asphalt; the heat of the reaction will melt the asphalt.
6. Pile about 15 to 20 grams of thermit black in the center of the skillet.
7. Before igniting the thermit igniting stick, again make sure no flammable material or people are within 10 feet vertically and 15 feet horizontally of the reaction vessel.
8. To ignite the thermit igniting stick, use a match or butane lighter. Once lit, the igniting stick will look like a sparkler, shooting sparks 6–12 inches away. Quickly plunge the igniting stick underneath the thermit mixture. Step back from the reaction. The heat and sparks from the igniting stick will ignite the thermit mixture. The igniting stick will only be active for 20–30 seconds. Never readjust the igniting stick after it’s in contact with the thermit. If the igniting stick fails to light the thermit, wait until it is extinguished, remove it and start again with a fresh igniting stick.
9. As soon as the thermit is activated, sparks, flames, smoke, and molten iron will be observed. The reaction is violent and proceeds quickly.
10. The molten iron produced by the reaction may weld itself to the cast iron skillet. If the iron is not welded to the skillet, then it may be picked up using metal tongs. The skillet should not be handled unless heat-protective gloves are worn. Allow the vessel to cool to ambient temperature. Don’t handle it unless absolutely necessary.
11. Not all the thermit may react. As the reaction progresses, it may lose temperature and some thermit may not react. This material can be reused.

Student Worksheet PDF

13050_Student1.pdf

Teacher Tips

• The thermit black must be dry in order to work properly. The slightest bit of moisture may be enough to prevent it from igniting. If a supply of this material is required in the future, store it in a Flinn Chem-Saf™ can with a dessicant like Drierite (Flinn No. D0011).
• As is always good practice, try this demonstration first with the aid of another science teacher before doing it in front of a class. Do this demonstration outdoors.
• The thermit igniting sticks are not easy to light—this is a safety feature. To light, keep a flame on the very tip for 10–20 seconds or until it ignites.
• Place the ignition stick under the thermit mixture. This will ensure the greatest degree of complete reaction.
• The reaction may generate enough heat to crack or burn a hole in the cast iron pan. Use an old cast iron pan.
• A common variation of this thermit reaction involves placing the thermit in clay flower pots; this is not recommended. The reaction is much better contained in a cast iron pan. The consequences of using clay, porcelain, glass or any other container instead of cast iron are unpredictable and have resulted in serious accidents. Other methods of igniting the thermit are not recommended. We recommend the thermit igniting sticks since they are more controllable.

Answers to Questions

1. What happened when the thermit black mixture was ignited?

   A great deal of sparks, flames and smoke were produced almost as soon as the thermit was lit. Molten iron could also be seen. The reaction was quick and did not last very long.

2. What is the difference between an endothermic and an exothermic reaction?

   An endothermic reaction is a reaction that takes in heat, using heat as a reactant. An exothermic reaction, on the other hand, is a reaction that gives off heat as a product.

3. Was the thermit reaction endothermic or exothermic? How do you know?

   The thermit reaction was exothermic because of the great amount of heat, mostly in the form of flames and sparks, that was released after the thermit was ignited.

4. Thermit black is a mixture of ferric oxide and aluminum powder. Write a balanced chemical equation for the reaction that occurred between them when the thermit was lit. Include heat on the correct side of the equation.

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

Discussion

Thermit black is a mixture of ferric oxide (Fe₂O₃) and very fine aluminum powder (Al). When the thermit black is ignited, aluminum oxide (Al₂O₃), elemental iron (Fe), and a large amount of heat are produced. The reaction is a highly exothermic, single replacement reaction (exothermic = heat producing). Aluminum is oxidized and iron is reduced. The melting point of iron is 1530 °C and the reaction temperature reaches approximately 2200 °C. (ΔH° = –849 kJ/mole).

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

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, 1983; Vol. 1, p 85.