The thermite reaction
Demonstrations to capture the student's imagination by Adrian Guy of Blundell's School. In this issue: The thermite reaction
The thermite reaction is spectacular and highly exothermic
© Adrian GUY
Industrially, it can be used for welding railway lines. The exothermic nature and extreme temperature generated, combined with the molten iron, make this reaction attractive to the military for incendiary devices, hand grenades, and also as a reasonably quiet means of disabling artillery by the blocking or damaging of barrels.
Also, a school favourite, the thermite reaction is guaranteed to impress - regardless of age - as the redox reaction between iron(III) oxide and aluminium produces a spectacular pyrotechnic effect and a molten ball of iron.
- 12g of iron(III) oxide;
- 4g of aluminium powder;
- 0.2g of magnesium powder;
- 1.7g of barium peroxide;
- 10cm piece of magnesium ribbon;
- 1 litre beaker filled with sand to 200ml and water to 800ml;
- pipe clay triangle;
- fluted filter paper;
- safety screens;
- heat resistant mats;
- heat resistant gloves.
For best results, use dry iron(III) oxide. To prepare a dry sample, heat iron(III) oxide in a crucible over a roaring Bunsen for a couple of hours. Leave to cool in a desiccator before weighing out 12g into a beaker. Add 4g of aluminium powder and mix by pouring repeatedly from one piece of paper to another. Do NOT mix with a metal spatula or similar. Place the mixture in a sealed specimen jar.
Mix 1.7g of barium peroxide with 0.2g of magnesium powder. Place the mixture in a sealed sample tube.
Place several heat resistant mats on a lab bench so all the surface is protected. Add sand to the 200ml mark on a 1 litre beaker - this prevents the molten lump of iron breaking the glass beaker. Make up to the 800ml mark with cold water.
If you can darken the room with blinds, the pyrotechnic effect is more spectacular.
After the reaction, the red hot ball of iron will continue to glow in the water, causing the water in contact to boil - this makes quite a bit of noise.
After a minute or so, making sure to wear the heat-resistant gloves, carefully remove the iron nugget for the class to view.
Fe2O3(s) + 2Al(s) Al2O3(s) + 2Fe(l)
At Key Stage 3, the simple, balanced equation provides an excellent opportunity to introduce reduction/oxidation (redox) reactions in terms of the loss or gain of oxygen. At Key Stage 4, oxidation numbers can also be used to discuss the process in terms of loss or gain of electrons: Iron is reduced, gaining three electrons and decreasing in oxidation number by three, and aluminium is oxidised, losing three electrons per atom and so increasing in oxidation number by three.
You can also use thermodynamics to calculate the very negative value for H°r and, as expected, a very positive value for S°total.
The ejection of molten lumps of very hot metal is the primary concern here. Safety screens must be used, with heat resistant mats to protect the bench. Pupils, wearing safety glasses, must be over 5m away. A high ceiling is also required. For the demonstrator a full face shield is advised and, after ignition of the magnesium ribbon, one must retreat behind a safety screen at least 5m.
- Barium peroxide is harmful by inhalation and ingestion of dust, and can produce explosive mixture with combustible materials which may ignite spontaneously if damp.
- Only add the ignition mixture when ready to use - do NOT store the ignition mixture or the thermite mixture.
- Powdered aluminium and magnesium can burn vigorously if blown into a flame.
- Dispose of unused mixtures as follows: thermite mixture, dissolve in 2M HCl; ignition mixture, dissolve in 1M H2SO4, filter off the precipitate and dispose of in refuse; a mixture which has failed to ignite should be dissolved in 2M HCl, then add MgSO4(s) to form the precipitate, which can be disposed of in refuse; all other solutions can go down the drain, if roughly neutral (pH 6-10).