TeacherExpt:The thermite reaction
From Learn Chemistry Wiki
This procedure is designed as a demonstration to be performed by a teacher or other qualified person.
This demonstration shows the highly exothermic reaction between aluminium and iron(III) oxide that produces molten iron. This is a competition reaction, showing aluminium to be a more reactive metal than iron. A redox reaction takes place.
The reaction is violent, but safe provided the procedures are followed exactly. Some teachers have had accidents when performing the procedure outside in a strong breeze; the powders blew into the flame, caught fire and caused burns to the hand and/or face. Siting the demonstration in a fume cupboard has caused damage to the cupboard.
The method described here is performed on a laboratory bench and does not produce many fumes. Do NOT do this demonstration in a fume cupboard or out of doors. It produces a result within seconds of setting it off because the water cools the iron down very quickly. A rehearsal is essential if this experiment has not been done before.
There have been occasional reported explosions when using methods similar to this. It is essential not to exceed the stated quantities and that the demonstrator and students are protected by safety screens.
The bench should be clear of combustible materials and protected with a sheet of hardboard or heat resistant mats. Pupils should not look directly at the glare of the burning magnesium, but cover their eyes with their fingers slightly apart. The demonstrator must have room to move quickly away to a safe distance.
The demonstration takes about 10 minutes to carry out if the apparatus is set up and the solid reagents are weighed in advance.
Apparatus and chemicals
- Eye protection
- Safety glasses for observers
- Goggles or face shield for the demonstrator.
The quantities given are for one demonstration
- Filter papers, 12 cm diameter (2)
- Pipe-clay triangle (or similar)
- Beaker, thick-walled (1 dm3)
- Dry sand (see diagram)
- Heat resistant mats
- Safety screens
- Small bar magnet
- Iron(III) oxide (Low hazard), 9 g (see notes 1 and 2)
- Aluminium powder (medium grade) (Highly flammable), 3 g (see note 3)
- Magnesium powder (Flammable), 0.2 g
- Barium nitrate (Harmful, Oxidising), 2 g (see note 4)
- Magnesium ribbon (Flammable), 10 cm length
Aluminium powder (Highly flammable) Refer to CLEAPSS® Hazcard 1
Magnesium powder (Highly flammable) Refer to CLEAPSS® Hazcard 59A
Barium nitrate (Harmful, Oxidising) Refer to CLEAPSS® Hazcard 11
Magnesium ribbon (Low hazard) Refer to CLEAPSS® Hazcard 59A
- It is important that the iron(III) oxide used in this demonstration is absolutely dry. An hour or so in a warm oven, or heating in an evaporating dish over a Bunsen flame, should suffice. The oxide should be allowed to cool completely before mixing.
- The weighed quantities of iron(III) oxide (9 g) and aluminium (3 g) may be thoroughly mixed beforehand by repeatedly pouring the mixture to-and-fro between two pieces of scrap paper, and then stored for the demonstration in a container labelled ‘Thermite mixture’.
- The demonstrator may wish (or be persuaded by the audience) to do a repeat demonstration. In this event it is important to keep the second set of materials well away from the first demonstration site.
- The weighed quantities of magnesium powder (0.2 g) and barium nitrate (2 g) may also be thoroughly mixed beforehand as indicated in note 2, and then stored for the demonstration in a container labelled ‘Igniter mixture’.
Health & Safety
A face shield or goggles and a laboratory coat (it can become messy at the end) should be worn by the demonstrator.
Students should stand further than 4 m from the reaction and wear eye protection. Safety screens must be used to surround the apparatus.
A. Fold two 12 cm diameter circles of filter paper into fluted cones and place one inside the other.
B. Into a 1 dm3, thick-walled beaker, pour dry sand until it is one-third full and then add water until it is two-thirds full.
C. Cover an area of the bench with several heat resistant mats and place the beaker in the centre. Set up the equipment as shown in the diagram above and surround it with safety screens. Add the Thermite mixture (see note 2) to the fluted filter paper cone sitting in the pipe clay triangle.
D. Make a depression in the Thermite mixture with a spatula and place the igniter mixture (see note 4) into it.
E. Insert a magnesium ribbon fuse upright into the igniter mixture. It must extend above the fluted filter paper. Light the magnesium fuse with a Bunsen burner flame and retreat to a safe distance behind the safety screens. A very vigorous reaction should follow, with some sparks flying upwards. The very hot residue containing molten iron will fall through into the water.
F. Once the reaction has stopped, remove the beaker and decant the water into the sink. Retrieve the iron formed with a magnet. Wash the iron under running water.
This shows that aluminium is above iron in the reactivity series.
The ‘Thermite’ mixture is stable until strong heating is applied, hence the need for an initiating reaction between the barium nitrate and magnesium powder. Once underway, the reaction is highly exothermic, rapidly reaching temperatures as high as 2000 °C, well in excess of the melting point of iron (1535 °C). The practical use of this reaction to weld railways lines together should be mentioned – see web link below.
Do not use potassium manganate(VII) and hot glycerol as an alternative to initiate the reaction in this version because the filter papers catch fire. Do not use any other metal oxides, such as copper oxides, chromium(VI) oxide, lead oxides or manganese(IV) oxide. However, chromium(III) oxide and [[Substance:Mn3O4|Mn3O4]] can be used.
This experiment has been adapted from Practical Chemistry: http://practicalchemistry.org/experiments/the-thermite-reaction,172,EX.html
A number of other experiments on the website also illustrate the competition principle.
- Experiment 38: Competition for oxygen
- Experiment 39: Displacement reactions between metals and their salts
- Experiment 40: Extracting metals with charcoal
- Experiment 42: The reaction between zinc and copper oxide
There are many video clips of Thermite reactions on the internet, some carried out on a scale and in a manner which is extremely hazardous. Two clips of reactions carried out safely using a different procedure to that outlined here can be found at:
Note that in the following reaction a much coarser mixture of the solids, as in commercial Thermite charges, is used. Using powdered solids on this scale would be extremely hazardous.
Details and pictures of the thermite welding of railway tracks can be found at: www.northeast.railfan.net/high_iron.html#thermite