The thermit (or thermite) reaction is the name given to a series of displacement1 reactions between powdered aluminium and various oxides or sulphides of metals that are below aluminium in the reactivity series of metals.
The name thermit is also given to the reaction mixtures themselves.
The ability of finely-divided aluminium to reduce certain metal oxides, with the evolution of sufficient heat to melt both the metal and the alumina produced, was noted by both Sainte-Claire Deville and Wöhler soon after the first isolation of aluminium by Wöhler in 1827.
In 1886 Héroult and Hall independently devised the electrolytic process for the extraction of aluminium from bauxite; a process which superseded the expensive sodium reduction process and which provided a quantum leap in the economics of aluminium supply.
Following this, in 1898 Dr Hans Goldschmidt of Essen made the aluminium reduction process commercially viable and used aluminium to obtain either the metal or metal alloys from the oxides of chromium, manganese, iron, copper, titanium, boron, tungsten, molybdenum, nickel, cobalt, zirconium, vanadium, niobium, tantalum, cerium, thorium, barium, calcium, sodium, potassium, lead and tin.
In a thermite reaction, a metallic compound is reduced by one of several metals or metallic alloys in such a way that when the mixture is ignited at one place, the reaction continues of its own accord, so that under complete oxidation of the reducing element, a fluid slag is formed, while the reduced metal is obtained as a homogeneous uniform regulus; if the oxide is used in excess, the reduced metal is free, or practically free, from the element used as a reducing agent.
In addition, Goldschmidt devised the means of initiating the reaction using a fuse, instead of heating the mixture until ignition took place. He also devised the equipment necessary to produce molten iron or steel for the in situ welding and repair of rails and machine parts; and for many years these have been a familiar sight on our railway systems.
Indeed, the most familiar of the thermit reactions is the reaction between iron(III) oxide and aluminium, where the oxide is reduced to the metal and the aluminium is oxidised to aluminium oxide. A temperature of about 3,000°C is attained and so the iron is obtained in molten form. Industrially, this is recovered and used for the welding together of steel railway lines in situ, for repairing defective castings, and in incendiary bombs. The process is also useful in the extraction of certain metals from their ores.
aluminium + iron(III) oxide ==> iron + aluminium oxide
2Al(s) + Fe2O3(s) ==> Al2O3(s) + 2Fe(s)
To Demonstrate this Reaction:
- Fume hood
- Two safety screens
- Sand tray
- Fire-clay crucible
- Dry test tube
- Iron(III) oxide (pre-dried in oven)
- Dry aluminium powder
- Magnesium powder
- Barium peroxide
- Magnesium ribbon
- Mix 5g of dry2 iron(III) oxide with an equal volume of dry aluminium powder in a fire-clay crucible (in practice use 3 - 4 spatulas-full of each), (NB: the maximum legal quantity to use in UK schools, according to CLEAPSS3 Hazcards4 is 20g of total mixture).
- Stand the crucible in a tin of sand between two safety screens. (An alternative is to place the mixture into a small filter paper cone set directly into a sand-tray).
- Meanwhile mix a spatula measure of magnesium powder with the same bulk of barium peroxide in a dry test-tube, in a fume hood. (Alternatively use 'Thermit Initiation Mixture', or finely ground potassium manganate(VII) + hot glycerol).
- Pour this mixture into a hollow made in the thermit mixture.
- Insert a length of magnesium ribbon fuse (pre-cleaned with sandpaper) vertically into the fuse mixture. It helps if the magnesium ribbon is 'feathered' at its upper end and folded over once or twice at its lower end.
- Ignite the fuse. (NB keep your hand below the level of the crucible in case of premature 'flashing' of the mixture).
- Retire rapidly and watch. Click here for some images.
- It is important that the thermite mixture is perfectly dry for the reasons given in the footnote.
- Temperatures very much higher than the melting point of iron (1,528°C) can be expected. Hence a CO2 fire extinguisher should be to hand. Water should not be used because potentially explosive hydrogen can be produced.
- As well as the safety screens, no one should be closer to the reaction than two metres.
...if any simple demonstration reveals the awesome power of chemistry, this one does.
- John Emsley