The Thermite Reaction

The Thermite Reaction is a reaction between aluminium and almost always iron(III) oxide. The products of the reaction are aluminium oxide and iron and this generates a lot of heat. It is a redox reaction because the aluminium gets oxidised and the iron oxide is reduced. The aluminium and the iron oxide are mixed as powders because powders have a greater surface area in contact and therefore there is a faster rate of reaction.

Although several metal oxides may be used in place of iron oxide depending on the application, aluminium is always used as the starting metal. Its large liquid temperature phase, relative to other reactive metals (over 1,859°C), makes the reaction more feasible and has a greater thermal contact when molten. It is also safer to handle due to the thin layer of aluminium oxide when exposed to air.

The reaction mixture, though unstable, does not proceed at room temperature because of the high activation enthalpy. The very high temperature required to get the reaction started is usually achieved by way of a lit magnesium ribbon, though caution should be taken not to be too close. The reaction can be dangerously unreliable. If magnesium is not available, adding glycerine to potassium permanganate crystals may be used instead, though in a less controlled manner.

It can be used to weld together broken railway lines, so-called thermite welding. Because iron is denser than aluminium, the molten aluminium prevents corrosion at the weld joint. It can also be used in other situations where welding equipment is unavailable.

Thermite is difficult to extinguish once alight since it needs no supplementary source of oxygen and introducing small quantities of water to the lit mixture just creates dangerous steam clouds which adds a further danger of scalding to anyone close by. It is best to keep ones distance until it has burnt itself out.

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