How train rails are welded on site
Thermite reaction · Redox · Single displacement · Exothermic
Aluminium powder and rust, lit with a fuse, pour out liquid iron hot enough to weld railway tracks.
Reactants approach
Drag to rotate
ΔH = -852 kJ · exothermic, releases heat
ΔH per mol Fe2O3.
Meet the molecules
- Reactantsolid
Fe2O3
iron(III) oxide
- Reactantsolid
Al
aluminium
- Productsolid
Al2O3
aluminium oxide
- Productsolid
Fe
iron
What you notice
Long, continuous railway tracks are often joined by pouring molten steel into the gap between two rail ends. Crews clamp a sand mould around the joint, set a crucible of thermite above it, and light it. Within seconds the mixture flares to about 2500 °C, and white-hot liquid iron runs down into the mould, fusing the rails together. Thermite is just aluminium powder mixed with iron oxide. It carries its own oxygen, in the oxide, so it cannot be smothered, and it is also used for heavy welding repairs and in demolition.
Safety: Thermite burns too hot to put out with water and sprays molten metal, so only trained crews use it.
Step by step
- 1
A hard start
Thermite needs a very hot igniter, such as a burning magnesium strip, because the oxide coating on aluminium makes the activation barrier high.
- 2
Aluminium takes the oxygen
Aluminium is more reactive than iron, so Al atoms give electrons to Fe3+ ions and bond to the oxide ions as Al2O3.
- 3
Iron is freed
Each Fe3+ ion gains three electrons and becomes an iron atom.
- 4
Molten metal
About 852 kJ is released per mole of Fe2O3, enough to melt both the iron (melting point 1538 °C) and the aluminium oxide slag, which floats on top.