IGCSE Chemistry 0620: The Thermite Reaction (Aluminium Displacing Iron)

A demonstration mixes powdered aluminium with powdered iron(III) oxide and ignites the mixture. The reaction proceeds without any external heat source, releasing an enormous amount of energy and producing molten iron together with solid aluminium oxide. This reaction (the "thermite" reaction) is sometimes used industrially to weld railway tracks together.

(a) Using the reactivity series, explain why aluminium is able to displace iron from iron(III) oxide in this reaction. [2]

(b) Write the balanced symbol equation, including state symbols, for this reaction. [2]

(c) Suggest why the fact that the iron produced is molten makes this reaction useful for welding two sections of railway track together. [2]

Part (a): Why aluminium displaces iron

In the reactivity series, aluminium is positioned above iron, meaning aluminium is more reactive and its atoms have a greater tendency to lose electrons and form positive ions than iron’s do. When aluminium is mixed with iron(III) oxide, aluminium atoms transfer electrons to the $\text{Fe}^{3+}$ ions within the oxide, reducing them to iron atoms, while the aluminium atoms are themselves oxidised to $\text{Al}^{3+}$ ions in aluminium oxide. Because the more reactive metal displaces the less reactive one from its oxide, aluminium takes the place of iron, and iron metal is released.

Part (b): Balanced symbol equation

Balancing the atoms on each side (2 aluminium atoms and 2 iron atoms are needed to balance the 3 oxygen atoms in $\text{Fe}_2\text{O}_3$ ):

$2\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\text{Fe}(l)$

Both aluminium and the iron(III) oxide are solids at the start of the reaction. The aluminium oxide product remains solid, but the enormous heat released by the reaction is enough to melt the iron produced, so it is given the state symbol $(l)$ .

Part (c): Why this is useful for welding rails

The thermite reaction is strongly exothermic, releasing enough heat to melt the iron it produces. This molten iron can flow into the gap left between two sections of railway track that need to be joined. As the molten iron cools and solidifies, it fuses the two rail sections together into one continuous, strong piece of metal, without needing an external furnace or power supply on site, which is especially useful for repairing track in remote locations.

Final answers

(a) Aluminium is more reactive than iron, with a greater tendency to form positive ions; it reduces $\text{Fe}^{3+}$ to iron while being oxidised itself, displacing iron from its oxide.
(b) $2\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\text{Fe}(l)$
(c) The molten iron flows into and fills the gap between the rail sections, then solidifies to join them into one continuous piece.