Displacement reactions of metal oxides

A more reactive metal will displace a less reactive metal from a compound. The thermite reaction is a good example of this. It is used to produce white hot molten (liquid) iron in remote locations for welding. A lot of heat is needed to start the reaction, but then it releases an incredible amount of heat, enough to melt the iron.

aluminium + iron(III) oxide → iron + aluminium oxide

2Al + Fe2O3 → 2Fe + Al2O3

Because aluminium is more reactive than iron, it displaces iron from iron(III) oxide. The aluminium removes oxygen from the iron(III) oxide:

Reactions between metals and metal oxides allow us to put a selection of metals into a reactivity series. Using metals A, B and C:

Metal AMetal BMetal C
A oxideXDisplaces ADisplaces A
B oxideNo reactionXNo reaction
C oxideNo reactionDisplaces CX

Metal A cannot displace either B or C - so it must be the least reactive and be at the bottom of this reactivity series.

Metal B displaces both A and C - so it must be the most reactive and be at the top of this reactivity series.

Metal C displaces A but cannot displace B - so it must be more reactive than A but less reactive than B, and be in between them in this reactivity series.

In general, the greater the difference in reactivity between two metals in a displacement reaction, the greater the amount of energy released.

Aluminium is much higher than iron in the reactivity series, so the thermite reaction releases a lot of energy. Magnesium is very high in the reactivity series, and copper is very low - so the reaction between magnesium and copper oxide is more violent.

Therefore, the order is:

 A list of letters from most reactive to least reactive: B, C and A,