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 A||Metal B||Metal C|
|A oxide||X||Displaces A||Displaces A|
|B oxide||No reaction||X||No reaction|
|C oxide||No reaction||Displaces C||X|
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: