A more reactive metal can remove oxygen from the oxide of a less reactive metal. The mixture needs to be heated together in order to react. For example, magnesium is more reactive than copper. It can remove oxygen from copper(II) oxide:
magnesium + copper(II) oxide → magnesium oxide + copper
Mg(s) + CuO(s) → MgO(s) + Cu(s)
Metals can be extracted from their oxides by competition reactions. The cheapest and most efficient way of doing this is to use carbon to react with the metal oxide.
Iron is extracted by heating iron ore with carbon in blast furnace. One reaction that takes place is the reaction between iron(III) oxide and carbon:
iron(III) oxide + carbon → iron + carbon dioxide
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)
The blast furnace is used to produce iron on a large scale. Sometimes, it is necessary to produce a small amount of iron more quickly, for example if railway workers need to produce molten metal to fix a broken rail.
The thermite reaction uses aluminium powder and iron(III) oxide. When ignited, the mixture reacts vigorously because of the large difference in reactivity between aluminium and iron. The heat produced in the reaction melts the iron produced.
Iron(III) oxide + aluminium → iron + aluminium oxide
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s)
In this reaction, iron(III) oxide loses oxygen to form iron - so iron(III) oxide is reduced. Carbon gains oxygen to form carbon dioxide - so carbon is oxidised.
Which substances are oxidised and reduced in the thermite reaction?
Iron(III) oxide is reduced, because it loses oxygen in this competition reaction. Aluminium gains oxygen and is oxidised.
A reactivity series can be produced by carrying out several competition reactions. A metal is mixed with a metal oxide and heated to see if a reaction takes place. Different combinations of metal and metal oxide are tested. The table shows the results of one of these investigations.
|A oxide||B oxide||C oxide||Reactions|
|Metal A||Not done||No visible reaction||Reaction occurs||1|
|Metal B||Reaction occurs||Not done||Reaction occurs||2|
|Metal C||No visible reaction||No visible reaction||Not done||0|
Metal B is the most reactive - it reacts with both A oxide and C oxide.
Metal C is the least reactive - it does not win the competition reaction with either A oxide or B oxide.
The order of reactivity of these metals is therefore B > A > C.