Metals and competition reactions

Competition reactions

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)

Extracting metals

Very few metals exist uncombined in the Earth's crust. Most metals are found in the form of compounds called ores. These compounds are often oxides.

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)

In this reaction, iron oxide loses oxygen to form iron - so iron(III) oxide is reduced. Carbon gains oxygen to form carbon dioxide - so carbon is oxidised.

Thermite reaction

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.

Question

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.

Using competition reactions to investigate reactivity

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 oxideB oxideC oxideReactions
Metal ANot doneNo visible reactionReaction occurs1
Metal BReaction occursNot doneReaction occurs2
Metal CNo visible reactionNo visible reactionNot done0

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.