Oxidation, reduction and displacement reactions

Reactions of metals with oxygen

Many metals react with oxygen to make metal oxides. For example, magnesium burns rapidly in air:

Magnesium + oyxgen → magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)

The reactions are oxidation reactions because the metal gains oxygen.

Oxygen can be removed from metal oxides in chemical reactions. For example:

Copper oxide + carbon → copper + carbon dioxide

2CuO(s) + C(s) → 2Cu(l) + CO2(g)

In this reaction, carbon is oxidised because it gains oxygen. At the same time, copper oxide is reduced because oxygen is removed from it.

Oxidation is the gain of oxygen. Reduction is the loss of oxygen.

Displacement in solutions

A more reactive metal can displace a less reactive metal from its compounds. For example, magnesium is more reactive than copper. It displaces copper from copper sulfate solution:

Magnesium + copper sulfate → magnesium sulfate + copper

Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)

In this displacement reaction:

  • magnesium becomes coated with copper
  • the blue colour of the solution fades as blue copper sulfate solution is replaced by colourless magnesium sulfate solution
Magnesium powder is added to blue copper sulfate solution and when stirred, they change into a colourless magnesium sulfate solution and copper powder.

Deducing a reactivity series

A reactivity series can be deduced by carrying out several displacement reactions. A piece of metal is dipped into a salt solution. Different combinations of metal and salt solution are tested. The table shows the results of one of these investigations.

Magnesium sulfate solutionCopper sulfate solutionIron sulfate solutionNumber of reactions
MagnesiumNot doneBrown coatingBlack coating2
CopperNo visible reactionNot doneNo visible reaction0
IronNo visible reactionBrown coatingNot done1
Question

Use the results in the table to deduce an order of reactivity, starting with the most reactive metal.

The order of reactivity is: magnesium > iron > copper. This is because magnesium displaced copper and iron, iron displaced copper only, but copper could not displace magnesium or iron.

Question

Explain why three combinations of metal and salt solution were not done in the investigation.

A metal cannot displace itself from a solution of one of its salts. There would be no reaction, so these combinations were not done.

Displacement reactions as redox reactions - Higher tier

A balanced equation for the reaction between magnesium and copper sulfate solution can be written in terms of the ions involved:

Mg(s) + Cu2+(aq) + SO42-(aq) → Mg2+(aq) + SO42-(aq) + Cu(s)

Sulfate ions, SO42-, appear on both sides of the equation, but they do not take part in the reaction. The equation can be rewritten without them:

Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)

This equation is an example of a balanced ionic equation. It can be split into two half equations :

Mg(s) → Mg2+(aq) + 2e- (oxidation)

Cu2+(aq) + 2e- → Cu(s) (reduction)

Notice that:

Reduction and oxidation happen at the same time, so the reactions are called redox reactions.

curriculum-key-fact
Oxidation is the loss of electrons, and reduction is the gain of electrons.

It helps to remember OIL RIG - oxidation is loss of electrons, and reduction is gain of electrons.

Displacement reactions are just one example of redox reactions. Electrolysis reactions are also redox reactions.