Metals and displacement reactions

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(II) 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(II) sulfate solution is replaced by colourless magnesium sulfate solution
Two beakers, one showing the state of magnesium in blue copper(II) sulfate solution and the other the beaker has been stirred and is now copper in colourless magnesium sulfate solution.

Determining 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 solutionLead nitrate solutionIron(II) sulfate solutionReactions
MagnesiumNot doneSilver coatingBlack coating2
LeadNo visible reactionNot doneNo visible reaction0
IronNo visible reactionSilver coatingNot done1

Example

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 > lead. This is because magnesium could displace lead and iron, iron could only displace lead, but lead 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

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

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

Notice that sulfate ions, SO42-, appear on both sides of the equation. They do not take part in the reaction and are called spectator ions. 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:

  • magnesium atoms lose electrons - they are oxidised
  • copper(II) ions gain electrons - they are reduced

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

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