Metals and displacement reactions

Displacement in solutions

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

magnesium + copper(II) 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 the blue copper(II) sulfate solution is replaced by a 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.

Determining a reactivity series

A reactivity series can be worked out 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(II) sulfate solutionIron(II) sulfate solutionNumber of reactions
MagnesiumNot doneBrown coatingBlack coating2
CopperNo visible reactionNot doneNo visible reaction0
IronNo visible reactionBrown coatingNot done1

Example

Use the results in the above 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 could displace copper and iron, iron could only displace copper, 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.

Writing ionic equations for displacement reactions - Higher

A balanced chemical equation for the reaction between magnesium and copper(II) sulfate solution is:

Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)

It can be written using the ions involved:

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

Notice that sulfate ions, SO42-(aq), appear on both sides of the equation. 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 balanced ionic equation. It shows only the atoms and ions that change during the reaction by losing or gaining electrons.

Question

The balanced chemical equation for the displacement reaction between iron and copper(II) sulfate is

Fe(s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)

Write the balanced ionic equation for the same reaction.

Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s)

Ionic equations:

  • show only the ions that change in the reaction
  • show the gain or loss of electrons
  • are useful for representing displacement reactions because they show what happens to the metal ions