Displacement and double displacement reactions

Displacement: an element takes a place

In a displacement reaction, an element takes the place of another element in a compound.

Example: iron and copper sulphate. An iron nail is put in blue copper sulphate solution. After some time, the nail is coated with a brown layer, and the solution turns pale green.

Iron has displaced copper. Iron is more reactive than copper.

Other examples:

• $\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}$
• $\text{Pb(s)} + \text{CuCl}_2\text{(aq)} \rightarrow \text{PbCl}_2\text{(aq)} + \text{Cu(s)}$

The reactivity series

Metals are not equally reactive. We can arrange them in a list, from the most reactive to the least reactive. This is the reactivity series.

A metal displaces another metal from its salt solution only if it is higher in the series. So iron displaces copper, but copper does not displace iron. Silver does not displace copper either.

You should find that a metal reacts only with the salt of a metal to its right, and never with its own salt.

Double displacement: partners are swapped

In a double displacement reaction, two compounds swap their partners.

Example. Sodium sulphate and barium chloride, both in water, are mixed. A white precipitate of barium sulphate forms.

When a double displacement gives a precipitate, it is called a precipitation reaction.

Neutralisation is also a double displacement. An acid and a base give a salt and water.

Practise with new numbers

Try it in Python

This program uses the reactivity series to say if a metal displaces another.

Remember these

• Displacement: $\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$. The more reactive element displaces the less reactive one.
• Double displacement: $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$. A precipitate often forms.
• A metal displaces another metal from a salt only if it is higher in the reactivity series.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Does lead displace copper from copper chloride solution?

2. Does copper displace zinc from zinc sulphate solution?

3. What type of reaction is $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$?

4. A silver ring falls into copper sulphate solution. What happens?