Displacement Reactions

Definition of Displacement Reactions

A displacement reaction happens when one metal replaces another metal from a compound in an aqueous solution. This involves a more reactive metal reacting with the ions of a less reactive metal in solution, causing the less reactive metal to be released as a solid.

For example, if metal A is more reactive than metal B, metal A will displace metal B from its compound in solution:

Metal A (solid) + Metal B compound (aqueous) 12; Metal A compound (aqueous) + Metal B (solid)

Reactivity Series and Displacement

The reactivity series ranks metals from most reactive to least reactive. A metal higher in the series can displace a metal lower in the series from its compound.

Common metals in the reactivity series (from most reactive to less reactive) include:

• Potassium (K)
• Sodium (Na)
• Calcium (Ca)
• Magnesium (Mg)
• Aluminium (Al)
• Zinc (Zn)
• Iron (Fe)
• Lead (Pb)
• Hydrogen (H) 11; included for reference
• Copper (Cu)
• Silver (Ag)
• Gold (Au)

If a metal is above another in this list, it can displace the lower metal from its aqueous compound. If it is below, no reaction occurs.

For example, zinc is above copper in the series, so zinc can displace copper from copper sulfate solution. But copper cannot displace zinc from zinc sulfate solution.

Examples of Displacement Reactions

Here are some key examples showing displacement reactions between metals and metal salt solutions:

Metal	Metal Salt Solution	Reaction	Observation
Zinc (Zn)	Copper sulfate (CuSO4)	Zn (s) + CuSO4 (aq) 12; ZnSO4 (aq) + Cu (s)	Blue solution fades; reddish-brown copper solid forms
Iron (Fe)	Copper sulfate (CuSO4)	Fe (s) + CuSO4 (aq) 12; FeSO4 (aq) + Cu (s)	Blue solution fades; copper solid forms
Magnesium (Mg)	Zinc sulfate (ZnSO4)	Mg (s) + ZnSO4 (aq) 12; MgSO4 (aq) + Zn (s)	Colourless solution remains; grey zinc solid forms

These examples show that a more reactive metal displaces a less reactive metal from its salt solution.

For instance, zinc is more reactive than copper, so zinc metal displaces copper ions from copper sulfate solution, forming zinc sulfate and copper metal.

The reverse does not happen: copper metal cannot displace zinc ions from zinc sulfate solution because copper is less reactive than zinc.

Observations and Equations

During displacement reactions, you can observe:

• Colour changes in the solution: For example, copper sulfate solution is blue due to Cu²⁺ ions. When zinc displaces copper, the blue colour fades as Cu²⁺ ions are removed and replaced by colourless Zn²⁺ ions.
• Formation of a solid metal: The displaced metal appears as a solid deposit, often as a different colour. For example, copper metal forms a reddish-brown solid.

Displacement reactions can be written as word equations and symbol equations.

Word equation example:

Zinc + copper sulfate 12; zinc sulfate + copper

Symbol equation example:

$$\mathrm{Zn (s) + CuSO_4 (aq) \rightarrow ZnSO_4 (aq) + Cu (s)}$$

Here, (s) means solid and (aq) means aqueous (dissolved in water).

The metal on the left is the solid metal reacting, and the metal on the right is the metal displaced from solution.

The ions in solution change accordingly: the metal ion displaced goes from aqueous to solid, and the more reactive metal goes into solution as ions.

For example, in the zinc and copper sulfate reaction, Cu²⁺ ions are reduced to copper metal, and zinc metal is oxidised to Zn²⁺ ions.

This is an example of a redox reaction, but the focus here is on displacement.

For instance, if you place a strip of zinc metal into blue copper sulfate solution, the blue colour fades and a reddish-brown copper solid forms on the zinc strip.

For example, the reaction between iron and copper sulfate solution can be written as:

$$\mathrm{Fe (s) + CuSO_4 (aq) \rightarrow FeSO_4 (aq) + Cu (s)}$$

Here, iron displaces copper because iron is more reactive.

If a metal is less reactive than the metal ion in solution, no reaction occurs. For example, copper metal placed in zinc sulfate solution will not react.

Learning Example

If magnesium metal is placed in zinc sulfate solution, will a displacement reaction occur? Magnesium is above zinc in the reactivity series.

Since magnesium is more reactive, it will displace zinc from the solution:

$$\mathrm{Mg (s) + ZnSO_4 (aq) \rightarrow MgSO_4 (aq) + Zn (s)}$$

You would observe grey zinc metal forming and the solution remaining colourless.

For instance, zinc metal reacts with copper sulfate solution causing the blue colour to fade and copper metal to form as a reddish-brown solid.