Displacement Reactions
When a more reactive metal takes the place of a less reactive one
What is a Displacement Reaction?
A more reactive metal can displace a less reactive metal from a solution of its salt. This happens because the more reactive metal more readily loses electrons and forms ions.
Example: If you put iron into copper sulfate solution:
Iron is more reactive than copper, so it displaces copper from the solution.
Predicting Displacement Reactions
Use the reactivity series to predict whether a displacement reaction will occur:
| Metal Added | Solution | Reaction? | Reason |
|---|---|---|---|
| Zinc | Copper sulfate | Yes | Zn is more reactive than Cu |
| Iron | Copper sulfate | Yes | Fe is more reactive than Cu |
| Copper | Iron sulfate | No | Cu is less reactive than Fe |
| Magnesium | Zinc chloride | Yes | Mg is more reactive than Zn |
| Copper | Silver nitrate | Yes | Cu is more reactive than Ag |
| Silver | Copper sulfate | No | Ag is less reactive than Cu |
Observations in Displacement Reactions
When a displacement reaction occurs, you can typically observe:
| Reaction | What You See |
|---|---|
| Iron + copper sulfate solution | Blue solution turns pale green; orange/brown coating of copper forms on the iron |
| Zinc + copper sulfate solution | Blue solution turns colourless; reddish-brown copper appears on the zinc |
| Magnesium + copper sulfate solution | Blue solution turns colourless; brown/pink copper coats the magnesium; solution gets hot |
| Copper + silver nitrate solution | Colourless solution turns blue; silvery crystals of silver form on the copper |
Balanced Symbol Equations with State Symbols
State symbols:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous (dissolved in water)
Ionic Equations H
In a displacement reaction, the spectator ions (ions that don't change) can be removed to give the ionic equation. This shows only the particles that actually react.
Full equation:
Written with ions:
The sulfate ions (SO42−) appear on both sides – they are spectator ions. Remove them:
Ionic equation:
Oxidation & Reduction: Electron Transfer H
Reduction is the gain of electrons.
Remember: OIL RIG – Oxidation Is Loss, Reduction Is Gain.
In the displacement reaction between iron and copper sulfate:
- Iron (Fe) loses 2 electrons → Fe2+ — iron is oxidised
- Copper ions (Cu2+) gain 2 electrons → Cu — copper is reduced
Half equations:
The more reactive metal is always the one that is oxidised (it loses electrons more readily). The less reactive metal ion is reduced (it gains electrons and becomes a metal atom).
Practice Questions
Which of the following pairs will produce a displacement reaction?
Zinc is more reactive than iron, so zinc can displace iron from iron sulfate solution. In all other options, the metal added is less reactive than the metal in the solution.
A student places a piece of magnesium into copper sulfate solution. Describe what they would observe and explain why this reaction occurs.
The blue copper sulfate solution would turn colourless [1 mark]. A brown/orange coating of copper would appear on the magnesium [1 mark]. This happens because magnesium is more reactive than copper, so it displaces the copper from the solution [1 mark].
In the reaction Fe + CuSO4 → FeSO4 + Cu, which species is reduced?
Copper ions (Cu2+) gain electrons to become copper atoms (Cu). Gaining electrons is reduction. Iron is oxidised (it loses electrons to form Fe2+).
Write a balanced symbol equation, with state symbols, for the displacement reaction between zinc and copper sulfate solution.
[1 mark for correct formulae and balancing, 1 mark for state symbols]
For the reaction between magnesium and copper sulfate solution:
- Write the ionic equation. [1 mark]
- Write the half equation for oxidation. [1 mark]
- Write the half equation for reduction. [1 mark]
- Explain which species is the reducing agent. [1 mark]
(a)
(b) Oxidation half equation:
(c) Reduction half equation:
(d) Magnesium is the reducing agent because it donates/loses electrons to the copper ions, causing the copper ions to be reduced.
A student adds copper to silver nitrate solution. What colour change would they observe in the solution?
Silver nitrate solution is colourless. As copper displaces the silver, copper ions (Cu2+) go into solution, turning it blue. Silver crystals form on the copper.
Explain why adding copper to magnesium sulfate solution does not produce a reaction.
Copper is less reactive than magnesium [1 mark]. A less reactive metal cannot displace a more reactive metal from a solution of its salt [1 mark].