Displacement Reactions

Displacement reactions occur when a more reactive element replaces a less reactive element in a compound. They are essential for confirming the reactivity series and the halogen reactivity trend.

The general pattern

more reactive element + compound of less reactive element -> compound of more reactive element + less reactive element

Metal displacement reactions

A more reactive metal displaces a less reactive metal from a solution of its salt.

Key examples and observations

Reaction	Equation	Observations
Mg in CuSO4	Mg + CuSO4 -> MgSO4 + Cu	Blue solution decolourises, brown copper deposited on Mg
Zn in CuSO4	Zn + CuSO4 -> ZnSO4 + Cu	Blue fades, brown copper deposited
Fe in CuSO4	Fe + CuSO4 -> FeSO4 + Cu	Blue fades to pale green, brown copper deposited
Mg in ZnSO4	Mg + ZnSO4 -> MgSO4 + Zn	Colourless stays colourless, grey zinc deposited
Mg in FeSO4	Mg + FeSO4 -> MgSO4 + Fe	Pale green fades, grey iron deposited

Reactions that do NOT happen

Attempt	Why no reaction
Cu in ZnSO4	Copper is less reactive than zinc
Cu in MgSO4	Copper is less reactive than magnesium
Fe in MgSO4	Iron is less reactive than magnesium
Ag in CuSO4	Silver is less reactive than copper

Redox in metal displacement

Every metal displacement is a redox reaction:

Example: Zn + CuSO4 -> ZnSO4 + Cu

• Oxidation (Zn): Zn -> Zn2+ + 2e- (loses electrons)
• Reduction (Cu2+): Cu2+ + 2e- -> Cu (gains electrons)

The more reactive metal acts as the reducing agent. The less reactive metal ion acts as the oxidising agent.

Metal + acid as displacement

Strictly, metal + acid reactions are also displacement reactions. The metal displaces hydrogen from the acid:

Zn + H2SO4 -> ZnSO4 + H2

Zinc displaces hydrogen because zinc is above hydrogen in the reactivity series.

Halogen displacement reactions

A more reactive halogen displaces a less reactive halide from solution. Reactivity of halogens: Cl2 > Br2 > I2.

Reaction	Equation	Observation
Cl2 + KBr	Cl2 + 2KBr -> 2KCl + Br2	Colourless to orange-brown
Cl2 + KI	Cl2 + 2KI -> 2KCl + I2	Colourless to brown
Br2 + KI	Br2 + 2KI -> 2KBr + I2	Orange to darker brown

Reactions that do NOT happen

Attempt	Why
Br2 + KCl	Bromine is less reactive than chlorine
I2 + KBr	Iodine is less reactive than bromine
I2 + KCl	Iodine is less reactive than chlorine

Redox in halogen displacement

Cl2 + 2KBr -> 2KCl + Br2

• Reduction (Cl2): Cl2 + 2e- -> 2Cl- (gains electrons)
• Oxidation (Br-): 2Br- -> Br2 + 2e- (loses electrons)

Note: in halogen displacement, the pattern is reversed compared to metals. The halogen that is reduced is the more reactive one.

Thermite reaction

The thermite reaction is a dramatic displacement:

2Al + Fe2O3 -> Al2O3 + 2Fe

Aluminium displaces iron because Al is more reactive. The reaction is highly exothermic — molten iron is produced.

How to recognise in exams

• A metal placed into a salt solution of another metal
• A halogen added to a halide solution
• Questions asking “will a reaction occur?” (compare positions in reactivity series)
• Questions asking for observations when metals are added to salt solutions

Iron is placed in copper(II) sulfate solution. Describe and explain the observations. [4 marks]

• The iron becomes coated with a brown/pink-brown solid (copper) [1]
• The blue colour of the solution fades / turns pale green [1]
• Iron is more reactive than copper, so it displaces copper [1]
• Fe + CuSO4 -> FeSO4 + Cu / iron is oxidised and copper ions are reduced [1]

Predict whether a reaction occurs when bromine water is added to potassium chloride solution. Explain your answer. [2 marks]

• No reaction occurs [1]
• Bromine is less reactive than chlorine, so it cannot displace chloride ions from solution [1]