Displacement reactions
Emmanuel Opoku
Teacher
What Is a Displacement Reaction?
A displacement reaction occurs when a more reactive element pushes out (displaces) a less reactive element from its compound.
This happens because more reactive elements form ions more easily - they either lose or gain electrons faster.
Displacement Between Metals
Metals high in the reactivity series can displace metals lower down from their compounds.
General pattern:
More reactive metal + Less reactive metal compound More reactive metal compound + Less reactive metal
Example 1:
Magnesium displaces copper because magnesium is more reactive.
Example 2:
no reaction
Copper is less reactive than zinc, so nothing happens.
No answer provided.
Key idea:
The more reactive metal forms positive ions more easily.
The less reactive metal is left as the element.
No answer provided.
Observations in Metal Displacement
If a reaction occurs | If no reaction occurs |
Colour change (often solution becomes paler or darker) | No visible change |
Heat may be released | Solution remains unchanged |
A solid metal forms on the surface of another | No new solid appears |
Example observation:
When magnesium is added to blue copper sulfate solution, the blue colour fades and brown copper metal forms.
No answer provided.
Displacement in Halogens (Group 7)
Halogens (Group 7) show the opposite trend - reactivity decreases down the group.
This means a more reactive halogen displaces a less reactive halogen from its halide compound.
General pattern:
More reactive halogen + Less reactive halide compound More reactive halide compound + Less reactive halogen
Example 1:
Chlorine displaces bromine because chlorine is more reactive.
Example 2:
no reaction
Iodine is less reactive than chlorine, so it cannot displace it.
No answer provided.
Trend:
Reactivity decreases down Group 7
Fluorine Chlorine Bromine Iodine.
No answer provided.
Colour Changes in Halogen Displacement
Reaction mixture | Colour change | Explanation |
|---|---|---|
Chlorine + Potassium bromide | Green Orange | Bromine forms |
Chlorine + Potassium iodide | Green Brown | Iodine forms |
Bromine + Potassium iodide | Orange Brown | Iodine forms |
Iodine + Potassium chloride | No change | No reaction |
Reminder: Colours of halogens in solution
Visual cue:
Chlorine: green gas / pale green solution
Bromine: orange solution
Iodine: brown solution (or purple vapour when heated)
No answer provided.
Why Displacement Happens?
Displacement depends on how easily atoms form ions:
Metals: more reactive = loses electrons more easily.
Halogens: more reactive = gains electrons more easily.
In each case, the more reactive element takes part in the reaction, and the less reactive one is forced out.
Example Summary
Type | Reaction | Does It Occur? | Reason |
Metal |
| Yes | is higher in the reactivity series than |
Metal | no reaction | No | is higher in the reactivity series than |
Halogen | Yes | is more reactive than | |
Halogen | no reaction | No | is less reactive than |
No answer provided.
Check Your Understanding
Recall
a) Define a displacement reaction.
Answer
a) A reaction where a more reactive element displaces a less reactive one from its compound.
b) Write one example of a metal displacement reaction.
Answer
b)
c) Write one example of a halogen displacement reaction.
Answer
c)
Apply
d) What colour change would you see if chlorine is bubbled into potassium iodide solution?
Answer
d) Green brown, because iodine forms.
e) Explain why copper does not react when placed in magnesium sulfate solution.
Answer
e) Copper is less reactive than magnesium, so it cannot displace magnesium from its compound.
Challenge
f) Explain, in terms of electrons, why magnesium displaces copper but copper cannot displace magnesium (HT only).
Answer
f) Magnesium loses electrons more easily than copper (higher in the reactivity series), forming ions faster and releasing energy; copper does not lose electrons easily.