Chemical Reactions and Equations · Lesson 5 of 9
Displacement Reaction
“A more reactive element shows up and politely removes someone else from the compound.”
• Define a displacement reaction. • Explain the observations when iron nails are placed in copper sulphate solution. • Identify which element displaces which in a reaction. • Connect displacement with relative reactivity. • Recognise displacement equations from their pattern.
A displacement reaction is easier to understand if the word displacement is taken literally: one element enters a compound and pushes another element out. Whether this happens depends on relative reactivity.
Iron Nails In Copper Sulphate Solution
Freshly prepared copper sulphate solution is blue. Clean iron nails are immersed in the solution while another nail is kept aside for comparison. After some time, the solution becomes less intensely blue and the immersed nails develop a brownish coating.
Iron enters the compound and forms iron sulphate. Copper is released as copper metal and deposits on the iron nail, producing the brownish coating. The fading blue colour is linked with consumption of copper sulphate solution.
A Displacement Reaction is a reaction in which a more reactive element displaces a less reactive element from its compound.
More Displacement Examples
Zinc and lead displace copper from its compounds in the examples given. This tells us that zinc and lead are more reactive than copper. The observation of displacement therefore provides information about relative reactivity.
How To Identify What Was Displaced
Start by locating the free element among the reactants. Then find the element inside the compound that appears free among the products. In Fe + CuSO₄ → FeSO₄ + Cu, iron begins free and becomes part of FeSO₄, while copper begins inside CuSO₄ and appears as free Cu. Therefore iron displaces copper.
Problem
In Zn + CuSO₄ → ZnSO₄ + Cu, identify the displaced element.
- 1.Zn begins as a free element.
- 2.Cu begins inside the compound CuSO₄.
- 3.In the products, Zn is now inside ZnSO₄ and Cu appears free.
- 4.Therefore zinc has displaced copper.
- 5.The equation also shows that zinc is more reactive than copper in this comparison.
Quiz
What type of reaction is Fe + CuSO₄ → FeSO₄ + Cu?
What causes the brownish coating on the iron nail?
Why does the blue colour of copper sulphate solution fade?
What does Zn + CuSO₄ → ZnSO₄ + Cu show?
Which description best matches displacement?
Practice Problems
- Explain every visible observation when iron nails are placed in copper sulphate solution.
- In Fe + CuSO₄ → FeSO₄ + Cu, identify the free element before and after the reaction and state what was displaced.
- Use Zn + CuSO₄ → ZnSO₄ + Cu to compare the reactivity of zinc and copper.
- Use Pb + CuCl₂ → PbCl₂ + Cu to identify the displaced element.
- Write a short rule for recognising displacement reactions from an equation and apply it to the three equations in this lesson.
Key Takeaways
• In a displacement reaction a more reactive element replaces a less reactive element from its compound. • Iron displaces copper from copper sulphate to form iron sulphate and copper. • The fading blue solution and brown copper coating are direct observations of the iron-copper sulphate reaction. • Zinc and lead also displace copper in the examples given. • Reading which element begins free and which element ends free helps identify the displacement.