Chapter 3 · Question 6
With reference to Activity 3.12, explain displacement reactions using the reaction of an iron nail with copper sulphate solution. Write the balanced equation and explain how such reactions help establish the reactivity series.
Q6
With reference to Activity 3.12, explain displacement reactions using the reaction of an iron nail with copper sulphate solution. Write the balanced equation and explain how such reactions help establish the reactivity series.
Answer Revealed
Direct Answer:
In Activity 3.12, an iron nail dipped in copper sulphate solution (blue) gets coated with a reddish-brown layer of copper metal, while the blue solution fades to pale green. A copper wire in iron sulphate solution shows no change. The reaction is: . Iron, being more reactive than copper, displaces copper from its salt solution. Copper cannot displace iron because it is less reactive. Such displacement reactions provide direct comparative evidence to arrange metals in the order of decreasing reactivity — a more reactive metal can always displace a less reactive metal from its salt solution.
Simple Explanation
Take an iron nail and put it in blue copper sulphate solution. In about 20 minutes, the nail turns reddish-brown — that is pure copper coating it. The blue colour fades because copper ions are being replaced by iron ions in the solution. If you try the reverse — a copper wire in iron sulphate solution — nothing happens. This proves iron is stronger (more reactive) than copper. The rule is simple: a more reactive metal can push a less reactive one out of its compound. By testing many pairs this way, we can build the reactivity series.
Exam-Ready Structure
Displacement reactions between metals and salt solutions are the definitive experimental basis for constructing the reactivity series, as demonstrated in NCERT Activity 3.12. 1. Activity 3.12 procedure: (a) Take a clean copper wire and an iron nail. (b) Place the copper wire in a test tube containing iron sulphate solution. (c) Place the iron nail in a test tube containing copper sulphate solution. (d) Observe after approximately 20 minutes (as illustrated in Fig. 3.4). 2. Observations: (a) In the iron nail + CuSO₄ tube: The blue colour of copper sulphate fades, and a brownish-red coating of copper metal deposits on the iron nail. A reaction has occurred. (b) In the copper wire + FeSO₄ tube: No visible change — no reaction occurs. 3. Balanced equation: . This is a single displacement reaction. 4. Explanation: Iron (Fe) is more reactive than copper (Cu) and displaces Cu²⁺ ions from the solution. The displaced Cu²⁺ ions gain electrons and deposit as metallic copper on the nail. Copper cannot displace Fe²⁺ from iron sulphate because it is less reactive. 5. General displacement equation: Metal A + Salt solution of B → Salt solution of A + Metal B, where A is more reactive than B. 6. Building the reactivity series: Displacement reactions provide better evidence for comparing reactivity than reactions with oxygen, water, or acids because the results are clearer and more binary — either a reaction happens or it does not. By systematically testing which metal displaces which, the relative order of reactivity is established.
Key Points
- Fe + CuSO₄ → FeSO₄ + Cu: iron nail gets brownish Cu coating, blue colour fades
- Cu + FeSO₄ → no reaction: copper is less reactive than iron
- Displacement rule: a more reactive metal displaces a less reactive metal from its salt solution
- This is a single displacement reaction: an element + a compound → new element + new compound
- Displacement reactions are the best evidence for arranging metals in the reactivity series
Related Questions
Q5
With the help of Activity 3.11, describe how metals react with dilute acids. Explain why hydrogen gas is not evolved when metals (except Mg and Mn) react with dilute nitric acid. What is aqua regia and what makes it special?
Q7