Which of the following pairs will give displacement reactions ?
AgNO3 solution and copper metal
Copper displaces silver from silver nitrate solution — option 1 :
\(\text{Cu}+2\text{AgNO}_{3}\rightarrow\text{Cu(NO}_{3}\text{)}_{2}+2\text{Ag}\)
The rule. A displacement reaction occurs only when the free metal is more reactive than the metal in the salt. The reactivity series, in decreasing order, runs :
K › Na › Ca › Mg › Al › Zn › Fe › Pb › H › Cu › Hg › Ag › Au
Apply it to each pair :
| Pair | Compare | Reaction ? |
|---|---|---|
| Copper + AgNO3 | Cu is above Ag | Yes — copper displaces silver |
| Copper + NaCl | Cu is far below Na | No |
| Silver + FeSO4 | Ag is below Fe | No |
| Aluminium + MgCl2 | Al is below Mg | No |
The classic demonstration. A bright copper wire left standing in colourless silver nitrate solution slowly acquires a coating of shining silver crystals, while the solution turns blue from the copper(II) ions passing into it. Both changes — the deposit and the colour — can be seen with the naked eye, which is why this is the textbook example of displacement.
What is really happening. It is a redox reaction. Copper is oxidised and silver is reduced :
\(\text{Cu}\rightarrow\text{Cu}^{2+}+2e^{-}\qquad\text{and}\qquad2\text{Ag}^{+}+2e^{-}\rightarrow2\text{Ag}\)
The more reactive metal is the one more willing to give up electrons, which is exactly what the reactivity series measures.
A related use. The same principle explains why iron nails placed in copper sulphate solution acquire a brown copper coating, and why zinc is used as a sacrificial anode to protect iron from rusting.
Hence, the answer is AgNO3 solution and copper metal.
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