In the silver plating of copper, $K[Ag(CN)_2]$ is used instead of $AgNO_3$. The reason is : -
A
A thin layer of Ag is formed on Cu
B
More voltage is required
C
$Ag^+$ ions are completely removed from solution
D
Less availability of $Ag^+$ ions, as Cu can not displace Ag from $[Ag(CN)_2]^-$ ion
The answer is D.
Solution
Copper is more reactive than silver. With $AgNO_3$ solution, copper would directly displace silver: $Cu + 2Ag^+ \rightarrow Cu^{2+} + 2Ag$. This gives a loose, uneven deposit instead of a smooth plating. In $K[Ag(CN)_2]$, silver is held in the stable complex ion $[Ag(CN)_2]^-$, so the concentration of free $Ag^+$ ions is very low. Copper cannot displace silver from this complex, and $Ag^+$ is supplied slowly during electrolysis. This gives a smooth, even and firmly adhering coating. So $K[Ag(CN)_2]$ is used because of less availability of $Ag^+$ ions, as Cu cannot displace Ag from the $[Ag(CN)_2]^-$ ion.
ConceptComplex ion in electroplating
Coordination Compounds in past papers
104 questions from this chapter have appeared across 27 exam years.
