Question

Give the formula of the complex used in electroplating of objects by silver.
A. $Na\left[ {Ag{{\left( {{H_2}O} \right)}_2}} \right]CN$
B. $Na\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]Cl$
C. $Na\left[ {AgC{l_2}} \right]$
D. $Na\left[ {Ag{{\left( {CN} \right)}_2}} \right]$

Answer 1

For electroplating, the electrolyte used must be properly soluble in water to form a uniform solution and it should not be a very strong electrolyte so that, it does not undergo very fast redox reaction to avoid non-uniform distribution of silver ions on the surface of object to be electroplated.

Complete step by step answer:
For electroplating of silver, an aqueous solution of sodium argentocyanide i.e. $Na\left[ {Ag{{\left( {CN} \right)}_2}} \right]$ is used, because in this electrolyte the migration of silver ions $\left( {A{g^ + }} \right)$ is slow and helps in even deposition of silver metal on the object.
The ionic compound, silver chloride solution i.e. $AgCl$ is a very strong electrolyte and is not properly soluble in water, thus, it is not used in electroplating of silver because it undergoes a very fast redox process which leads to non-uniform distribution of silver metal on the object.
$Ag{\left( {N{H_3}} \right)_2}$ is a weak electrolyte, so it does not completely dissociate in aqueous solution and is not used in silver electroplating.

Hence, option D is the correct option.

Additional information:
In the electroplating of silver, a silver bar is placed on the anodic side and the object to be electroplated is placed on the cathodic side with an electrolyte which helps in avoiding the non-uniform distribution of silver ions on the surface of the object. Thus, the anode bar gradually dissolves to replenish the silver ions in the solution. The net result is that silver metal has been transferred from the anode to the cathode. This process continues until the desired coating thickness is built up on the object.

Note: It is to be noted that, in option D the cyanide ions are present inside the square bracket, which will dissociate from silver ion and thus the silver ion will deposit on the object. In option A, the cyanide ion is present outside the square bracket and should not be considered while electroplating is taken in account.