In the electroplating of iron by nickel, nickel sulphate solution is taken as an electrolyte. What is the acid that is added to the electrolyte during the process? (a) \(\mathrm{HCl}\) (b) \(\mathrm{HCN}\) (c) \(\mathrm{HNO}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Electroplating is a process in which a metal ion in solution is reduced to a metal and deposited onto a surface, such as iron in this case. The reduction of the metal ion occurs at the cathode (negative electrode), while oxidation of the metal occurs at the anode (positive electrode). We will examine how nickel ions are reduced and deposited onto the iron in the presence of an appropriate acid.

We need to choose an acid that can be compatible with the nickel sulfate solution and ensure successful electroplating. We should consider the acidity of the electrolyte and the possible complex formation or precipitation during the process. (a) \(\mathrm{HCl}\): Adding hydrochloric acid to the nickel sulfate solution will produce nickel chloride, which is highly soluble in water. However, chloride ions tend to form complexes with nickel ions, reducing the availability of nickel ions during the electroplating process. Therefore, this option might not be ideal for electroplating. (b) \(\mathrm{HCN}\): Hydrogen cyanide (HCN) reacts with nickel ions to form a very stable and tightly coordinated complex, making the nickel ions unavailable for reduction and deposition during the electroplating process. Thus, HCN is not suitable for electroplating in this case. (c) \(\mathrm{HNO}_{3}\): Nitric acid is a powerful oxidizing agent that would oxidize the metal ions and inhibit the deposition of nickel onto iron. Also, nitrate ions can form complexes with nickel ions, making them unavailable for reduction during the electroplating process. Therefore, nitric acid is not suitable for this process. (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\): Sulfuric acid is compatible with the nickel sulfate solution, adding sufficient acidity to the electrolyte without forming undesired complexes or precipitates. Furthermore, sulfate ions facilitate electroplating by providing a more conductive environment during the reaction.

Therefore, the most suitable acid to add to the electrolyte during the electroplating of iron by nickel is \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (sulfuric acid). The solution is: (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\)