The electrolytic bath used in gold plating of copper articles contains — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Understand the Objectives of Electroplating Electroplating is an electrochemical process used to deposit a thin, uniform, and highly adherent layer of a superior metal (such as gold, \ce{Au}) onto the surface of a base metal object (such as copper, \ce{Cu}). To achieve a smooth, mirror-like finish that adheres strongly to the base metal without peeling, cracking, or forming a rough, powdery deposit, the rate of metal deposition must be very slow and highly controlled. Step 2 - Analyze the Behavior of Simple Metal Salt Solutions If we use a simple, fully dissociated aqueous solution of a gold salt such as gold(III) chloride (\ce{AuCl3}): $$\ce{AuCl3(aq) -> Au^3+(aq) + 3Cl^-(aq)}$$ The concentration of free \ce{Au^3+} ions in the solution will be very high. During electrolysis, these abundant free cations migrate rapidly to the cathode and undergo extremely fast reduction: $$\ce{Au^3+(aq) + 3e^- -> Au(s)}$$ This rapid, uncontrolled rate of deposition causes gold crystals to grow haphazardly, resulting in a coarse, uneven, loose, and dark-colored powdery coating that lacks physical adherence and easily flakes off. Step 3 - Explain the Role of Complexing Agents (\ce{NaCN}) To overcome this issue, a complexing agent like sodium cyanide (\ce{NaCN}) is added to the electrolyte bath. When \ce{AuCl3} is mixed with an excess of aqueous \ce{NaCN}, they undergo a chemical reaction to form a highly stable, soluble coordination complex, sodium dicyanoaurate(I), \ce{Na[Au(CN)2]}: $$\ce{Au^3+(aq) + 3CN^-(aq) + H2O(l) -> [Au(CN)2]^-(aq) + CNO^-(aq) + 2H^+(aq)}$$ The sodium dicyanoaurate(I) complex exists in dynamic equilibrium with a tiny concentration of free gold(I) ions in the solution: $$\ce{[Au(CN)2]^-(aq) <=> Au^+(aq) + 2CN^-(aq)}$$ Because the stability constant ($K_{\text{f}}$) of this complex is exceptionally high ($K_{\text{f}} \approx 10^{38}$), the dissociation equilibrium lies almost completely to the left. Consequently, the concentration of free, uncomplexed \ce{Au^+} ions is kept extremely low (typically around $10^{-10}\text{ M}$ to $10^{-15}\text{ M}$). Step 4 - Describe the Cathodic Reduction Mechanism During the electroplating process, only the free \ce{Au^+} ions in the immediate vicinity of the cathode (the copper article) are reduced: $$\ce{Au^+(aq) + e^- -> Au(s)}$$ As these free \ce{Au^+} ions are discharged, the complex \ce{[Au(CN)2]^-} slowly dissociates to replenish them according to Le Chatelier's principle. This continuous, controlled release of extremely low concentrations of gold ions ensures that the metal deposits slowly, atom-by-atom, allowing them to form a beautifully smooth, dense, uniform, and highly adherent gold film on the copper substrate. Step 5 - Evaluate the Options * **Option (A) is incorrect:** Molten gold requires temperatures exceeding $1064^\circ\text{C}$. This is extremely dangerous, expensive, and would melt or damage the copper article (copper melts at $1085^\circ\text{C}$). * **Option (B) is incorrect:** Copper sulphate (\ce{CuSO4}) is used for copper plating, which would not result in gold deposition. * **Option (C) is incorrect:** Plain aqueous \ce{AuCl3} yields a high concentration of free \ce{Au^3+} ions, leading to a poor, non-adherent, and powdery gold deposit. * **Option (D) is correct:** The mixture of \ce{AuCl3} and \ce{NaCN} forms the stable complex \ce{Na[Au(CN)2]}, providing a highly controlled concentration of gold ions for a smooth and uniform plating. $$\text{Correct Option: } \boxed{\text{D}}$$