Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

In the process of extraction of gold, roasted gold ore \(+\mathrm{CN}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{O}_{2}}{\longrightarrow}[\mathrm{X}]+[\mathrm{Y}]+\mathrm{Zn}\) \(+\mathrm{Au},[\mathrm{X}]\) and \([\mathrm{Y}]\) are (a) \([\mathrm{X}]=\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-},[\mathrm{Y}]=\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{-2}\) (b) \([\mathrm{X}]=\left[\mathrm{Au}(\mathrm{CN})_{4}\right]^{-3},[\mathrm{Y}]=\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{-2}\) (c) \([\mathrm{X}]=\left[\mathrm{Au}(\mathrm{CN})_{2}\right],[\mathrm{Y}]=\left[\mathrm{Zn}(\mathrm{CN})_{6}\right]^{4}\) (d) \([\mathrm{X}]=\left[\mathrm{Au}(\mathrm{CN})_{4}\right]^{-},[\mathrm{Y}]=\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{-2}\)

Step-by-Step Solution

Answer

The correct answer is (a).

Step 1: Understand the Reaction Involved

In the reaction provided, roasted gold ore is treated with cyanide ions \(\mathrm{CN}^{-}\), water \(\mathrm{H_2O}\), and oxygen \(\mathrm{O_2}\). This reaction is part of the cyanidation process, which is a common method for gold extraction.

Step 2: Identify the Products of Cyanidation

In the cyanidation process, gold reacts with \(\mathrm{CN}^{-}\) ions in the presence of oxygen to form a soluble complex, \([\mathrm{Au}(\mathrm{CN})_2]^{-}\). This step exploits the formation of the gold-cyanide complex.

Step 3: Determine the Role of Zinc

Zinc is used in the displacement reaction where it reduces the gold complex ions. The displacement results in the formation of a zinc-cyanide complex, \([\mathrm{Zn}(\mathrm{CN})_4]^{-2}\), while gold is displaced and precipitated.

Step 4: Match the Products with the Options

Based on the typical reactions and products: \([\mathrm{X}]\) is \([\mathrm{Au}(\mathrm{CN})_2]^{-}\) and \([\mathrm{Y}]\) is \([\mathrm{Zn}(\mathrm{CN})_4]^{-2}\). Therefore, the correct choice is option (a).

Key Concepts

Gold Extraction

The process of extracting gold from its ore involves treating the ore with cyanide ions (CN\(^-\)). This procedure is part of the broader cyanidation process. When gold ore is roasted, it becomes more reactive, facilitating its interaction with cyanide ions and oxygen. The primary goal of gold extraction through cyanidation is to retrieve pure gold from the ore. One of the main advantages of this method is that cyanidation is efficient in the dissolution of gold. This method, called hydrometallurgy, allows miners to obtain gold through a combination of chemical reactions and solubility principles:

• Gold reacts with cyanide ions and oxygen to form a soluble gold-cyanide complex.
• This ensures that gold can be separated and later retrieved from the solution.
• It is a cost-effective way to process low-grade ores.

Zinc-Cyanide Complex

In the gold extraction process, zinc plays a crucial role after gold has transformed into its soluble form, the gold-cyanide complex. This is a displacement reaction known as the "Merrill-Crowe process," where zinc is introduced to the solution.When zinc is added, it reacts with the gold-cyanide complex ions. This results in the precipitation of gold from the solution because zinc has a stronger affinity for the cyanide ions, thus forming a zinc-cyanide complex. The chemical reaction can be summarized as follows:

• The gold is displaced and precipitated out of the solution.
• The remaining zinc-cyanide complex is represented as \([\mathrm{Zn}(\mathrm{CN})_4]^ {-2}\).
• Zinc effectively "captures" the cyanide ions, separating them from the gold.

This stage ensures that the gold can be collected from the precipitate, allowing for the recovery of pure gold.

Gold-Cyanide Complex

In the cyanidation process, the formation of the gold-cyanide complex is a key step. Upon mixing the roasted gold ore with cyanide ions and oxygen, gold reacts to form a soluble complex, represented as \([\mathrm{Au}(\mathrm{CN})_2]^ {-}\). This complex is central to gold recovery, as it allows for the precious metal to dissolve into the solution. The characteristics of the gold-cyanide complex include:

• Gold is stabilized within the cyanide solution, forming a complex ion.
• Its solubility in water aids in the separation of gold from other materials in the ore.
• Enables easy filtration and recovery of gold from the solution post-reaction.

The understanding of this complex is essential for not only its role in gold extraction but also its chemical behavior, which is predictable and useful for efficient recovery practices.