Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

In the process of extraction of gold roasted gold ore \(+\mathrm{CN}^{-}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{O}_{2} \rightarrow}[\mathrm{X}]+\mathrm{OH}^{-}\) \([\mathrm{X}]+\mathrm{Zn} \longrightarrow[\mathrm{Y}]+\mathrm{Au}\) \([\mathrm{X}]\) 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), with \([\text{X}]=[\text{Au(CN)}_2]^-\) and \([\text{Y}]=[\text{Zn(CN)}_4]^{-2}\).

Step 1: Understanding the Gold Extraction Process

In the process of gold extraction using cyanide, gold ore reacts with cyanide and water in the presence of oxygen. This forms complex ions of gold which are soluble.

Step 2: Formation of Complex Ion [X]

When gold reacts with cyanide ions, the gold forms a complex ion. The balanced chemical equation for this part is:\[ \text{Au} + 2\text{CN}^- + \frac{1}{2} \text{O}_2 + \text{H}_2\text{O} \rightarrow [\text{Au(CN)}_2]^- + \text{OH}^- \]This shows that the complex ion \([X]\) is \([\text{Au(CN)}_2]^-\).

Step 3: Reaction with Zinc to Form [Y] and Gold

The complex ion [X] \([\text{Au(CN)}_2]^-\) is then treated with zinc metal to displace the gold:\[ [\text{Au(CN)}_2]^- + \text{Zn} \rightarrow [\text{Zn(CN)}_4]^{2-} + \text{Au} \]Here, [Y] is formed as \([\text{Zn(CN)}_4]^{2-}\).

Step 4: Matching Options with Derived Results

The derived complex ions \([X] = [\text{Au(CN)}_2]^-\) and \([Y] = [\text{Zn(CN)}_4]^{2-}\) match option (a).

Key Concepts

Complex Ion Formation

In chemistry, complex ions are formed when a metal ion bonds with one or more ligands, which are ions or molecules that can donate electrons. In the context of gold extraction, complex ion formation occurs when gold reacts with cyanide ions. This process results in the formation of a soluble gold cyanide complex, specifically \([\text{Au(CN)}_2]^-\).

• The gold atom accepts electron pairs from cyanide ions.
• The bond formed is a coordinate covalent bond.
• This complex is soluble in water, allowing it to be easily separated from the ore.

This process makes it possible to extract gold in a liquid form using mere chemical reactions. Thus, complex ion formation is a crucial step that transforms gold from an inert metal to a reactive participant in the cyanide leaching process.

Cyanide Leaching

Cyanide leaching, often simply called 'leaching', is a hydrometallurgical technique used to extract gold from ore. Here's a breakdown of the process:

• Gold ore is mixed with water and cyanide in the presence of oxygen.
• The cyanide ions facilitate the dissolution of gold from the ore by forming a complex ion \([\text{Au(CN)}_2]^-\).
• This method requires oxygen to be successful, as it assists in the oxidation of gold.

The process is particularly valued because it allows for efficient gold recovery even from low-grade ores. Despite toxicity concerns, strict environmental and safety measures help manage and mitigate risks associated with cyanide use, making cyanide leaching a widely adopted practice in the industry.

Displacement Reaction

A displacement reaction in the context of gold extraction with cyanide involves the replacement of one element by another within a compound. This is an excellent illustration of how displacement reactions work in nature. Following the formation of the gold-cyanide complex ion, a further chemical reaction is utilized to recover gold metal.

• Zinc is introduced to the solution containing the complex ion \([\text{Au(CN)}_2]^-\).
• Zinc displaces the gold from the complex ion because it is more reactive, leading to gold being released as a solid.
• The balanced chemical equation for this reaction is: \[ [\text{Au(CN)}_2]^- + \text{Zn} \rightarrow [\text{Zn(CN)}_4]^{2-} + \text{Au} \]

This step is crucial as it leads to the recovery of gold from the solution. It highlights the use of some basic principles of reactivity found in displacement reactions, specifically the use of a more reactive metal (zinc) to liberate a less reactive one (gold).