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

Question

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

(a) \([X]=[\text{Au(CN)}_2]^−\), [Y]=[\text{Zn(CN)}_4]^{-2}\)

Step 1: Identify the reaction context

The chemical reaction involves the extraction of gold using cyanide, water, and oxygen. This forms a complex compound, identified as \([X]\), followed by a displacement reaction to isolate gold using zinc, forming another complex \([Y]\).

Step 2: Deduce the first reaction product [X]

In the given problem, gold cyanidation, also known as the MacArthur-Forrest process, forms complex ions. The gold ore reacts with cyanide to form a soluble gold complex. The gold cyanide complex is typically \([\text{Au}( ext{CN})_2]^−\). This is the common form of gold in cyanide leaching.

Step 3: Analyze the subsequent displacement reaction

In the second reaction, the complex \([X]\) reacts with zinc to form \([Y]\) along with metallic gold. This indicates the precipitation of gold, and usually, zinc forms a tetrahedral zinc cyanide complex \([\text{Zn}( ext{CN})_4]^{-2}\).

Step 4: Match the derived compounds with the options

From the reactions deduced: \([X] = [\text{Au}( ext{CN})_2]^−\) and \([Y] = [\text{Zn}( ext{CN})_4]^{-2}\). Option (a) matches with these complexes.

Key Concepts

MacArthur-Forrest Process

The MacArthur-Forrest Process revolutionized the mining industry by providing an efficient method to extract gold from low-grade ores. This process, developed in the late 19th century, involves the use of cyanide as a key reagent. The core idea here is to dissolve the gold from the ore, which is often in small quantities and embedded within other materials, into a liquid form. By converting gold into a soluble complex, it becomes easier to isolate and recover.

The process starts with the ore being finely ground. The ground ores are then subjected to a mixture containing cyanide ions ( containers - usually sodium or potassium cyanide). In the presence of oxygen and water, the cyanide solution effectively dissolves the gold, forming soluble complexes, typically vided ext{Au}( ext{CN} )_2^− ntifies . This aspect of the process is often referred to as 'cyanide leaching', which we’ll dive into next.

Overall, the MacArthur-Forrest process is an ingenious application of chemistry, converting problematic solid gold ores into a form that can be easily separated and purified.

Cyanide Leaching

Cyanide leaching is a key component in the gold extraction process, particularly when employing the MacArthur-Forrest method. This chemical process takes advantage of the properties of cyanide, which forms a strong complex with gold, aiding in its dissolution from the ore.

Here’s a step-by-step breakdown of this fascinating process:

• First, finely ground gold ore is mixed with a cyanide solution. This solution must be alkaline, meaning it has a pH greater than 7, to prevent the release of toxic hydrogen cyanide gas.
• Oxygen is then introduced. In the presence of cyanide and water, gold dissolves into the solution and forms the gold-cyanide complex xt ext{Au}( ext{CN} )_2^−
• This complex ion is stable in the solution, allowing it to be separated from the remaining solid material in the ore.

The dissolved gold can later be recovered by additional chemical reactions. Throughout the leaching process, careful control of conditions—like pH and cyanide concentration—is crucial to ensuring safety and effectiveness.

Metal Displacement Reactions

In metal extraction processes like gold mining, metal displacement reactions play a crucial role. Within the context of the MacArthur-Forrest Process, displacement reactions are employed to recover gold from its cyanide complex.

Once gold is dissolved in the cyanide solution, we achieve the ark complex plex ext{Au}( ext{CN} )_2^− . The next step involves transferring this gold back into its metallic form. This is where zinc comes into the picture.

Through a displacement reaction, zinc is introduced to the gold-cyanide complex. Here's what happens:

• Zinc, being more reactive than gold, displaces gold from its complex, which forms metallic gold ( ls it into its tangible form). This reaction can be written as:
  [ ext{Au}( ext{CN} )_2]^{-} + ext{Zn} ightarrow [ ext{Zn}( ext{CN} )_4]^{-2} + ext{Au}
• As a result, zinc replaces the position of gold, forming a stable zinc-cyanide complex ck [ ext{Zn}( ext{CN} )_4]^{-2} .

This reaction ensures that we can collect and purify gold effectively. Understanding metal displacement reactions is essential for grasping how raw, valuable metals can be isolated from complex mixtures in nature.