Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

\([X] = [\text{Au(CN)}_2]^-\) and \([Y] = [\text{Zn(CN)}_4]^{2-}\) from options (c) and (a).

Step 1: Understand the Reaction

In the gold extraction process, roasted gold ore reacts with cyanide ions and water in the presence of oxygen to form a complex denoted as \([X]\) and hydroxide ions. The complex \([X]\) then reacts with zinc to form another complex \([Y]\) and releases gold.

Step 2: Identify products and reactants in the first reaction

The reaction \( \text{Roasted gold ore} + \text{CN}^- + \text{H}_2\text{O} \stackrel{\text{O}_2}{\longrightarrow} [X] + \text{OH}^- \) is typical for cyanidation, forming a complex with gold. Given the common product in such reactions is \([\text{Au(CN)}_2]^-\), the possible complexes are either \([\text{Au(CN)}_2]^-\) or \([\text{Au(CN)}_4]^{3-}\).

Step 3: Match complex [X] with options

Option (c) provides \(X = [\text{Au(CN)}_2]^-\), which aligns with the standard process of gold cyanidation, thus identifying \([X] = [\text{Au(CN)}_2]^-\).

Step 4: Examine the Second Reaction

The complex \([X]\) reacts with zinc to form \([Y]\) and gold. In practice, \( [Y] \) is typically a zinc cyanide complex like \([\text{Zn(CN)}_4]^{2-}\) or \([\text{Zn(CN)}_6]^{4-}\). By elimination and matching options, the common complex \([\text{Zn(CN)}_4]^{2-}\) aligns with (a), identifying \([Y] = [\text{Zn(CN)}_4]^{2-}\).

Step 5: Confirm the Answer with Options

Based on our deductions: \([X] = [\text{Au(CN)}_2]^-\) matches with option (c), and \([Y] = [\text{Zn(CN)}_4]^{2-}\) corresponds to option (a). Thus, options (a) and (c) correctly identify the complexes.

Key Concepts

Cyanidation process

The cyanidation process is a crucial technique employed in the extraction of gold from ore. This process involves the transformation of gold into a water-soluble coordination complex through a reaction with cyanide ions. The primary goal is to convert gold particles contained within the ore into a form that can be easily extracted and purified.

During the cyanidation process, gold reacts with cyanide ions (CN⁻) in an aqueous environment. The presence of oxygen is essential, as it facilitates the oxidation of gold metal to ionized form, creating the gold-cyanide complex. This allows the metal to dissolve in the solution and separate from unwanted impurities within the ore.

Gold-cyanide complex

The gold-cyanide complex is a key component in the gold extraction process, especially during cyanidation. It refers to the gold ion bonded with cyanide ions in the solution. The primary complex formed in this method is \( ext{Au(CN)}_2^-\), a stable compound that is integral in making the gold soluble.

Formation of this complex occurs when gold, in the presence of cyanide ions, reacts to form a coordination compound. This process is facilitated by the oxygen in the solution, which assists in oxidizing metallic gold to an ionic form. The resulting \([ ext{Au(CN)}_2]^-\) is easily separated from the mixture, enabling the extraction and recovery of gold from the solution through further processing.

Zinc cyanide complex

In the gold extraction process, after forming the gold-cyanide complex, zinc plays an essential role in recovering gold. The zinc cyanide complex is formed during the final stages of the cyanidation process. When zinc metal is added to the cyanide solution containing the gold complex, a chemical reaction takes place.

Zinc displaces the gold from the \([ ext{Au(CN)}_2]^-\) complex due to its higher reactivity, converting it into metallic gold. The leftover compound becomes a zinc cyanide compound, typically \([ ext{Zn(CN)}_4]^{2-}\). This allows gold to precipitate out of the solution and be collected for further refinement.

Oxidation-reduction reactions

Oxidation-reduction reactions, often known as redox reactions, are fundamental to the cyanidation process used for gold extraction. These reactions involve the transfer of electrons between substances, resulting in a change in oxidation state. In the cyanidation process, redox reactions are crucial at various stages.

Initially, oxygen aids the oxidation of gold metal by cyanide ions, transforming it into a soluble gold-cyanide complex \([ ext{Au(CN)}_2]^-\). Subsequently, another redox reaction happens when zinc is introduced to the solution. Zinc, being more reactive, donates electrons to reduce the gold complexes back to metallic gold, while zinc itself is oxidized, forming the zinc cyanide complex. These interactions are key in efficiently separating gold from the ore and recovering it from solution.