Problem 83 - 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) \([\mathrm{X}]=[\mathrm{Au}(\mathrm{CN})]]^{-},[\mathrm{Y}]=\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{-2}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding the Chemical Reaction

The given chemical reaction involves the extraction of gold using cyanide ion \(\mathrm{CN}^{-}\) and oxygen \(\mathrm{O}_2\). The equation indicates the presence of gold in complex ion form \([\mathrm{X}]\) and another complex compound \([\mathrm{Y}]\). We need to determine the identities of \([\mathrm{X}]\) and \([\mathrm{Y}]\).

Step 2: Identify [X], the Gold Complex Ion

In the extraction process using cyanide leaching, gold typically forms the complex ion \([\mathrm{Au}( ext{CN})_2]^-\). This is consistent with the fact that gold forms a stable complex with two cyanide ions.

Step 3: Identify [Y], the Zinc Complex Ion

Zinc is used in the gold extraction process to oxidize and form complexes with cyanide. After the reaction, zinc typically forms the complex ion \([\mathrm{Zn}( ext{CN})_4]^{2-}\). This is the expected oxidation state of zinc complexed with four cyanide ions.

Step 4: Match [X] and [Y] with Given Options

Now, match the identified complexes \([\mathrm{X}] = [\mathrm{Au}(\text{CN})_2]^-\) and \([\mathrm{Y}] = [\mathrm{Zn}(\text{CN})_4]^{2-}\) with the given choices. Option (a) corresponds exactly to these complexes.

Key Concepts

Cyanide Leaching

Cyanide leaching is a critical process in the extraction of gold from raw ore. It involves using a cyanide solution, typically sodium cyanide or potassium cyanide, to dissolve and isolate gold. This is done because gold has a strong affinity for cyanide ions, which makes it easier to extract it from ore.

To perform cyanide leaching, the raw, crushed ore is soaked in the cyanide solution. Oxygen is often added to enhance the reaction, forming a soluble gold-cyanide complex. This chemical reaction is key to dissolving gold, making it available for separation.

The equation for cyanide leaching in gold extraction is as follows:

• 4 Au + 8 NaCN + O_2 + 2 H_2O → 4 Na[Au(CN)_2] + 4 NaOH

This reaction shows how gold bonds with cyanide to form a stable, soluble complex that can be separated from other materials in the ore.

Gold Complex Ion

In the context of gold extraction, the term 'gold complex ion' refers to the compound formed when gold interacts with cyanide ions during leaching. Specifically, gold typically forms the complex ion

• [Au(CN)_2]^-

This complex ion is soluble in water, making it easier to separate from the ore during processing.

Gold, being inert and unreactive, does not dissolve readily in most chemicals. However, when cyanide ions surround gold atoms, it stabilizes and dissolves them. The oxidation state of the gold in this complex is +1, which is stabilized by the bonding with cyanide ions.

The creation of this gold-cyanide complex is a cornerstone of the cyanide leaching process, which is why it is utilized so extensively in the industry. It allows for efficient extraction from the low gold-grade ore found in deposits around the world.

Zinc Complex Ion

Zinc complex ions play an important role in the extraction of gold due to their interaction with cyanide during the leaching process. After the gold has been transformed into its complex ion form, zinc is often introduced as part of the recovery process.

Zinc reacts in a slightly different way by forming a stable complex ion known as

• [Zn(CN)_4]^{-2}.

This complex ensures that the cyanide ion continues to be effectively utilized in the leaching cycle and stabilizes the reaction environment.

Zinc's ability to form this complex becomes especially valuable when considering its role in the purification stage, often referred to as the 'Merrill-Crowe process.' In this stage, zinc acts as a reducing agent where it reverts the gold-cyanide complex back to solid gold, allowing the metal to be filtered from the solution. This practice highlights zinc's integral role in the gold extraction industry.