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

Question

In the process of extraction of gold Roasted gold ore \(+\mathrm{CN}^{-}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{O}_{\longrightarrow}}{\longrightarrow}[\mathrm{X}]+\mathrm{OH}^{-}\) \([\mathrm{X}]+\mathrm{Zn} \longrightarrow[\mathrm{Y}]+\mathrm{Au}\) Identify the complexes of \([\mathrm{X}]\) and [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

(a) and (c) are the correct complexes for [Y] and [X].

Step 1: Understand the Reaction Context

The reaction involves the treatment of roasted gold ore with cyanide ions (\(\mathrm{CN}^-\)) and water in the presence of oxygen to form a complex \([X]\) and release \(\mathrm{OH}^-\). This is typically the process of gold cyanidation, where gold forms a stable complex with cyanide ions.

Step 2: Identify Complex \([X]\) From Gold Cyanidation

In the process of gold cyanidation, gold reacts with cyanide ions to form a soluble complex. The most common complex formed is the dicyanoaurate ion \(\left[\mathrm{Au} (\mathrm{CN})_2\right]^-\), which corresponds to option (c).

Step 3: Reaction with Zinc to Identify \([Y]\)

\([X]\) reacts with zinc to displace gold, forming a new complex \([Y]\). In this displacement reaction, zinc replaces gold in its complex, forming \(\left[\mathrm{Zn}(\mathrm{CN})_4\right]^{2-}\), which corresponds to option (a).

Step 4: Choose the Correct Options

Based on the analysis, the complex \([X]\) is \(\left[\mathrm{Au} (\mathrm{CN})_2\right]^-\) and \([Y]\) is \(\left[\mathrm{Zn} (\mathrm{CN})_4\right]^{2-}\). Thus, the correct answers are (a) and (c).

Key Concepts

Gold Cyanidation

Gold cyanidation is a process widely used for extracting gold from ore. The core of this method involves the leaching of roasted gold ore with a cyanide solution. Because gold itself is not readily soluble in water, you need a chemical that can interact with it to form a compound that is water-soluble. Enter cyanide ions, \((\mathrm{CN}^-)\), which have an affinity for gold and can form stable compounds called complexes. The presence of oxygen is key as it helps in oxidizing the gold, allowing it to bond with the cyanide ions, optimizing the efficiency of the process. This method is prevalent due to its ability to yield up to 97% of the gold from the ore under ideal conditions.

• First, the gold ore is crushed and roasted.
• Then it is treated with a cyanide solution.
• Oxygen acts as a catalyst in the formation of \([\mathrm{X}]\), facilitating a faster reaction.

This results in the formation of the gold-cyanide complex, essential for subsequent steps in the extraction process.

Cyanide Complexes

Cyanide, when reacted with metals such as gold or zinc, forms complexes that are highly stable due to the strong ligands cyanide ions serve as. In the case of gold, cyanide effectively ligates the gold to form \([\mathrm{Au}(\mathrm{CN})_2]^ -\). This is a typical result from the cyanidation process, where two cyanide ions coordinate around a gold ion, creating a negatively charged complex that dissolves in water.

• The \([\mathrm{Au}(\mathrm{CN})_2]^ -\) complex is common in gold extraction due to its solubility.
• Its negative charge means it can be processed further by adding different elements, such as zinc.

Cyanide complexes are important because they turn insoluble metals into soluble forms, advancing the extraction, purification, and the eventual economic retrieval of the metal.

Displacement Reaction

Displacement reactions are an exciting and pivotal part of gold extraction. Once the gold-cyanide complex is formed, a displacement reaction is used to separate the gold from the solution. This is achieved by introducing zinc to the mixture. Zinc has a higher affinity for cyanide ions, thus when it's added to the solution containing the dicyanoaurate complex, it competes for the cyanide ions and wins, effectively pushing out the gold and taking its place.

• The \([\mathrm{Zn}(\mathrm{CN})_4]^{2-}\) complex forms as zinc replaces the gold.
• This forceful exchange releases pure gold, which can then be filtered out from the solution.

Through this reaction mechanism, you’re left with metallic gold, ready for further refinement or sale.

Coordination Chemistry

In the realm of coordination chemistry, the interactions between metal ions and ligands, such as cyanide ions, allow us to create stable complexes. Coordination chemistry plays a foundational role in the extraction of metals via processes like cyanidation. A coordination complex forms when central metal atoms or ions are bonded to a collection of atoms, known as ligands, via coordinate covalent bonds.
For example, the gold ion combines with the cyanide ions through such interactions to create \([\mathrm{Au}(\mathrm{CN})_2]^-\).

• This complex is an example of how transition metals engage in coordination.
• Coordination compounds can often switch between having more than one mode of bonding, making them versatile in reactions.

Understanding coordination chemistry is crucial for predicting the formation and behavior of such compounds in various scenarios, like in the industrial extraction of gold.