Problem 29 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

The correct option is (a): \(X=[\text{Au(CN)}_2]^-\), \(Y=[\text{Zn(CN)}_4]^{2-}\).

Step 1: Understand Gold Cyanidation Reaction

The extraction of gold from its ore involves a cyanidation reaction where gold ions form a complex with cyanide ions in the presence of oxygen. The reaction is: \[ \text{4 Au} + 8 \text{CN}^- + \text{O}_2 + 2 \text{H}_2\text{O} \rightarrow 4 [\text{Au(CN)}_2]^- + 4 \text{OH}^- \] This implies that the gold complex \([X]\) formed is \([\text{Au(CN)}_2]^-\).

Step 2: Zinc Precipitation Reaction

In the process, zinc is added to recover gold from the gold cyanide complex. The gold is recovered and substituted by zinc in the complex: \[ 2 [\text{Au(CN)}_2]^- + \text{Zn} \rightarrow 2 \text{Au} + [\text{Zn(CN)}_4]^{2-} \]Thus, the complex \([Y]\) formed is \([\text{Zn(CN)}_4]^{2-}\).

Step 3: Match Complexes with Options

Now, compare the complexes found \([X] = [\text{Au(CN)}_2]^-\) and \([Y] = [\text{Zn(CN)}_4]^{2-}\) with the given options:- Option (a): \(X=[\text{Au(CN)}_2]^-\), \(Y=[\text{Zn(CN)}_4]^{2-}\) — Correct- Option (b): \(X=[\text{Au(CN)}_4]^{3-}\), \(Y=[\text{Zn(CN)}_4]^{2-}\) — Incorrect- Option (c): \(X=[\text{Au(CN)}_2]^-\), \(Y=[\text{Zn(CN)}_6]^{4-}\) — Incorrect- Option (d): \(X=[\text{Au(CN)}_4]^{-}\), \(Y=[\text{Zn(CN)}_4]^{2-}\) — Incorrect Only option (a) matches the identified complexes.

Key Concepts

Cyanidation Process

The cyanidation process, also known as the cyanide leaching process, is one of the most efficient methods for extracting gold from its ores. It involves treating the gold-containing ore with a cyanide solution in the presence of oxygen to dissolve the gold. This process takes advantage of the chemical reactivity of gold with cyanide ions, forming a water-soluble complex that can easily be recovered later.

Here's the basic reaction in the cyanidation process:
- Gold reacts with cyanide and oxygen to form a gold cyanide complex.- Simultaneously, hydroxide ions are produced.
The chemical equation for this reaction can be expressed as:\[ 4 \text{Au} + 8 \text{CN}^- + \text{O}_2 + 2 \text{H}_2\text{O} \rightarrow 4 [\text{Au(CN)}_2]^- + 4 \text{OH}^- \]This process has several advantages:

• High yield of gold recovery.
• Efficiency in utilizing less gold ore compared to traditional methods.
• The final process can be controlled and monitored for precision.

The cyanidation process is central to modern gold mining and, while effective, also requires careful and environmentally safe handling of cyanide.

Complex Ions

Complex ions are compounds consisting of a central metal atom or ion that is attached to surrounding molecules or ions, known as ligands. In the context of gold extraction, we encounter complex ions with gold and zinc.

During the cyanidation process, the gold forms a complex ion with cyanide, resulting in a soluble complex known as the cyanide complex of gold. This complex, represented as \([\text{Au(CN)}_2]^-\), indicates gold's preference for the stable linear coordination provided by the cyanide ion.
Cyanide ions are excellent ligands for forming complexes due to their ability to donate a pair of electrons.
After forming, these complex ions facilitate the separation of gold from the ore by making it soluble in the aqueous phase.When zinc is introduced later in the process, it forms its own complex ion with cyanide, represented as \([\text{Zn(CN)}_4]^{2-}\). These complex ions play a critical role in the chemistry of gold extraction, allowing for the effective separation and recovery from the ore.“Complex Ions" like \([\text{Au(CN)}_2]^-\) and \([\text{Zn(CN)}_4]^{2-}\) are integral to the reaction pathways that lead to gold’s separation.

Zinc Precipitation

Zinc precipitation is a crucial step in the gold extraction process following cyanidation. It involves adding zinc to the gold cyanide solution in order to recover gold. This step is based on the principle of displacement reactions, where a more reactive metal displaces a less reactive metal from its compound.

When zinc is added to the solution, it reacts with the gold cyanide complex to precipitate gold back to its elemental form. Here's how the reaction unfolds:

• Zinc displaces gold from the [\text{Au(CN)}_2]^- complex.
• The displaced gold is recovered as solid, leaving the zinc to form its own complex.

The chemical equation for the zinc precipitation reaction is:
\[ 2 [\text{Au(CN)}_2]^- + \text{Zn} \rightarrow 2 \text{Au} + [\text{Zn(CN)}_4]^{2-} \]This shows that zinc not only frees the gold but also creates a stable zinc-cyanide complex, \([\text{Zn(CN)}_4]^{2-}\), which remains dissolved. This method of gold recovery by zinc precipitation is not only efficient but overcomes many drawbacks of older methods that relied on mercury.