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

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

Step 1: Understand the Reaction

The exercise involves two chemical reactions that take place during the extraction of gold. The first equation is:Roasted gold ore + cyanide ions + water, in the presence of oxygen, produces a gold cyanide complex (\([X]\)) and hydroxide ions. The second equation involves this gold cyanide complex \([X]\) reacting with zinc to produce the zinc cyanide complex \([Y]\) and liberating gold.

Step 2: Identify Complex [X]

In the first reaction, gold reacts with cyanide ions forming a stable anion complex. The common complex of gold with cyanide is \([\text{Au}(\text{CN})_2]^-\). This corresponds to option (c).

Step 3: Identify Complex [Y]

In the second reaction, \([X]\) reacts with zinc to form a zinc cyanide complex and gold. The reaction typically produces \([\text{Zn}(\text{CN})_4]^{2-}\) as the zinc complex. This corresponds to option (a).

Key Concepts

Cyanide Process

The cyanide process is a technique primarily used to extract gold from ore in a cost-effective and efficient manner. It is also known as cyanidation. This chemical process involves the dissolution of gold by turning it into a complex compound that can easily be separated from other minerals in the ore.
Gold is naturally mixed within the ore, making it challenging to access it in its pure form. By introducing cyanide ions ( abla ext{CN}^-), the gold can form a water-soluble compound, which simplifies its extraction. Oxygen is an essential component in this process as it helps to oxidize and dissolve the gold. The resulting solution contains complex ions that hold the key to separating gold from its ore surroundings.

Complex Ion Formation

Complex ion formation is the heart of the cyanide process where complex compounds ensure efficient metal extraction. A complex ion refers to a structure made up of a metal ion at its center surrounded by molecules or anions, called ligands. In this context, ligands are the cyanide ions that latch onto the gold ions.
When cyanide ions attach themselves to gold, they engage in a chemical bonding that results in a complex ion. Such transformations allow gold to stay in a solution form, making it easier to separate out from other impurities or metals present in the ore.

Gold Cyanide Complex

The gold cyanide complex is a core outcome of the cyanide process where gold ions unite with cyanide ions to form a soluble entity. This specific complex is critical because it allows gold to remain in water instead of its typical metallic form. In chemical terms, the gold cyanide complex is expressed as \([ ext{Au}( ext{CN})_2]^-\).
This negatively charged complex is stable and keeps gold in a dissolved state. Through this method, gold can be effectively separated from its ore via filtration or other separation techniques as it remains in a liquid-phase complex. This complex formation is what makes the cyanidation both efficient and effective.

Zinc Cyanide Complex

Once the gold has been transformed into the gold cyanide complex, the next step is retrieving the gold while further processing other metals found within the ore. This involves introducing zinc to the solution now containing the dissolved gold cyanide complex.
Zinc, being more reactive than gold, replaces the gold within the cyanide complex. As zinc reacts with the complex, gold precipitates out as a solid, and a new complex is formed: the zinc cyanide complex \([ ext{Zn}( ext{CN})_4]^{2-}\).
This reaction, known as displacement or cementation, is crucial as it regenerates free cyanide ions, allowing further operation of the process while producing a distinct complex that can be treated or disposed of accordingly.