Question

Statement-I : Leaching of gold with cyanide ion in absence of air/O₂ leads to cyano complex of Au(III).
Statement-II : Zinc is oxidized during the displacement reaction carried out for gold extraction.
In the light of the above statements, choose the correct answer from the options given below.

• A. Both statement-I and statement-II are correct
• B. Both statement-I and statement-II are incorrect
• C. Statement-I is correct but statement-II is incorrect
• D. Statement-I is incorrect but statement-II is correct

Answer 1

The Correct Option is D

To determine the correctness of the statements provided, let's analyze each statement individually:

Statement-I: Leaching of gold with cyanide ion in absence of air/O₂ leads to cyano complex of Au(III).

Leaching is a process of extracting a substance from a solid by dissolving it in a liquid. The cyanide leaching process involves treating the ore with various cyanide salts to form a soluble gold-cyano complex.

For gold leaching, the chemical reaction occurs in the presence of oxygen:

4 \text{Au} + 8 \text{CN}^- + \text{O}_2 + 2 \text{H}_2\text{O} \rightarrow 4 \text{Au(CN)}_2^- + 4\text{OH}^-\

In the absence of oxygen, the reaction that forms the cyano complex does not proceed effectively. Therefore, the leaching of gold without the presence of air/O₂ is incorrect in leading to Au(III) cyano complex. The correct and effective formation of the gold-cyano complex requires oxygen, and typically forms Au(I) complex rather than Au(III).

Conclusion: Statement-I is incorrect because it disregards the necessary presence of oxygen for the effective formation of the gold-cyano complex.

Statement-II: Zinc is oxidized during the displacement reaction carried out for gold extraction.

Once the gold from the ore is in solution as a gold-cyanide complex, it needs to be recovered. This is often done by a displacement reaction where zinc is used to displace gold from its cyanide complex:

2\text{Au(CN)}_2^- + \text{Zn} \rightarrow 2\text{Au} + \text{Zn(CN)}_4^{2-}

Here, zinc acts as a reducing agent and is oxidized, while gold is reduced and precipitates out of the solution. Therefore, zinc becomes oxidized to form zinc-cyano complex, which confirms that zinc is oxidized during the displacement reaction.

Conclusion: Statement-II is correct because zinc, indeed, gets oxidized during the gold extraction completion process by displacement.

Therefore, considering the analysis, the correct answer is: Statement-I is incorrect but statement-II is correct.