Read the passage carefully and answer the questions.
The coordination compounds are of great importance. These compounds are widely present in the mineral, plants and animal world and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. For example, coordination compounds find use in many qualitative and quantitative chemical analysis. Some important metal extraction and purification make use of complex formation. Hardness of water can be estimated by stable complex formation of \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \)
From which coordination complex, gold is recovered by reducing this complex with Zn?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( [\text{Au}(\text{CN})_2]^{-} \)
Some metals are extracted by turning them into a soluble complex and then reducing that complex to the metal. In the cyanide process, gold is leached with a dilute cyanide solution in the presence of air (oxygen) and water. Gold is oxidised to Au(I) and dissolves as the dicyanidoaurate(I) ion:
\( \displaystyle \ce{4Au + 8CN^- + 2H2O + O2 -> 4[Au(CN)2]^- + 4OH^-} \)
The oxidation state of gold in the complex is found from the charges:
\( \displaystyle x + 2(-1) = -1 \Rightarrow x = +1 \)
Zinc, a more reactive metal, is then added. It is oxidised from 0 to +2 and reduces Au(I) to metallic gold, which separates:
\( \displaystyle \ce{2[Au(CN)2]^- + Zn -> [Zn(CN)4]^2- + 2Au} \)
So gold is recovered from \( \ce{[Au(CN)2]^-} \).
NCERT (2026-27 edition):
Importance and Applications of Coordination Compounds – …
