In the extraction of gold, the metal is first brought into aqueous solution by treating it with cyanide ions, and gold is then separated from this solution in metallic form by adding zinc. Which of the following equations correctly represents the first step?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( \ce{4Au + 8CN- + O2 + 2H2O -> 4[Au(CN)2]- + 4OH-} \)
Gold is extracted by complex formation. In the presence of oxygen and water, gold combines with cyanide to form the soluble coordination entity \( \ce{[Au(CN)2]^-} \), in which gold is +1.
Oxygen is the oxidising agent: each O2 takes four electrons, so four gold atoms are oxidised per O2.
\( \displaystyle \ce{4Au + 8CN- + O2 + 2H2O -> 4[Au(CN)2]- + 4OH-} \)
Check: Au 4 = 4; CN 8 = 8; O 2 + 2 = 4 (in 4OH−); H 4 = 4; charge −8 = −4 − 4.
In the second step zinc, a more reactive metal, is added and gold separates in metallic form.
Why the other options are incorrect
Option (2) \( \ce{4Au + 16CN- + 3O2 + 6H2O -> 4[Au(CN)4]- + 12OH-} \) — Incorrect. The equation is balanced, but it forms \( \ce{[Au(CN)4]^-} \) with gold in the +3 state. The entity formed in this process is \( \ce{[Au(CN)2]^-} \), with gold(I).
Option (3) \( \ce{2Au + 4CN- + 2H2O -> 2[Au(CN)2]- + 2OH- + H2} \) — Incorrect. Here water alone oxidises gold and hydrogen is set free. Gold is a noble metal; the process needs oxygen as the oxidising agent, and no hydrogen is formed.
Option (4) \( \ce{4Au + 4CN- + O2 + 2H2O -> 4AuCN + 4OH-} \) — Incorrect.
AuCN is a neutral cyanide that does not dissolve in water, so it would not bring gold into solution.
Gold dissolves as the complex ion \( \ce{[Au(CN)2]^-} \), which needs two cyanide ions per gold atom.
NCERT (2026-27 edition):
5.7 Importance and Applications of Coordination Compounds – “Some important extraction processes of metals, like those of silver and gold, make use of complex formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN)2]– in aqueous solution. Gold can be separated in metallic form from this solution by the addition of zinc.”