In the cyanide extraction process of silver, the oxidizing and reducing agents used are:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(O_2\) and \(Zn\) dust respectively
In the cyanide process (leaching), silver is dissolved from the ore by dilute NaCN solution in the presence of air. Oxygen oxidises Ag (0) to \( \ce{Ag^+} \) (+1), which is held as the soluble complex \( \ce{[Ag(CN)2]^-} \). The metal is then recovered by adding zinc dust, a more electropositive metal: Zn (0) is oxidised to Zn (+2) in \( \ce{[Zn(CN)4]^2-} \) and so reduces silver back to the metal.
So the oxidising agent is O2 and the reducing agent is Zn dust.
Why the other options are incorrect
Option (1) \(O_2\) and \(CO\) respectively — Incorrect. CO is the reducing agent for iron oxide in the blast furnace, not for the cyanide complex of silver.
Option (3) \(H_2O\) and \(Zn\) dust respectively — Incorrect. Water is only the medium; it is not the oxidising agent, which is \( \ce{O2} \).
Option (4) \(H_2O\) and \(NaCN\) respectively — Incorrect. NaCN is the leaching (complexing) agent that dissolves \( \ce{Ag^{+}} \); it does not reduce anything.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
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