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The following reactions occur during extraction of gold by hydrometallurgy: Au + CN⁻ + H2O --(O2)--> X X + Zn --> Y + Au Identify X and Y.

1. [Au(CN)2]⁻ and [Zn(CN)4]⁴-
2. [Au(CN)2]⁻ and [Zn(CN)4]²-
3. [Au(CN)4]³- and [Zn(CN)6]⁴-
4. [Au(CN)2]⁻ and [Zn(CN)4]²-

Correct answer: [Au(CN)2]⁻ and [Zn(CN)4]²-

Solution

In hydrometallurgical gold extraction (MacArthur-Forrest process), Au is oxidised by O2 in the presence of CN⁻ to form [Au(CN)2]⁻. Zinc (more electropositive) then reduces Au back: Zn + 2[Au(CN)2]⁻ -> [Zn(CN)4]²- + 2Au. X = [Au(CN)2]⁻, Y = [Zn(CN)4]²-.

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