Question

The metal extracted by leaching with a cyanide is:
A. $Mg$
B. $Ag$
C. $Cu$
D. $Na$

Answer 1

Hint: We know that leaching is a process which is used widely in metallurgy where the ore is treated with chemicals to convert the valuable metals present within the ore into soluble salts while the impurities remain insoluble. The impurities can be easily washed out and then processed to give pure metal.

Complete answer:
Leaching is mainly used if the ore is present in soluble in some suitable solvent. While performing the process of metallurgy of silver and gold, the respective metal is leached with a dilute solution of $NaCN$ or $KCN$ in the presence of air. Air provides oxygen from which the metal can be obtained later in the process by replacement.
The reaction for the general process takes place as:
$Au/Ag + C{N^ - } + {H_2}O + {O_2} \to {[Au/Ag{(CN)_2}]^ - } + O{H^ - }$
The silver or gold complex formed with cyanide is a water-soluble complex and it further undergoes the process of reduction by reaction with zinc to give us the extracted metal.
${[Ag/Au{(CN)_2}]^ - } + Zn \to {[Zn{(CN)_4}]^{2 - }} + Au/Ag$
The complex formed between the zinc and cyanide is also a water-soluble complex.
Here we will see that how is silver extracted by the process of leaching with a cyanide:
So here the silver ore will form a soluble complex with $NaCN$ from which silver will be precipitated out using scrap zinc.
$A{g_2}S + 2NaCN \to Na[Ag{(CN)_2}]$
$Na[Ag{(CN)_2}] + Zn \to N{a_2}[Zn{(CN)_4}] + Ag \downarrow $
The complex formed is called sodium argent cyanide which is a soluble complex.
Hence, the correct answer is, ‘B. $Ag$’.

Note: The precipitated silver can be removed by the filtration process. Also, the impurities of silver can be removed by liquefaction, distillation or zone refining. For the process of metallurgy, we need to understand the electrochemical series because we know that the more electropositive metal, the greater will be its reducing power to displace a metal from the solution.