Problem 86 - A Complete Resource Book in Chemistry for JEE Main

Question

Name the metal \(\mathrm{M}\) which is extracted on the basis of following reactions: \(4 \mathrm{M}+8 \mathrm{CN}^{-}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \longrightarrow 4[\mathrm{M}(\mathrm{CN})]^{-1}+4 \mathrm{OH}^{-}\) \(2\left[\mathrm{M}(\mathrm{CN})_{2}\right]^{-1}+\mathrm{Zn} \longrightarrow\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2^{-}}+2 \mathrm{M}\) (a) \(\mathrm{Ag}\) (b) \(\mathrm{Cu}\) (c) \(\mathrm{Hg}\) (d) \(\mathrm{Ni}\)

Step-by-Step Solution

Answer

The metal M is silver (Ag).

Step 1: Understand the chemical equations

The first equation represents the reaction of metal M with cyanide ions in the presence of water and oxygen to form a metal cyanide complex and hydroxide ions. The second equation represents the displacement of metal M from its cyanide complex by zinc.

Step 2: Analyze the first reaction

The first reaction suggests that metal M forms a cyanide complex. In metallurgy, silver (Ag) is commonly extracted using cyanidation, where silver forms complexes like \[\mathrm{Ag(CN)}_{2}^{-}\]. This indicates that metal M could be silver.

Step 3: Analyze the second reaction

In the second equation, zinc displaces metal M from the \[\mathrm{M(CN)}_{2}^{-}\] complex. Zinc is above silver in the electrochemical series and can displace it, reinforcing that metal M could be silver (Ag).

Step 4: Compare with options

Among the options (a) Ag, (b) Cu, (c) Hg, and (d) Ni, silver (Ag) is the known metal often extracted using the described reactions involving cyanide complexes.

Key Concepts

Cyanidation

Cyanidation is a crucial process in the field of metallurgy, especially for extracting precious metals like gold and silver from ores. It involves the use of cyanide ions, which, despite being highly toxic, are effective in dissolving these metals from their ores. The basic principle of cyanidation is the transformation of metal into a cyanide complex.

• In the presence of cyanide ions, water, and oxygen, metals such as silver form stable complexes.
• Silver is known to create a soluble complex, specifically \[ \mathrm{Ag(CN)}_{2}^{-} \], during this process.
• The reaction typically results in the formation of metal complexes along with hydroxide ions.

This cyanide complex reaction is reversible, and its success significantly depends on the control of pH and oxygen availability.
Cyanidation is a leading method for silver extraction, making it an essential piece of knowledge for metallurgists handling silver-rich ores.

Electrochemical Series

The electrochemical series—also called the activity series—ranks elements (usually metals) based on their standard electrode potentials. This series indicates how easily an element can undergo oxidation or reduction.

• Elements at the top, like lithium, are more likely to lose electrons and are thereby more reactive.
• Zinc is higher in the series compared to silver, displaying its capability to displace silver from its compounds.

In the context of metal extraction, the electrochemical series is valuable in predicting the feasibility of displacement reactions.
When zinc displaces silver from its cyanide complex, its a direct application of knowledge from the electrochemical series. Recognizing this, metallurgists can determine and predict the extraction processes of various metals.

Displacement Reaction

A displacement reaction is a type of chemical reaction where a more reactive element displaces a less reactive element from its compound. This principle is widely utilized in metallurgy to recover metals from their ores or compounds.

• In the provided reactions, zinc displaces silver from the \[ \mathrm{Ag(CN)}_{2}^{-} \] complex.
• This occurs because zinc is more reactive and higher in the electrochemical series than silver.
• The general form of the reaction is \[ ext{Element A} + ext{Compound of Element B} \rightarrow ext{Compound of Element A} + ext{Element B} \]. \

Displacement reactions are routinely used to extract metals like silver and are fundamental to understanding basic metallurgical procedures.
They demonstrate how reactivity differences can be exploited to recover valuable elements from complex chemical forms.

Silver Extraction

Silver extraction through cyanidation is an advanced and efficient methodology employed by metallurgists.

• Initially, silver is dissolved out of its ore using cyanide to form the \[ \mathrm{Ag(CN)}_{2}^{-} \] complex in solution.
• Subsequently, zinc is utilized to displace the silver from this complex—a defining step in the extraction process.
• Finally, silver is collected in its pure metallic form after displacement.

This precise method leverages principles such as cyanidation and knowledge from the electrochemical series.
Through careful regulation of conditions during each step, the extraction process becomes highly efficient.
Understanding these steps is crucial for those interested in the silver industry, ensuring effective recovery of this precious metal from its ores.