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: \(4 \mathrm{M}+8 \mathrm{CN}^{-}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \longrightarrow 4\left[\mathrm{M}(\mathrm{CN})_{2}\right]^{-1}+4 \mathrm{OH}^{-}\) \(2\left[\mathrm{M}(\mathrm{CN})_{2}\right]^{-1}+\mathrm{Zn} \longrightarrow\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2^{-}}+2 \mathrm{M}\) (a) \(\mathrm{Ag}\) (b) \(\mathrm{Cu}\) (c) \(\mathrm{Hg}\) (d) \(\mathrm{Ni}\)

Step-by-Step Solution

Answer

The metal M is Ag (silver).

Step 1: Analyze the First Reaction

The first reaction shows that the metal M reacts with cyanide ions, water, and oxygen to form a complex [M(CN)_2]^{-1} along with the production of hydroxide ions OH^{-}. This reaction is commonly associated with metals that form stable cyanide complexes, such as Ag(CN)_2^{-}.

Step 2: Analyze the Second Reaction

The second reaction describes a displacement where [M(CN)_2]^{-1} complex reacts with Zn, forming [Zn(CN)_4]^{2-} and releasing metal M. This suggests that metal M can be displaced by zinc, which is typical for metals less active than zinc, such as silver (Ag).

Step 3: Evaluate Possible Metals

Among the options, Ag (silver) is known to form stable cyanide complexes (AgCN), and it can be displaced by Zn in a reactivity series. The formation of a cyanide complex, as shown in the first reaction, also suggests the association of M with noble metals like silver.

Step 4: Conclude the Identity of Metal M

Since silver forms the complex [Ag(CN)_2]^{-1} in cyanidation and can be displaced by Zn, metal M is most likely silver (Ag) based on the reactions provided.

Key Concepts

Silver Extraction

Silver extraction is an intriguing topic, especially when we delve into its chemistry involving complex formations and displacement reactions. The process typically begins with silver forming a complex ion that allows it to be more easily separated from its ore. This is commonly done using cyanide, a process known for forming stable complexes with silver, such as \([Ag(CN)_2]^{-}\). Here are the general steps followed in the extraction of silver:

• Cyanidation: Involves the dissolution of silver ore into a cyanide solution to create a silver cyanide complex.
• Concentration: The dissolved silver is concentrated by processes like flotation or leaching.
• Recovery: The concentrated solution is treated to recover silver, often involving displacing the silver complex with another metal like zinc.

These steps reveal the intricate nature of silver's journey from ore to pure metal. Understanding the chemical reactions involved is crucial for efficient extraction.

Metal Displacement Reactions

Metal displacement reactions are fascinating chemical processes in which a more reactive metal displaces a less reactive metal from its compound. This type of reaction is fundamental to metallurgy and the extraction of various metals. A classic example is the use of zinc to displace silver from its cyanide complex: \( 2[Ag(CN)_2]^{-} + Zn \rightarrow [Zn(CN)_4]^{2-} + 2Ag \)This equation reveals several key aspects:

• The zinc atom donates electrons, and thus becomes oxidized, bonding with cyanide to form a zinc cyanide complex.
• The silver ions are reduced, releasing them as elemental silver.
• This reaction takes advantage of zinc's higher reactivity compared to silver, shifting silver from its ionic state back to metal.

Such reactions are significant in industrial processes, showcasing the elegance of chemistry in recovering and refining precious metals.

Cyanidation Process

The cyanidation process is essential for extracting not just silver, but also gold and other noble metals from their ores. It is sometimes referred to as the "cyanide process" and is primarily used due to its efficiency in separating metals from ore.The process generally involves these steps:

• Ore is crushed and ground to expose gold or silver particles.
• The pulverized ore is then mixed with a dilute solution of sodium cyanide, causing the metals to dissolve into the solution, forming complexes like \([Ag(CN)_2]^{-}\).
• Oxygen is often bubbled through the mixture to speed up the reaction.
• Once in solution, the metal complexes are then processed to recover the metal, typically by adding a more reactive metal like zinc to displace the silver or gold.

Though highly effective, the process must be managed carefully due to the hazardous nature of cyanide. Environmental and safety regulations are key to ensuring safe cyanidation in metal extraction.