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 is \(\mathrm{Ag}\) (silver).

Step 1: Analyze the Reaction Types

Evaluate the given reactions. The first reaction is an oxidation in which metal M reacts with cyanide ions (\(\mathrm{CN}^-\)), water (\(\mathrm{H}_2\mathrm{O}\)), and oxygen (\(\mathrm{O}_2\)), forming a cyanide complex \([\mathrm{M}(\mathrm{CN})_2]^-\) and hydroxide ions (\(\mathrm{OH}^-\)). The second equation involves reduction, in which \([\mathrm{M}(\mathrm{CN})_2]^-\) is reduced to metal M using zinc, and forms a zinc-cyanide complex \([\mathrm{Zn}(\mathrm{CN})_4]^{2-}\).

Step 2: Identify the Metal Extraction Process

Recognize that this process involves the 'cyanide process' or 'MacArthur-Forrest process', typically used to extract precious metals. It includes complex formation with cyanides and further displacement by a less noble metal like zinc.

Step 3: Relate Reactions to Metal Types

Identify metals that form stable cyanide complexes. - Silver (\(\mathrm{Ag}\)) forms \([\mathrm{Ag}(\mathrm{CN})_2]^-\) which is known from cyanide extraction processes.- Copper (\(\mathrm{Cu}\)), mercury (\(\mathrm{Hg}\)), and nickel (\(\mathrm{Ni}\)) can also form complexes but are less common in this specific process without additional context.

Step 4: Select the Most Likely Metal

Given the typical cyanide complexation and extraction process, silver (\(\mathrm{Ag}\)) is a fitting candidate in the MacArthur-Forrest extraction process, often extracted using sodium or potassium cyanide and zinc dust.

Key Concepts

Metal Extraction

In metal extraction, refining is crucial to obtain pure forms of metals from their ores. Various processes are used based on the characteristics of each metal, but they all aim to separate metal from undesired materials.
Once the ore is obtained, the extraction process can start, often involving chemical reactions to isolate the metal. Here are a few general steps involved:

• Mining the ore from the Earth's crust.
• Breaking down the ore into smaller pieces through crushing and grinding.
• Using physical or chemical methods to concentrate the ore, separating metal from the substrate.
• Undergoing reduction, where oxygen is removed from the compound to leave pure metal.

Each metal varies in its reactivity and thus the extraction method needs to be matched to these characteristics. For example, highly reactive metals require electrolysis, while others may only need heating in a furnace along with reduction agents.

MacArthur-Forrest Process

The MacArthur-Forrest Process is a method of extracting gold from ore using cyanide. This technique revolutionized the mining industry when it was invented in the late 19th century.
The process involves several key steps:

• Ore is crushed and ground to allow the cyanide solution to access the gold particles.
• The ground ore is then mixed with a dilute solution of sodium or potassium cyanide and oxygen, forming a gold cyanide complex.
• Zinc dust is added to the mix, causing a chemical reaction that reduces the gold complex back to elemental gold, which precipitates out of the solution.

This process allows for the efficient extraction of gold, even from low-grade ores, and has significantly reduced costs associated with gold mining.

Metal Complexes

Metal complexes, often termed coordination compounds, consist of a central metal atom bonded to a surrounding array of molecules or ions. These molecules or ions are known as ligands.
Metal complexes have a variety of applications and properties, depending on their composition. Here are some characteristics:

• The metal usually serves as a positive center, capable of binding with negatively charged ligands.
• These complexes can exhibit a range of chemical behaviors, often used in catalysis or as metal medications.
• The geometry and coordination number (number of bonds to the metal) determine the complex's structure and properties.

Many transition metals, like silver in the context of extraction processes, form stable cyanide complexes, crucial for separation and purification processes in metallurgy.