Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

Magnesium can be obtained by (a) reducing magnesium salt solution with \(\mathrm{Fe}\) (b) electrolysis of fused magnesium salt (c) electrolysis of \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) solution (d) reducing \(\mathrm{MgO}\) with coke

Step-by-Step Solution

Answer

Magnesium can be obtained by electrolysis of fused magnesium salt (option b).

Step 1: Evaluate Magnesium Extraction Options

To find how magnesium can be obtained, we need to examine each option in the context of typical metallurgical practices. Options involve different chemical and electrochemical processes and conditions.

Step 2: Analyze Reduction Processes

(a) Reducing magnesium salt solution with Fe: Fe is less reactive than Mg, so it cannot reduce Mg ions. Therefore, this method is not suitable for obtaining magnesium.

Step 3: Consider Electrolysis of Fused Salts

(b) Electrolysis of fused magnesium salt: This is a common method for obtaining magnesium. Electrolysis of molten magnesium salts isolates Mg as a metal due to the mobility and complete dissociation of ions in molten state.

Step 4: Evaluate Electrolysis of Solutions

(c) Electrolysis of Mg(NO3)2 solution: Because magnesium is reactive and tends to remain in solution during electrolysis, it does not deposit out easily in aqueous solutions. Thus, this method is not effective for obtaining magnesium.

Step 5: Analyze Reduction with Coke

(d) Reducing MgO with coke: The thermodynamic instability and high reactivity of Mg prevent its extraction this way. Coke is used mainly for reducing less reactive metal oxides.

Step 6: Determine Viable Method

After reviewing all options, the most practical method for obtaining magnesium is electrolysis of a fused magnesium salt, as in option (b). This method capitalizes on the properties of molten salts to isolate metals such as Mg.

Key Concepts

Electrolysis of Fused Salts

When it comes to extracting magnesium, one of the most effective methods is through electrolysis of fused salts. This process involves using electrical energy to drive a chemical reaction in molten magnesium salts. The molten state is key because it allows the ions within the salts to move freely. This fluid mobility and complete dissociation of ions when heated are crucial.
Electrolysis in molten salts works by applying an electric current that forces magnesium ions ( Mg^{2+} ) to migrate towards the cathode, where they gain electrons and form magnesium metal.

• The electrolytic cell used often consists of a graphite (carbon) anode and a steel cathode.
• Molten magnesium chloride ( MgCl_2 ) is a typical salt used during this process.
• At the cathode, magnesium ions ( Mg^{2+} ) gain 2 electrons ( 2e^{-} ) and are reduced to magnesium metal ( Mg ).
• At the anode, chloride ions ( Cl^{-} ) lose electrons and form chlorine gas ( Cl_2 ).

Through this direct electrolysis technique, magnesium, which is highly reactive and difficult to isolate from aqueous solutions, can be efficiently extracted as a metal.

Reduction Processes

Reduction processes play a vital role in extracting various metals, although they are less suitable for magnesium. Reduction is a chemical reaction that involves gaining electrons. However, magnesium’s high reactivity makes it a challenging candidate for many traditional reduction techniques.

• In the extraction of less reactive metals, elements such as iron or coke are often used as reducing agents.
• Magnesium cannot typically be extracted by reduction with coke because it forms stable oxides that are difficult to break.

For magnesium, the reduction process is not viable due to these factors:

• Thermodynamic instability: Magnesium forms stable compounds like MgO that resist breakdown.
• High reactivity: This characteristic limits reduction methods since Mg reacts strongly with reduction agents themselves.

Thus, while reduction processes are fundamental in metallurgy, their application for magnesium extraction is limited due to these unique chemical properties.

Metallurgical Practices

Metallurgical practices encompass a wide array of processes and techniques for extracting and refining metals, such as magnesium, from their natural states. These practices combine science, engineering, and technology to successfully isolate metals.
Magnesium extraction exemplifies the subtle complexity of metallurgical work. Its extraction from magnesium salts requires careful management of chemical reactions, high temperatures, and sometimes intricate electrochemical setups. The choice of technique rests heavily on understanding each metal's unique properties and behavior under various conditions.

• For magnesium, electrolysis in molten salt is prioritized due to its high reactivity, which renders aqueous electrolysis ineffective.
• Environmental and economic considerations guide the choice of extraction methods. High yields, energy efficiency, and minimal environmental impact are all desirable features of a strong metallurgical process.

Through continuous advancements, metallurgical practices continue to evolve, leading to more efficient, sustainable, and cost-effective methods of metal extraction.