Problem 39 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give briefly the isolation of magnesium from sea water by the Dow process. Give equations for the steps involved.

Step-by-Step Solution

Answer

Mg is isolated from seawater in the Dow process by precipitating Mg(OH)₂, converting it to MgCl₂, and electrolyzing the MgCl₂.

Step 1: Formation of Magnesium Hydroxide

In the Dow process, magnesium is first precipitated as magnesium hydroxide from sea water by adding lime (calcium hydroxide). The reaction is:\[\text{Mg}^{2+}_{(aq)} + \text{Ca(OH)}_{2(s)} \rightarrow \text{Mg(OH)}_{2(s)} + \text{Ca}^{2+}_{(aq)}\]This forms solid magnesium hydroxide.

Step 2: Conversion to Magnesium Chloride

The magnesium hydroxide is then reacted with hydrochloric acid to form soluble magnesium chloride. The reaction is:\[\text{Mg(OH)}_{2(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{MgCl}_{2(aq)} + 2\text{H}_2\text{O}_{(l)}\]This step transforms the magnesium hydroxide into magnesium chloride.

Step 3: Electrolysis of Magnesium Chloride

The magnesium chloride solution is subjected to electrolysis. During this process, magnesium is deposited at the cathode and chlorine gas is released at the anode:\[\text{MgCl}_{2(aq)} \rightarrow \text{Mg}_{(s)} + \text{Cl}_2_{(g)}\]This step isolates magnesium metal from the solution.

Key Concepts

Dow Process

The Dow Process is a chemical method specifically designed for isolating magnesium from sea water. This process is economically feasible due to the abundance of resources, such as magnesium ions found in sea water and lime (calcium hydroxide), which are used in the extraction process.

The basic principle involves a series of reactions that convert magnesium ions to magnesium metal. It begins with extracting magnesium hydroxide from sea water, then converting it into magnesium chloride, and finally, isolating magnesium metal through electrolysis. This method stands out because of its practicality and efficiency in producing magnesium on a large scale.

Magnesium Hydroxide Formation

In the first step of the Dow Process, magnesium hydroxide is precipitated from sea water. This is achieved by adding lime to the sea water, which contains magnesium ions in solution.

The chemical reaction is fairly straightforward:

• Calcium hydroxide (\( ext{Ca(OH)}_2\)) is introduced to the sea water.
• Magnesium ions (\( ext{Mg}^{2+}\)) from the sea water react with calcium hydroxide, forming an insoluble compound known as magnesium hydroxide (\( ext{Mg(OH)}_2\)).
• The reaction can be represented as \( ext{Mg}^{2+}_{(aq)} + ext{Ca(OH)}_{2(s)} ightarrow ext{Mg(OH)}_{2(s)} + ext{Ca}^{2+}_{(aq)}\).

This reaction results in the precipitation of solid magnesium hydroxide, which can be easily separated from the rest of the solution. It's a key step that moves us closer to obtaining pure magnesium.

Electrolysis of Magnesium Chloride

After converting magnesium hydroxide into magnesium chloride in the previous step, electrolysis is performed to extract the magnesium metal.

The dissolved magnesium chloride (\( ext{MgCl}_2\)) in the solution undergoes electrolysis. This process breaks down the compound into its components:

• Magnesium metal is deposited at the cathode.
• Chlorine gas is released at the anode.

The overall reaction during electrolysis is represented as \( ext{MgCl}_{2(aq)} ightarrow ext{Mg}_{(s)} + ext{Cl}_2_{(g)}\).

This method effectively extracts magnesium as a solid, allowing it to be collected in its pure metallic form. Electrolysis is a powerful technique that not only separates the magnesium but also ensures that it's done with precise control over the chemical process involved.