Problem 80 - Chemistry Principles and Practice

Question

The commercial production of magnesium is accomplished by electrolysis of molten \(\mathrm{MgCl}_{2}\) (a) Why is electrolysis of an aqueous solution of \(\mathrm{MgCl}_{2}\) not used in this process? (b) Write the anode and cathode half-reaction in the electrolysis of molten \(\mathrm{MgCl}_{2}\)

Step-by-Step Solution

Answer

(a) Electrolysis of aqueous MgCl2 is not used because water would interfere by being reduced or oxidized, forming gases instead of magnesium. (b) Cathode: \(\mathrm{Mg^{2+} + 2e^- \rightarrow Mg}\); Anode: \(2\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2e^-\)."}

Step 1: Understanding Electrolysis and Magnesium Chloride

Electrolysis involves the use of an electrical current to drive a non-spontaneous chemical reaction. In the case of magnesium production, we are specifically dealing with the electrolysis of magnesium chloride \(MgCl_2\). In this process, magnesium is extracted from its compound.

Step 2: Analyzing Aqueous vs. Molten MgCl2

In an aqueous solution, \(MgCl_2\) is dissolved in water. Water, being a potential reactant, can interfere with the electrolysis process. Specifically, water can be reduced and oxidized during electrolysis, which could lead to the production of hydrogen gas and oxygen instead of magnesium metal. As a result, molten (liquid) \(MgCl_2\), which does not contain water, is used to ensure the direct production of magnesium.

Step 3: Cathode Reaction of Molten MgCl2

During electrolysis, \(Mg^{2+}\) ions are reduced to magnesium metal at the cathode. The half-reaction for this process is: \[ \text{Cathode:} \quad \mathrm{Mg^{2+} + 2e^- \rightarrow Mg} \] This describes the gain of electrons (reduction) by magnesium ions to form magnesium metal.

Step 4: Anode Reaction of Molten MgCl2

At the anode, \(Cl^-\) ions undergo oxidation to form chlorine gas. The half-reaction for this process is: \[ \text{Anode:} \quad 2\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2e^- \] This represents the loss of electrons (oxidation) by chloride ions, resulting in the formation of chlorine gas.

Key Concepts

Magnesium Production

Magnesium is a highly versatile metal used frequently in industries ranging from automotive to electronic devices. One of the key methods for extracting magnesium is through the use of electrolysis, specifically targeting magnesium chloride (\(\mathrm{MgCl}_{2}\)).
In the commercial setting, magnesium production is achieved by decomposing molten \(\mathrm{MgCl}_{2}\) via electrical current. This process effectively separates magnesium metal from its chloride ions, enabling its collection for industrial use.
The benefits of electrolysis for magnesium production include its ability to produce pure magnesium and operate at relatively efficient energy costs, making it a preferred method over other extraction techniques.

Aqueous vs. Molten Solutions

In the realm of electrolysis, choosing between aqueous and molten solutions of \(\mathrm{MgCl}_{2}\) is crucial. An aqueous solution involves dissolving \(\mathrm{MgCl}_{2}\) in water, whereas a molten solution is simply the compound in its liquid form at high temperatures.
While an aqueous solution might seem convenient, it introduces water as a potential reactant. This complicates the process, as water can undergo electrolysis itself, leading to the production of hydrogen gas instead of magnesium metal at the cathode. Therefore, to avoid such interference, molten \(\mathrm{MgCl}_{2}\) is preferred. It ensures that the electrolysis process directly targets only the magnesium ions without any disruptive side reactions involving water.

Half-Reaction

Electrolysis involves two crucial half-reactions: one occurring at the cathode and the other at the anode. These half-reactions enable the separation of the desired product, in this case, magnesium.
At the cathode, \(\mathrm{Mg^{2+}}\) ions gain electrons in the reduction process to form magnesium metal. The equation for this reaction is given by:

• Cathode: \(\mathrm{Mg^{2+} + 2e^- \rightarrow Mg}\)

Conversely, at the anode, \(\mathrm{Cl^{-}}\) ions lose electrons to form chlorine gas. This oxidation process is represented by:

• Anode: \(2\mathrm{Cl^{-}} \rightarrow \mathrm{Cl_2} + 2e^-\)

Together, these half-reactions complete the process of electrolysis, efficiently extracting magnesium while producing chlorine gas as a byproduct.

Chemical Reaction

A chemical reaction occurs during electrolysis when an electrical current induces a change in the chemical substances involved.
In the electrolysis of molten \(\mathrm{MgCl}_{2}\), the principal reactions involve the transformation of magnesium ions into solid magnesium and chloride ions into chlorine gas.
The introduction of electricity provides the necessary energy to break chemical bonds and rearrange electrons in these substances. For the reaction:

• \(\mathrm{MgCl_2 (l) \rightarrow Mg (s) + Cl_2 (g)}\)

Here, molten \(\mathrm{MgCl}_{2}\) is split into magnesium and chlorine gas, showcasing how electrolysis can effectively drive a non-spontaneous chemical reaction, thus highlighting its importance in industrial applications.