Question

The last step in the Dow process for the production of magnesium metal involves clectrolysis of molten \(\mathrm{MgCl}_{2}\). (a) Why isn't the electrolysis carried out with aqueous \(\mathrm{MgCl}_{2}\) ? What are the products of this aqueous electrolysis? (b) Do the high temperatures required to melt \(\mathrm{MgCl}_{2}\) favor products or reactants? (Hint: Consider the \(\Delta H_{\mathrm{f}}^{\circ}\) of \(\mathrm{MgCl}_{2}\).)

Short answer

Electrolysis isn't done with aqueous \(\text{MgCl}_2\) because it produces \(\text{H}_2\) and \(\text{O}_2\). High temperatures favor product formation.

Step-by-step solution

Understanding Electrolysis of Molten \(\text{MgCl}_2\)

Electrolysis is a process by which electrical energy is used to cause a non-spontaneous chemical reaction. When molten \(\text{MgCl}_2\) is electrolyzed, it decomposes into magnesium and chlorine gas.

Electrolysis of Aqueous \(\text{MgCl}_2\)

Electrolysis of \(\text{MgCl}_2\) in an aqueous solution is not performed because water also gets electrolyzed along with \(\text{MgCl}_2\). This would lead to the formation of hydrogen gas at the cathode and oxygen gas at the anode, instead of magnesium metal and chlorine gas.

Products of Aqueous Electrolysis

The products of aqueous electrolysis of \(\text{MgCl}_2\) are hydrogen gas \(\text{H}_2\) and oxygen gas \(\text{O}_2\). Magnesium ions do not get reduced to magnesium metal because the reduction of water is more favorable.

High Temperatures for Electrolysis of Molten \(\text{MgCl}_2\)

Magnesium chloride has a high melting point. The heat needed to melt \(\text{MgCl}_2\) allows the electrolysis to proceed efficiently. This process favors the formation of products (Mg and Cl_2) over the reactants due to the high temperature, which helps in overcoming the activation energy barrier.

Consideration of \(\Delta H_{\text{f}}^\circ\)

The standard enthalpy of formation \(\Delta H_{\text{f}}^\circ\) for \(\text{MgCl}_2\) indicates how much energy is required to form \(\text{MgCl}_2\) from its elements. A high melting temperature means breaking the ionic bonds to form Mg and Cl_2 is energetically favorable, shifting the equilibrium towards the products.

Key concepts

Electrolysis

Electrolysis is a chemical process that uses electrical energy to drive a non-spontaneous reaction. It involves passing an electric current through a substance to cause its decomposition. In the context of electrolysis of magnesium chloride (MgCl_2), electrical energy is used to break down molten MgCl_2 into its elemental components: magnesium (Mg) and chlorine gas (Cl_2). This process is critical in industries for extracting and purifying metals from their compounds.
During electrolysis, two electrodes are immersed in the electrolyte, which is the molten or dissolved substance. The positive electrode, called the anode, attracts negatively charged ions (anions), while the negative electrode, called the cathode, attracts positively charged ions (cations). The external power source drives the flow of electrons through the circuit, causing the chemical reactions at the electrodes.

Magnesium Chloride

Magnesium chloride (MgCl_2) is a chemical compound composed of magnesium and chloride ions. It is commonly found in its hydrated form, MgCl_2 * 6H_2O. MgCl_2 is used in various applications, including de-icing roads, acting as a coagulant in tofu production, and in the production of magnesium metal via electrolysis.
In its molten state, MgCl_2 can be electrolyzed to produce magnesium metal and chlorine gas. The high melting point of MgCl_2 (about 712°C) makes it necessary to use high temperatures during the electrolysis process. This is due to the strong ionic bonds between magnesium and chloride ions, which require significant energy to break.

Aqueous Electrolysis

In aqueous electrolysis, the substance to be electrolyzed is dissolved in water. However, performing electrolysis on aqueous magnesium chloride (MgCl_2) presents challenges. When MgCl_2 is dissolved in water and an electric current is applied, water molecules also undergo electrolysis. This leads to the production of hydrogen gas (H_2) at the cathode and oxygen gas (O_2) at the anode instead of magnesium metal and chlorine gas.
The reason for this is that the reduction of water to hydrogen gas is thermodynamically more favorable than the reduction of magnesium ions to magnesium metal in an aqueous solution. As a result, aqueous electrolysis of MgCl_2 does not yield the desired products for industrial purposes.

Thermodynamics

Thermodynamics plays a crucial role in understanding the electrolysis of MgCl_2, especially the influence of temperature on the reaction. The high temperatures required to melt MgCl_2 favor the formation of products (Mg and Cl_2) over the reactants. This is because the energy provided by the heat helps to overcome the activation energy barrier, making the decomposition of MgCl_2 into Mg and Cl_2 more efficient.
The standard enthalpy of formation, \(\text{ΔH}_{\text{f}}^\text{°}\), of MgCl_2 indicates the energy required to form MgCl_2 from its elemental components. A high \(\text{ΔH}_{\text{f}}^\text{°}\) value means a significant amount of energy is needed to break the ionic bonds in MgCl_2. The high melting point of MgCl_2 ensures that sufficient energy is available to shift the equilibrium towards the products, making the electrolysis process effective.