Q22.35P - Chemistry: Molecular Nature Of Matter And Change

Question

The last step in the Dow process for the production of magnesium metal involves electrolysis of molten ${\mathbf{\text{MgCl}}}_{\mathbf{\text{2}}}$.

(a) Why isn't the electrolysis carried out with aqueous ${\mathbf{\text{MgCl}}}_{\mathbf{\text{2}}}$? What are the products of this aqueous electrolysis?

(b) Do the high temperatures required to melt ${\mathbf{\text{MgCl}}}_{\mathbf{\text{2}}}$ favor products or reactants? (Hint: Consider the ${{\mathbf{\Delta H}}_{\mathbf{f}}}^{\mathbf{0}}$ of ${\mathbf{\text{MgCl}}}_{\mathbf{\text{2}}}$.)

Step-by-Step Solution

Answer

(a) From the electrode potential values, water shows more reduction than Mg metal. So, hydrogen gas will be released at the cathode instead of the metal Mg.

(b) High temperatures favor the formation of magnesium metal and chlorine gas.

Step 1: Metallurgy of magnesium

The magnesium metal is extracted from seawater. It involves various steps like mining, converting to mineral, compound, and electrochemical redox reactions.

Step 2: Subpart (a)

- ${\text{Mg}}^{\text{2 + }}$ is more difficult to reduce than water, so ${\mathrm{H}}_{2\left(\mathrm{g}\right)}$ would be produced instead of

magnesium metal at the cathode.

- At the anode, chlorine gas is produced.

Step 3: Subpart (b)

The chemical reaction leads to the formation of magnesium chloride is given by

 ${\text{MgCl}}_{2( \text{s})}\to {\text{Mg}}_{\left(\text{s}\right)}+{\text{Cl}}_{2( \text{g})} \Delta {{\text{H}}_{\text{f}}}^{\text{o}}=+641.6 \text{kJ}/\text{mol}$

The delta H value clearly shows that the forward reaction is endothermic.

High temperatures favor the forward reaction(endothermic), so high temperatures favor the formation of magnesium metal and chlorine gas.