Question

Metallic Magnesium is prepared by______

• A. Reduction of $\mathrm{MgO}$ by coke
• B. Electrolysis of aqueous solution of $\mathrm{Mg}{\left({\mathrm{NO}}_3\right)}_2$
• C. Displacement of Magnesium by iron from ${\mathrm{MgSO}}_4$ solution
• D. Electrolysis of molten ${\mathrm{MgCl}}_2$

Answer 1

The correct option is D

Electrolysis of molten ${\mathrm{MgCl}}_2$

The correct option is option D

The explanation for the correct option:

1. Metallic magnesium is prepared either by electrolysis of molten ${\mathrm{MgCl}}_2$ or by metallothermic reduction of its halides by alkali or alkaline-earth metals.
2. ${\mathrm{MgCl}}_2$is electrolyzed in two steps: first, the molten state is prepared, and then the molten state is electrolyzed.
3. Preparation of molten magnesium chloride:
4. ${\mathrm{Mg}}^{2+}+\underset{\mathrm{Calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{l}\right)}\to \underset{\mathrm{Magnesium}\mathrm{hydroxide}}{\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}+\underset{\mathrm{Calcium}\mathrm{chloride}}{{\mathrm{CaCl}}_2\left(\mathrm{s}\right)}\stackrel{\mathrm{dil}\mathrm{HCl}}{\to }\underset{\mathrm{Magnesium}\mathrm{chloride}}{{\mathrm{MgCl}}_2.6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$,
5. Calcium hydroxide is added as a source of hydroxide ions and magnesium is precipitated as Magnesium Hydroxide.
6. It is further reacted with dilute $\mathrm{HCl}$ to generate magnesium chloride.
7. Now, the electrolysis of the molten mixture of ${\mathrm{MgCl}}_2$ is done that produces $\mathrm{Mg}$ at the cathode, and ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ at the anode.
8. Cathode:${\mathrm{Mg}}^{+2}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\to \mathrm{Mg}\left(\mathrm{s}\right)$, Anode: $2{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{Cl}}_2\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}$

The explanation for the incorrect options:

Option (A): Reduction of $\mathrm{MgO}$ by coke

1. $\mathrm{MgO}$ cannot be reduced by coke because it has greater reducing power than carbon.
2. Hence, it is an incorrect option.

Option B: Electrolysis of aqueous solution of $\mathrm{Mg}{\left({\mathrm{NO}}_3\right)}_2$

1. ${\mathrm{Mg}}^{+2}$ ions are not reduced at the cathode during the electrolysis of solution$\mathrm{Mg}{\left({\mathrm{NO}}_3\right)}_2$ , instead, hydrogen is reduced in preference because its electrode potential is lower than that of ${\mathrm{Mg}}^{+2}$
2. Hence, it is an incorrect option.

Option C: Displacement of Magnesium by iron from ${\displaystyle MgS{O}_4}$ solution

1. Because magnesium is more electropositive than iron, iron cannot displace magnesium from a solution of ${\displaystyle MgS{O}_4}$.
2. Hence, it is an incorrect option.

Final answer: Metallic magnesium is prepared by the electrolysis of molten ${\mathrm{MgCl}}_2$