Magnesium is a valuable, light weight metal used as a structural material as well as in alloys, batteries, and in chemical synthesis. Although magnesium is plentiful in Earth's crust, it is mainly found in the sea water (after sodium). There is about 1.3 g of magnesium in every kilogram of sea water. The process for obtaining magnesium from sea water employs all three types of reactions, i.e., precipitation, acid-base, and redox-reactions.
Redox reaction takes place (in the extraction of Mg) :

Options

A. in the electrolytic cell when fused `MgCl_(2)` is subjected to electrolysis
B. when fused `MgCO_(3)` is heated
C. When fused `MgCO_(3)` is strongly heated
D. none of the above

Answer

To extract magnesium (Mg) from seawater, we utilize a process known as the Dow's process, which involves several steps including precipitation, acid-base reactions, and redox reactions. Here is a step-by-step breakdown of the redox reaction that occurs during the extraction of magnesium:

### Step 1: Formation of Anhydrous Magnesium Chloride (MgCl2)
1. **Source of Magnesium Ions**: Seawater contains magnesium ions (Mg²⁺) in aqueous form.
2. **Precipitation Reaction**: When magnesium ions from seawater react with slaked lime (calcium hydroxide, Ca(OH)₂), magnesium hydroxide (Mg(OH)₂) precipitates out:
\[ \text{Mg}^{2+}_{(aq)} + \text{Ca(OH)}_{2(aq)} \rightarrow \text{Mg(OH)}_{2(s)} + \text{Ca}^{2+}_{(aq)} \]
3. **Acid-Base Reaction**: The precipitated magnesium hydroxide is then treated with dilute hydrochloric acid (HCl) to form magnesium chloride in aqueous form:
\[ \text{Mg(OH)}_{2(s)} + 2 \text{HCl}_{(aq)} \rightarrow \text{MgCl}_{2(aq)} + 2 \text{H}_2\text{O}_{(l)} \]
4. **Crystallization**: The aqueous magnesium chloride is crystallized to obtain magnesium chloride hexahydrate (MgCl₂·6H₂O):
\[ \text{MgCl}_{2(aq)} \rightarrow \text{MgCl}_{2} \cdot 6\text{H}_2\text{O}_{(s)} \]
5. **Dehydration**: The hexahydrate is then heated with dry HCl or steam to produce anhydrous magnesium chloride (MgCl₂):
\[ \text{MgCl}_{2} \cdot 6\text{H}_2\text{O}_{(s)} \xrightarrow{\text{heat}} \text{MgCl}_{2(s)} + 6 \text{HCl}_{(g)} \]

### Step 2: Electrolysis of Anhydrous Magnesium Chloride
1. **Electrolysis Setup**: The anhydrous MgCl₂ is subjected to electrolysis in a molten state, often mixed with calcium chloride (CaCl₂) and sodium chloride (NaCl) to improve conductivity and lower the melting point.
2. **Dissociation**: In the molten state, MgCl₂ dissociates into magnesium ions (Mg²⁺) and chloride ions (Cl⁻):
\[ \text{MgCl}_{2(l)} \rightarrow \text{Mg}^{2+}_{(l)} + 2 \text{Cl}^-_{(l)} \]
3. **Redox Reactions**:
- **At the Anode (Oxidation)**: Chloride ions lose electrons to form chlorine gas (Cl₂):
\[ 2 \text{Cl}^- \rightarrow \text{Cl}_{2(g)} + 2 e^- \]
- **At the Cathode (Reduction)**: Magnesium ions gain electrons to form molten magnesium:
\[ \text{Mg}^{2+} + 2 e^- \rightarrow \text{Mg}_{(l)} \]

### Conclusion
The overall redox reaction during the electrolysis of molten MgCl₂ can be summarized as:
\[ \text{Mg}^{2+} + 2 \text{Cl}^- \rightarrow \text{Mg}_{(l)} + \text{Cl}_{2(g)} \]
This indicates that during the electrolysis of fused MgCl₂, redox reactions take place, with oxidation occurring at the anode and reduction at the cathode.