During the electrolysis of carnallite, `MgCl_(2)` is decomposed and not KCl. This is because of :

Options

A. lower decomposition voltage of `MgCl_(2)` than that of KCl
B. reverse reaction `MgCl_(2)+2K to Mg +2KCl ` if KCl is decomposed under other experimental condition
C. both (a) and (b)
D. none of the above

Answer

To solve the question regarding the electrolysis of carnallite and the decomposition of MgCl₂ instead of KCl, we can break down the reasoning into clear steps:

### Step-by-Step Solution:

1. **Understanding the Composition of Carnallite**:
- Carnallite is a mineral with the formula KCl·MgCl₂·6H₂O. It contains both potassium ions (K⁺) and magnesium ions (Mg²⁺) along with chloride ions (Cl⁻).

**Hint**: Identify the components of the mineral and their charges.

2. **Electrolysis Process**:
- During electrolysis, cations migrate to the cathode (negative electrode) and anions migrate to the anode (positive electrode).
- The ions present in carnallite are K⁺, Mg²⁺, and Cl⁻.

**Hint**: Recall the movement of ions during electrolysis.

3. **Discharge of Ions**:
- The discharge of ions at the electrodes depends on their electropositivity. Magnesium (Mg²⁺) is less electropositive than potassium (K⁺), meaning it is preferentially discharged at the cathode.

**Hint**: Compare the electropositivity of magnesium and potassium.

4. **Decomposition Voltage**:
- The decomposition voltage is the minimum voltage required to decompose an ionic compound. MgCl₂ has a lower decomposition voltage compared to KCl. This means that MgCl₂ can be decomposed more easily during electrolysis.

**Hint**: Understand the concept of decomposition voltage and its implications.

5. **Electrolysis Reactions**:
- At the cathode, the reaction is:
\[ Mg^{2+} + 2e^- \rightarrow Mg \]
- At the anode, the reaction is:
\[ 2Cl^- \rightarrow Cl_2 + 2e^- \]

**Hint**: Write down the half-reactions occurring at each electrode.

6. **Conclusion**:
- Since Mg²⁺ is preferentially discharged over K⁺ due to its lower decomposition voltage, MgCl₂ is decomposed during the electrolysis of carnallite instead of KCl.

**Hint**: Summarize the reasons why MgCl₂ is decomposed preferentially.

### Final Answer:
The reason MgCl₂ is decomposed instead of KCl during the electrolysis of carnallite is that MgCl₂ has a lower decomposition voltage than KCl, allowing Mg²⁺ to be preferentially discharged at the cathode.