Which of the following metals cannot be obtained by reduction of its metal oxide by aluminium?

Options

A. Cr
B. Mn
C. Fe
D. Mg

Explanation

To determine which of the following metals cannot be obtained by the reduction of its metal oxide by aluminium, we will analyze each option step by step.

### Step 1: Analyze Chromium
- **Metal Oxide**: Chromium oxide (Cr2O3)
- **Reaction**: 2 Cr2O3 + 4 Al → 4 Cr + 2 Al2O3
- **Conclusion**: Chromium can be obtained by reduction with aluminium.

**Hint**: Check the reactivity of chromium and its oxide with aluminium.

### Step 2: Analyze Manganese
- **Metal Oxide**: Manganese oxide (MnO2)
- **Reaction**: 3 MnO2 + 4 Al → 3 Mn + 2 Al2O3
- **Conclusion**: Manganese can also be obtained by reduction with aluminium.

**Hint**: Look for the balanced reaction to confirm the reduction process.

### Step 3: Analyze Iron
- **Metal Oxide**: Iron oxide (Fe2O3)
- **Reaction**: Fe2O3 + 2 Al → 2 Fe + Al2O3
- **Conclusion**: Iron can be obtained by reduction with aluminium.

**Hint**: Consider the thermodynamic feasibility of the reaction.

### Step 4: Analyze Magnesium
- **Metal Oxide**: Magnesium oxide (MgO)
- **Reaction**: MgO + Al → No reaction (Mg is more reactive than Al)
- **Conclusion**: Magnesium cannot be obtained by reduction with aluminium.

**Hint**: Remember that more reactive metals cannot be reduced by less reactive metals.

### Final Conclusion
The metal that cannot be obtained by reduction of its metal oxide by aluminium is **Magnesium (Option 4)**.