Which reaction is not used in the preparation of `H_(2)`?

Options

A. `Zn+NaOHto`
B. `Mg+NaOHto`
C. `Al+NaOHto`
D. `Be+NaOHto`

Explanation

To determine which reaction is not used in the preparation of hydrogen gas (H₂), we will analyze the reactions of various metals with sodium hydroxide (NaOH) and see if they produce hydrogen gas.

### Step-by-Step Solution:

1. **Identify the Metals and Reactions**: We will consider the reactions of the following metals with sodium hydroxide:
- Zinc (Zn)
- Magnesium (Mg)
- Aluminium (Al)
- Beryllium (Be)

2. **Reaction of Zinc with NaOH**:
- The reaction is:
\[ \text{Zn} + 2 \text{NaOH} + 2 \text{H}_2\text{O} \rightarrow \text{Na}_2\text{Zn(OH)}_4 + \text{H}_2 \]
- This reaction produces hydrogen gas (H₂). Therefore, zinc is a valid method for hydrogen preparation.

3. **Reaction of Magnesium with NaOH**:
- The reaction is:
\[ \text{Mg} + 2 \text{NaOH} + 2 \text{H}_2\text{O} \rightarrow \text{Na}_2\text{Mg(OH)}_4 + \text{H}_2 \]
- However, magnesium does not react with NaOH to produce hydrogen gas. Thus, this reaction does not yield H₂.

4. **Reaction of Aluminium with NaOH**:
- The reaction is:
\[ 2 \text{Al} + 2 \text{NaOH} + 6 \text{H}_2\text{O} \rightarrow 2 \text{Na}_3\text{Al(OH)}_6 + 3 \text{H}_2 \]
- This reaction produces hydrogen gas (H₂). Therefore, aluminium is also a valid method for hydrogen preparation.

5. **Reaction of Beryllium with NaOH**:
- The reaction is:
\[ \text{Be} + 2 \text{NaOH} \rightarrow \text{Na}_2\text{BeO}_2 + \text{H}_2 \]
- This reaction also produces hydrogen gas (H₂). Thus, beryllium is a valid method for hydrogen preparation.

6. **Conclusion**:
- Among the metals considered, magnesium does not produce hydrogen gas when reacted with sodium hydroxide. Therefore, the reaction of magnesium with NaOH is not used in the preparation of H₂.

### Final Answer:
The reaction that is not used in the preparation of H₂ is the reaction of **Magnesium with NaOH**.

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