Which of following acid can not be prepared by using conc. `H_(2) SO_(4)` from its corresponding salt ?

Options

A. HF
B. HBr
C. `H_(2)S`
D. HI

Answer

To determine which acid cannot be prepared using concentrated sulfuric acid (H₂SO₄) from its corresponding salt, we need to analyze the behavior of concentrated H₂SO₄ as an oxidizing agent and the nature of the acids formed from their salts.

### Step-by-Step Solution:

1. **Understanding the Reactivity of Concentrated H₂SO₄**:
- Concentrated sulfuric acid is a strong oxidizing agent. This means that it can oxidize certain substances while itself being reduced.

2. **Identifying the Acids and Their Corresponding Salts**:
- Let's consider the acids mentioned in the options: HBr (from NaBr), HI (from NaI), and H₂S (from Na₂S).
- The salts are NaBr, NaI, and Na₂S respectively.

3. **Reactions with Concentrated H₂SO₄**:
- When NaBr is treated with concentrated H₂SO₄, the initial reaction produces HBr:
\[ \text{NaBr} + \text{H}_2\text{SO}_4 \rightarrow \text{NaHSO}_4 + \text{HBr} \]
- However, HBr is a good reducing agent and can react with H₂SO₄:
\[ 2 \text{HBr} + \text{H}_2\text{SO}_4 \rightarrow \text{Br}_2 + \text{SO}_2 + 2 \text{H}_2\text{O} \]
- This means that instead of forming HBr, it gets oxidized to Br₂.

4. **Similar Reactions for HI and H₂S**:
- For NaI:
\[ \text{NaI} + \text{H}_2\text{SO}_4 \rightarrow \text{NaHSO}_4 + \text{HI} \]
- Again, HI is a strong reducing agent and will react with H₂SO₄:
\[ 2 \text{HI} + \text{H}_2\text{SO}_4 \rightarrow \text{I}_2 + \text{SO}_2 + 2 \text{H}_2\text{O} \]
- For Na₂S:
\[ \text{Na}_2\text{S} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{HSO}_4 + \text{H}_2\text{S} \]
- H₂S can also be oxidized:
\[ \text{H}_2\text{S} + \text{H}_2\text{SO}_4 \rightarrow \text{S} + \text{SO}_2 + 2 \text{H}_2\text{O} \]

5. **Conclusion**:
- Since all three acids (HBr, HI, and H₂S) are reducing agents, they cannot be formed from their corresponding salts using concentrated H₂SO₄ because they will be oxidized instead.

Thus, the answer is that none of the acids (HBr, HI, H₂S) can be prepared from their corresponding salts using concentrated sulfuric acid.