Which reactions are used for the preparation of the halogen acid?

Options

A. `2KBr+ underset((conc.))(H_(2)SO_(4)) to K_(2)SO_(4)+2HBr`
B. `CaF_(2)+underset((conc.))(H_(2)SO_(4)) to CaSO_(4)+2HF`
C. `NaCl+underset((conc.))(H_(2)SO_(4)) to NaHSO_(4)+HCl`
D. `2KI+underset((conc.))(H_(2)SO_(4)) to K_(2)SO_(4)+2HI`

Answer

To prepare halogen acids, we can utilize specific reactions involving ionic salts and concentrated sulfuric acid (H₂SO₄). Below is a step-by-step breakdown of the reactions that can and cannot be used for the preparation of halogen acids.

### Step-by-Step Solution:

1. **Understanding Halogen Acids**:
Halogen acids are binary acids formed from halogens and hydrogen. Common examples include HCl (hydrochloric acid), HBr (hydrobromic acid), and HI (hydroiodic acid).

2. **Reactions with Ionic Salts**:
The typical method to prepare halogen acids is by reacting an ionic salt (like KBr or KI) with concentrated sulfuric acid (H₂SO₄). The general reaction can be represented as:
\[ \text{Salt} + \text{Concentrated H₂SO₄} \rightarrow \text{Halogen Acid} + \text{Other Products} \]

3. **Identifying Concentrated H₂SO₄ as an Oxidizing Agent**:
Concentrated H₂SO₄ is not only a strong acid but also a strong oxidizing agent due to sulfur being in the +6 oxidation state. This property can lead to unwanted oxidation reactions.

4. **Analyzing Specific Reactions**:
- **Reaction with KBr**:
\[ 2 \text{KBr} + \text{H₂SO₄ (conc.)} \rightarrow \text{K₂SO₄} + 2 \text{HBr} \]
Here, Br⁻ can be oxidized to Br₂, which means we won't get the desired HBr but rather bromine gas. Thus, this reaction is not suitable.

- **Reaction with KI**:
\[ 2 \text{KI} + \text{H₂SO₄ (conc.)} \rightarrow \text{K₂SO₄} + 2 \text{HI} \]
Similarly, I⁻ can be oxidized to I₂, leading to the same issue as above. Therefore, this reaction is also unsuitable.

5. **Suitable Reactions**:
- **Reaction with CaF₂**:
\[ \text{CaF₂} + \text{H₂SO₄ (conc.)} \rightarrow \text{CaSO₄} + 2 \text{HF} \]
In this case, fluoride ions (F⁻) are not oxidized, and we successfully obtain HF.

- **Reaction with NaCl**:
\[ \text{NaCl} + \text{H₂SO₄ (conc.)} \rightarrow \text{NaHSO₄} + \text{HCl} \]
Here, chloride ions (Cl⁻) are also not oxidized, allowing us to obtain HCl.

6. **Conclusion**:
The reactions that can be used for the preparation of halogen acids are those involving CaF₂ and NaCl, while those involving KBr and KI with concentrated H₂SO₄ should be avoided due to the oxidation of halide ions.

### Summary of Suitable and Unsuitable Reactions:
- **Unsuitable Reactions**:
- KBr + H₂SO₄ (conc.) → Unwanted oxidation to Br₂
- KI + H₂SO₄ (conc.) → Unwanted oxidation to I₂

- **Suitable Reactions**:
- CaF₂ + H₂SO₄ (conc.) → HF
- NaCl + H₂SO₄ (conc.) → HCl