In which of the following reactions conc. `H_(2)SO_(4)` is used as an oxidising reagent?

Options

A. `CaF_(2)+H_(2)SO_(4)toCaSO_(4)+2HF`
B. `2HI+H_(2)SO_(4)toI_(2)+SO_(2)+2H_(2)O`
C. `Cu+2H_(2)SO_(4)toCuSO_(4)+SO_(2)+2H_(2)O`
D. `NaCl+H_(2)SO_(4)toNaHSO_(4)+HCl`

Explanation

To determine in which of the given reactions concentrated \( H_2SO_4 \) is used as an oxidizing reagent, we need to analyze each reaction to see if \( H_2SO_4 \) is oxidizing another species. An oxidizing agent is a substance that gains electrons (is reduced) while causing another substance to lose electrons (is oxidized).

### Step-by-Step Solution:

1. **Identify the Reactions**: We are given four reactions. We need to analyze each one to see if \( H_2SO_4 \) acts as an oxidizing agent.

2. **Understand Oxidation and Reduction**:
- **Oxidation**: Loss of electrons or an increase in oxidation state.
- **Reduction**: Gain of electrons or a decrease in oxidation state.

3. **Analyze Each Reaction**:
- **Reaction 1**: Calcium fluoride \( (CaF_2) \) with \( H_2SO_4 \).
- Calcium \( (Ca) \) is \( +2 \) and Fluorine \( (F) \) is \( -1 \) in both reactants and products. No change in oxidation states means no oxidation occurs. Therefore, \( H_2SO_4 \) is not acting as an oxidizing agent here.

- **Reaction 2**: Hydroiodic acid \( (HI) \) with \( H_2SO_4 \).
- In \( HI \), Hydrogen is \( +1 \) and Iodine is \( -1 \). In the product \( I_2 \), Iodine is \( 0 \). Here, Iodine goes from \( -1 \) to \( 0 \) (loss of electrons), indicating oxidation. Thus, \( H_2SO_4 \) is acting as an oxidizing agent.

- **Reaction 3**: Copper \( (Cu) \) with \( H_2SO_4 \).
- Copper in its elemental form is \( 0 \) and in \( CuSO_4 \), it is \( +2 \). The oxidation state of Copper increases from \( 0 \) to \( +2 \), indicating oxidation. Therefore, \( H_2SO_4 \) is acting as an oxidizing agent in this reaction as well.

- **Reaction 4**: Sodium chloride \( (NaCl) \) with \( H_2SO_4 \).
- Sodium is \( +1 \) and Chlorine is \( -1 \) in both reactants and products. No change in oxidation states means no oxidation occurs. Thus, \( H_2SO_4 \) is not acting as an oxidizing agent here.

4. **Conclusion**: The reactions in which \( H_2SO_4 \) acts as an oxidizing agent are:
- Reaction 2: \( HI \) to \( I_2 \)
- Reaction 3: \( Cu \) to \( Cu^{2+} \)

### Final Answer:
Concentrated \( H_2SO_4 \) is used as an oxidizing reagent in reactions 2 and 3.

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