Which amongst the following reactions cannot be used for the respective preparation?

Options

A. `2KBr+H_(2)SO_(4)(conc.)to K_(2)SO_(4)+2Br`
B. `NaCl+H_(2)SO_(4)(conc.)to NaHSO_(4)+HCl`
C. `NaHSO_(4)+NaCl to Na_(2)SO_(4)+HCl`
D. `CaF_(2)+H_(2)SO_(4)to CaSO_(4)+2HF`

Answer

To solve the question of which reaction cannot be used for the respective preparation, we will analyze the given options step by step.

### Step 1: Identify the reactions
We need to examine the reactions provided in the options (A, B, C, D) to determine which one cannot be used for the respective preparation.

### Step 2: Analyze Option A
- The reaction given is:
\[ 2KBr + H_2SO_4 \rightarrow K_2SO_4 + 2HBr \]
- Here, we are trying to prepare HBr using concentrated H2SO4 and KBr.
- However, concentrated H2SO4 is a strong oxidizing agent and can oxidize HBr to Br2, which means that this reaction cannot proceed as written.

### Step 3: Conclusion for Option A
- Since concentrated H2SO4 can oxidize HBr to Br2, the preparation of HBr from KBr and concentrated H2SO4 is not feasible. Therefore, Option A is the reaction that cannot be used for the respective preparation.

### Step 4: Analyze Other Options (B, C, D)
- Without the specific details of options B, C, and D, we can generally state that:
- Reactions involving NaCl and Cl2 with concentrated H2SO4 are valid.
- Other halides like HI can also be prepared using different methods, but they may not involve concentrated H2SO4 due to similar oxidation issues.

### Final Answer
Thus, the reaction that cannot be used for the respective preparation is **Option A**.

---