In the prepartion of `HBr` or `HI, NaX (X=Br,I)` is treated with `H_(3)PO_(4)` and not by concentrated `H_(2)SO_(4)` since,

Options

A. `H_(2)SO_(4)` makes the reaction reversible
B. `H_(2)SO_(4)` oxidises `HX` to `X_(2)(Br_(2),I_(2))`
C. `Na_(2)SO_(4)` is water soluble and `Na_(3)PO_(4)` is water insoluble
D. `Na_(3)PO_(4)` is water insoluble and `Na_(2)SO_(4)` is water soluble

Answer

To understand why `H3PO4` is used instead of concentrated `H2SO4` in the preparation of `HBr` or `HI` from `NaX` (where `X` is `Br` or `I`), we can break down the reasoning into the following steps:

### Step 1: Identify the Reactants
We start with sodium halides, `NaX`, where `X` can be either `Br` or `I`. The two acids we are considering are `H3PO4` (phosphoric acid) and `H2SO4` (sulfuric acid).

### Step 2: Reaction with `H3PO4`
When `NaX` reacts with `H3PO4`, the reaction can be represented as:
\[ \text{NaX} + \text{H}_3\text{PO}_4 \rightarrow \text{HX} + \text{Na}_3\text{PO}_4 \]

In this reaction, `HX` (either `HBr` or `HI`) is formed along with sodium phosphate (`Na3PO4`).

### Step 3: Properties of `H3PO4`
`H3PO4` is a weak acid and a weak oxidizing agent. This means that it does not have the ability to further oxidize the `HX` produced in the reaction. Therefore, the `HX` remains stable and is not converted into any further products.

### Step 4: Reaction with Concentrated `H2SO4`
Now, if we consider the reaction of `NaX` with concentrated `H2SO4`, the reaction can be written as:
\[ \text{NaX} + \text{H}_2\text{SO}_4 \rightarrow \text{HX} + \text{Na}_2\text{SO}_4 \]

However, in this case, concentrated `H2SO4` is a strong acid and a strong oxidizing agent. This means that after the formation of `HX`, the strong oxidizing nature of `H2SO4` can lead to the oxidation of `HX` to produce halogen gas (`X2`), which is not desired.

### Step 5: Conclusion
The key reason for using `H3PO4` instead of `H2SO4` is that `H3PO4` does not oxidize `HX` to `X2`, allowing for the stable production of `HBr` or `HI`. In contrast, `H2SO4` would oxidize `HX` to `X2`, leading to the loss of the desired hydrogen halide product.

### Final Answer
In the preparation of `HBr` or `HI`, `NaX` is treated with `H3PO4` instead of concentrated `H2SO4` because `H3PO4` is a weak acid and does not oxidize `HX` to `X2`, while concentrated `H2SO4` is a strong oxidizing agent that would oxidize `HX` to `X2`.

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