Presence of which reagent will affect the reversibility of the following reaction, and change it to a irreversible reaction :
`CH_(4) + I_(2) underset("Reversible")overset(h v)hArr CH_(3) - I + HI`

Options

A. HOCl
B. dilute `HNO_(2)`
C. Liquid `HNO_(2)`
D. Concentrated `HNO_(3)`

Answer

To determine which reagent will affect the reversibility of the reaction:

\[ CH_4 + I_2 \underset{\text{Reversible}}{\overset{h \nu}{\rightleftharpoons}} CH_3I + HI \]

and change it to an irreversible reaction, we need to analyze the components involved in the reaction.

### Step-by-Step Solution:

1. **Identify the Reaction Components**:
The reaction involves methane (\(CH_4\)) reacting with iodine (\(I_2\)) to produce methyl iodide (\(CH_3I\)) and hydrogen iodide (\(HI\)).

2. **Understand the Nature of the Reaction**:
The reaction is reversible, meaning that the products can react to form the reactants again. This equilibrium can be shifted by changing the conditions or adding certain reagents.

3. **Determine the Role of HI**:
In this reaction, \(HI\) acts as a reducing agent. If we can remove \(HI\) from the reaction, it will drive the equilibrium towards the products, making the reaction irreversible.

4. **Identify Suitable Reagents**:
To make the reaction irreversible, we need an oxidizing agent that can react with \(HI\) and remove it from the equilibrium. Common oxidizing agents include concentrated \(HIO_3\) and concentrated \(HNO_3\).

5. **Reactions of Oxidizing Agents**:
- **Concentrated \(HIO_3\)**: This oxidizing agent can react with \(HI\) to produce iodine (\(I_2\)) and water, effectively removing \(HI\) from the system and shifting the equilibrium towards the products.
- **Concentrated \(HNO_3\)**: Similarly, concentrated \(HNO_3\) can react with \(HI\) to produce iodine (\(I_2\)), nitrogen oxides, and water, also removing \(HI\) from the reaction.

6. **Conclusion**:
Since both concentrated \(HIO_3\) and concentrated \(HNO_3\) can make the reaction irreversible by removing \(HI\), we can conclude that the presence of either of these reagents will affect the reversibility of the reaction.

7. **Select the Correct Option**:
From the options provided, if concentrated \(HNO_3\) is listed, it will be the correct answer.

### Final Answer:
**The presence of concentrated \(HNO_3\) will affect the reversibility of the reaction and change it to an irreversible reaction.**

---