Which of the following reactions is reversible?

Options

A. `H_(2)(g)+I_(2)(g) to 2HI (g)`
B. `Ba(OH)_(2)(I)+H_(2)SO_(4)(I) to BaSO_(4)(s)+2H_(2)O(I)`
C. `NaCl(I)+AgNO_(3)(I) to AgCl(s) +NaNO_(3)(I)`
D. `Fe(s)+S(s) to FeS(s)`

Answer

To determine which of the following reactions is reversible, we need to analyze the nature of the reactions provided. A reversible reaction is one where the products can revert back to the reactants under certain conditions.

### Step-by-Step Solution:

1. **Identify the Reactions**:
- We need to look at the given reactions and identify their states (solid, liquid, gas) and the nature of the products formed.

2. **Analyze Reaction 1**:
- Consider the reaction: H2(g) + I2(g) ⇌ 2 HI(g).
- All reactants and products are in the gaseous state.
- The reaction can proceed in both directions: H2 and I2 can combine to form HI, and HI can decompose back into H2 and I2.
- **Conclusion**: This reaction is reversible.

3. **Analyze Reaction 2**:
- Consider the reaction: A(l) + B(aq) → C(s) + D(l).
- Here, a solid precipitate (C) is formed from liquid and aqueous reactants.
- The formation of a solid from liquids typically does not allow for easy reversion without the input of energy.
- **Conclusion**: This reaction is not reversible under normal conditions.

4. **Analyze Reaction 3**:
- Consider the reaction: E(aq) + F(aq) → G(s).
- Similar to the previous reaction, a solid is formed from aqueous reactants.
- The reverse reaction would require significant energy input to break down the solid back into its aqueous components.
- **Conclusion**: This reaction is not reversible.

5. **Analyze Reaction 4**:
- Consider the reaction: H(s) + I(s) → J(s).
- All reactants and products are solids.
- The formation of products from solids typically does not allow for easy reversion without additional energy.
- **Conclusion**: This reaction is not reversible.

### Final Conclusion:
- The only reversible reaction among the options provided is **Reaction 1**: H2(g) + I2(g) ⇌ 2 HI(g).