Which of the following reactions is reversible?

Options

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

Answer

To determine which of the given reactions is reversible, we need to analyze each reaction based on the characteristics of reversible and irreversible reactions.

### Step-by-Step Solution:

1. **Understand Reversible and Irreversible Reactions:**
- A reversible reaction can proceed in both forward and backward directions, represented by a double arrow (⇌). This means that reactants can form products, and products can revert back to reactants.
- An irreversible reaction only proceeds in one direction, represented by a single arrow (→). This means that once the reactants form products, they cannot revert back to the original reactants.

2. **Analyze the Given Reactions:**
- **Reaction 1:** Hydrogen (H₂) + Iodine (I₂) ⇌ Hydrogen Iodide (HI)
- This reaction is reversible because both reactants (H₂ and I₂) can combine to form products (HI), and HI can decompose back into H₂ and I₂.

- **Reaction 2:** Barium Hydroxide (Ba(OH)₂) + Sulfuric Acid (H₂SO₄) → Barium Sulfate (BaSO₄) + Water (H₂O)
- This reaction is irreversible because it produces a precipitate (BaSO₄) and cannot revert back to the original reactants.

- **Reaction 3:** Sodium Chloride (NaCl) + Silver Nitrate (AgNO₃) → Silver Chloride (AgCl) + Sodium Nitrate (NaNO₃)
- This reaction is also irreversible because it produces a precipitate (AgCl) and does not revert back to the original reactants.

- **Reaction 4:** Iron (Fe) + Sulfur (S) → Iron Sulfide (FeS)
- This reaction is irreversible because once iron and sulfur react to form iron sulfide, they do not revert back to iron and sulfur.

3. **Conclusion:**
- Among the four reactions analyzed, the only reversible reaction is:
- **Hydrogen + Iodine ⇌ Hydrogen Iodide**

### Final Answer:
The reversible reaction is **Hydrogen + Iodine → Hydrogen Iodide**.

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