Which of these reaction is correct?

Options

A. `Cl^(-)+Br_(2) to Br^(-)+Cl_(2)`
B. Mohr's salt `overset("NaOH solution")toNH_(3)uarr(g)`
C. `K_(2)Cr_(2)O_(7)` solution`overset(SO_(3))to`Green colour solution
D. `FeCl_(2) overset(NaOH) to ` (ppt. coloured)`overset("Excess NaOH")to`Soluble complex

Explanation

To determine which of the given reactions is correct, we will analyze each option step by step.

### Step 1: Analyze Reaction A
**Reaction A:** Cl⁻ + Br₂ → Br⁻ + Cl₂

- This reaction suggests that chloride ions (Cl⁻) are oxidizing bromine (Br₂) to bromide ions (Br⁻) while being oxidized to chlorine gas (Cl₂).
- To assess feasibility, we need to check the standard cell potential (E°cell) for this reaction.
- The half-reaction for Cl⁻ to Cl₂ has a standard reduction potential of -1.36 V, while the half-reaction for Br₂ to Br⁻ has a standard reduction potential of +1.07 V.
- The overall cell potential (E°cell) can be calculated, and it turns out to be negative, indicating that this reaction is not feasible.

**Conclusion:** Reaction A is not correct.

### Step 2: Analyze Reaction B
**Reaction B:** Mohr's salt (NH₄)₂Fe(SO₄)₂·6H₂O with NaOH gives ammonia gas.

- Mohr's salt, when treated with sodium hydroxide (NaOH), will produce ammonia (NH₃) gas.
- The reaction can be represented as:
\[ NH₄^+ + OH^- → NH₃ + H₂O \]
- This is a well-known reaction where ammonium ions are converted to ammonia gas in the presence of a strong base.

**Conclusion:** Reaction B is correct.

### Step 3: Analyze Reaction C
**Reaction C:** K₂Cr₂O₇ with SO₃ gives a green color solution.

- K₂Cr₂O₇ (potassium dichromate) is a strong oxidizing agent.
- The reaction with SO₃ (sulfur trioxide) does not occur because SO₃ is already in a high oxidation state (+6) and cannot be oxidized further.
- Therefore, this reaction is not feasible.

**Conclusion:** Reaction C is not correct.

### Step 4: Analyze Reaction D
**Reaction D:** FeCl₂ with NaOH gives a colored precipitate and with excess NaOH gives a soluble complex.

- The reaction of FeCl₂ with NaOH produces ferrous hydroxide (Fe(OH)₂), which is a green precipitate:
\[ FeCl₂ + 2 NaOH → Fe(OH)₂ + 2 NaCl \]
- However, when excess NaOH is added, Fe(OH)₂ does not dissolve to form a soluble complex; instead, it remains as a precipitate.
- Therefore, this reaction is also not feasible as described.

**Conclusion:** Reaction D is not correct.

### Final Conclusion
The only correct reaction from the options provided is **Option B**.

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