Ammonia gas is not prepared by :

Options

A. reduction of sodium nitrite or sodium nitrate by the reaction of zinc dust and sodium hydroxide
B. hydrolysis of calcium cyanamide
C. heating ammonium chloride with slaked lime
D. heating of `(NH_4)_2 Cr_2O_7`

Explanation

To determine which reaction does not produce ammonia gas, let's analyze each option step by step:

1. **Reduction of Sodium Nitrite or Sodium Nitrate with Zinc Dust and Sodium Hydroxide:**
- Sodium nitrite (NaNO2) and sodium nitrate (NaNO3) can be reduced by zinc dust in the presence of sodium hydroxide (NaOH).
- The reduction of nitrite (NO2-) and nitrate (NO3-) leads to the formation of ammonia (NH3) and water (H2O).
- **Conclusion:** This reaction can produce ammonia.

2. **Hydrolysis of Calcium Cyanamide:**
- Calcium cyanamide (CaCN2) reacts with water (hydrolysis) to produce calcium carbonate (CaCO3) and ammonia (NH3).
- **Conclusion:** This reaction can also produce ammonia.

3. **Heating Ammonium Chloride with Slaked Lime:**
- When ammonium chloride (NH4Cl) is heated with slaked lime (Ca(OH)2), it decomposes to form ammonia (NH3), calcium chloride (CaCl2), and water (H2O).
- The balanced reaction is:
\[ 2 \text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \rightarrow 2 \text{NH}_3 + \text{CaCl}_2 + 2 \text{H}_2\text{O} \]
- **Conclusion:** This reaction can produce ammonia.

4. **Heating Ammonium Dichromate (NH4)2Cr2O7:**
- When ammonium dichromate is heated, it decomposes into nitrogen gas (N2), chromium(III) oxide (Cr2O3), and water (H2O).
- The balanced reaction is:
\[ 4 \text{(NH}_4)_2\text{Cr}_2\text{O}_7 \rightarrow 2 \text{N}_2 + \text{Cr}_2\text{O}_3 + 4 \text{H}_2\text{O} \]
- **Conclusion:** This reaction does not produce ammonia.

**Final Answer:**
Ammonia gas is not prepared by heating ammonium dichromate \((NH_4)_2Cr_2O_7\).

---