The following flow diagram represent the industrial preparation of nitric acid from ammonia:
`NH_(3)+underset(("excess air"))(O_(2)) underset(900^(@)C)overset((X))NO underset(air)overset((Y))to(Z)overset("water")toHNO_(3)+NO`
Q. When (Z) is dissolved in `H_(2)O` then formation of `HNO_(3)` takes place through various reactions. select the reaction not observed in this step:

Options

A. `NO_(2)+H_(2)O toHNO_(3)+HNO_(2)`
B. `HNO_(2) to H_(2)O+NO+NO_(2)`
C. `NO_(2)+H_(2)O to HNO_(3)+NO`
D. none of these

Answer

To determine the reaction that is not observed in the hydrolysis of compound Z (which is NO2) to form nitric acid (HNO3), we can analyze the steps involved in the Ostwald process for the industrial preparation of nitric acid from ammonia.

### Step-by-Step Solution:

1. **Identify the Initial Reaction**:
- Ammonia (NH3) reacts with oxygen (O2) in excess air at high temperatures (900°C) to produce nitrogen monoxide (NO).
- **Reaction**:
\[ 4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O \]

2. **Oxidation of NO**:
- The nitrogen monoxide (NO) is then oxidized in the presence of air to form nitrogen dioxide (NO2).
- **Reaction**:
\[ 2NO + O_2 \rightarrow 2NO_2 \]

3. **Identify Compound Z**:
- From the above step, we identify that compound Z is nitrogen dioxide (NO2).

4. **Hydrolysis of NO2**:
- When NO2 is dissolved in water (H2O), it undergoes hydrolysis to form nitric acid (HNO3) and nitrous acid (HNO2).
- **Reaction**:
\[ 3NO_2 + H_2O \rightarrow 2HNO_3 + HNO_2 \]

5. **Decomposition of Nitrous Acid**:
- The nitrous acid (HNO2) can further decompose into water, nitrogen dioxide, and nitric oxide.
- **Reaction**:
\[ 2HNO_2 \rightarrow H_2O + 2NO + O_2 \]

6. **Final Products**:
- The final products from the hydrolysis of NO2 are nitric acid (HNO3) and nitrogen monoxide (NO).
- **Overall Reaction**:
\[ NO_2 + H_2O \rightarrow HNO_3 + NO \]

### Conclusion:
From the analysis, the reactions involved in the hydrolysis of NO2 include:
- The formation of HNO3 and HNO2 from NO2 and H2O.
- The decomposition of HNO2.

To answer the question, we need to identify which reaction is not observed in this step. The reactions that are typically observed in the hydrolysis of NO2 to form HNO3 are the ones mentioned above. Therefore, any reaction that does not involve the formation of HNO3 or HNO2 during the hydrolysis of NO2 would be the one not observed.