In which reaction, there is change in oxidation number of N atom?

Options

A. `2NO_(2) iff N_(2)O_(4)`
B. `2NO_(2)+H_(2)O rarr HNO_(3)+HNO_(2)`
C. `NH_(4)OH rarr NH_(4)^(+)+OH^(-)`
D. `N_(2)O_(5)+H_(2)O rarr 2HNO_(3)`

Answer

To determine in which reaction there is a change in the oxidation number of the nitrogen atom, we will analyze the oxidation states of nitrogen in the given reactions.

### Step-by-Step Solution

1. **Identify the Reactions**: We need to analyze the oxidation states of nitrogen in the provided reactions. The reactions we will consider are:
- A: \(2NO_2 \rightarrow N_2O_4\)
- B: \(NO_2 \rightarrow HNO_3\)
- C: \(NH_3 \rightarrow NH_4^+\)
- D: \(N_2O_5 \rightarrow HNO_3\)

2. **Calculate Oxidation State in Reaction A**:
- For \(NO_2\): Let the oxidation state of nitrogen be \(x\).
\[ x + 2(-2) = 0 \implies x - 4 = 0 \implies x = +4 \]
- For \(N_2O_4\):
\[ 2x + 4(-2) = 0 \implies 2x - 8 = 0 \implies 2x = 8 \implies x = +4 \]
- **Conclusion**: Oxidation state of nitrogen remains +4 in both \(NO_2\) and \(N_2O_4\). No change.

3. **Calculate Oxidation State in Reaction B**:
- For \(NO_2\):
\[ x + 2(-2) = 0 \implies x - 4 = 0 \implies x = +4 \]
- For \(HNO_3\):
\[ x + 1 + 3(-2) = 0 \implies x + 1 - 6 = 0 \implies x - 5 = 0 \implies x = +5 \]
- **Conclusion**: Oxidation state of nitrogen changes from +4 in \(NO_2\) to +5 in \(HNO_3\). There is a change.

4. **Calculate Oxidation State in Reaction C**:
- For \(NH_3\):
\[ x + 3(1) = 0 \implies x + 3 = 0 \implies x = -3 \]
- For \(NH_4^+\):
\[ x + 4(1) = +1 \implies x + 4 = 1 \implies x = -3 \]
- **Conclusion**: Oxidation state of nitrogen remains -3 in both \(NH_3\) and \(NH_4^+\). No change.

5. **Calculate Oxidation State in Reaction D**:
- For \(N_2O_5\):
\[ 2x + 5(-2) = 0 \implies 2x - 10 = 0 \implies 2x = 10 \implies x = +5 \]
- For \(HNO_3\):
\[ x + 1 + 3(-2) = 0 \implies x + 1 - 6 = 0 \implies x - 5 = 0 \implies x = +5 \]
- **Conclusion**: Oxidation state of nitrogen remains +5 in both \(N_2O_5\) and \(HNO_3\). No change.

6. **Final Conclusion**: The only reaction where the oxidation number of nitrogen changes is in Reaction B, where it goes from +4 in \(NO_2\) to +5 in \(HNO_3\).

### Answer
The reaction in which there is a change in the oxidation number of the nitrogen atom is:
**B: \(NO_2 \rightarrow HNO_3\)**.