Which of the following represents an exothermic reaction :-

Options

A. `N_(2)(g)+O_(2)(g)rarr 2NO(g), Delta H=180.5 KJ`
B. `H_(2)O(g)+C(s)rarr CO(g)+H_(2)(g), Delta E=131.2KJ`
C. `2HgO(s)+180.4 KJ rarr 2Hg(l)+O_(2)(g)`
D. `2Zn(s)+O_(2)(g)rarr 2ZnO(s), Delta E = -693.8 KJ`

Answer

To determine which of the given reactions represents an exothermic reaction, we need to analyze the changes in enthalpy (ΔH) or internal energy (ΔU) for each reaction. An exothermic reaction is characterized by the release of heat, which is indicated by a negative change in enthalpy or internal energy.

### Step-by-Step Solution:

1. **Analyze the First Reaction:**
- Reaction: \( N_2 + O_2 \rightarrow 2NO \)
- Change in Enthalpy (ΔH): Positive
- Conclusion: Since ΔH is positive, this reaction is endothermic (heat is absorbed).

2. **Analyze the Second Reaction:**
- Reaction: \( H_2O + C \rightarrow CO + H_2 \)
- Change in Internal Energy (ΔU): Positive
- Conclusion: A positive ΔU indicates that this reaction is also endothermic (heat is absorbed).

3. **Analyze the Third Reaction:**
- Reaction: Heat is added (not specified, but implied).
- Conclusion: The addition of heat indicates that this reaction is endothermic.

4. **Analyze the Fourth Reaction:**
- Reaction: \( Zn + O_2 \rightarrow ZnO \)
- Change in Internal Energy (ΔU): Negative
- Conclusion: A negative ΔU indicates that heat is released during the reaction, thus this reaction is exothermic.

### Final Answer:
The reaction that represents an exothermic reaction is the fourth one: \( Zn + O_2 \rightarrow ZnO \).