Which one of the following is an exothermic reaction?

Options

A. `N_(2)(g) _ O_(2)(g) + 180.8 kJ to 2NO(g)`
B. `N_(2)(g) + 3H_(2)(g) - 90 kJ to 2 NH_(3)(g)`
C. `C(g) + H_(2)O(g) to CO(g) + H_(2)(g) - 131.1 kJ`
D. `C("graphite") + 2S(s) to CS_(2) (l) + 91.9 kJ`

Explanation

To determine which of the given reactions is exothermic, we need to analyze the heat changes associated with each reaction. An exothermic reaction is characterized by the release of heat to the surroundings, while an endothermic reaction requires heat input.

### Step-by-Step Solution:

1. **Identify the Reactions**:
We have four reactions to analyze. Let's denote them as A, B, C, and D.

2. **Analyze Reaction A**:
- Reaction A: N2 + O2 + heat → 2NO
- Here, heat is required for the reaction to proceed, indicating that this reaction absorbs heat.
- **Conclusion**: Reaction A is endothermic.

3. **Analyze Reaction B**:
- Reaction B: N2 + 3H2(g) → 2NH3(g) + 90 kJ
- The presence of "+ 90 kJ" indicates that 90 kJ of heat is released as a product of the reaction.
- **Conclusion**: Reaction B is exothermic.

4. **Analyze Reaction C**:
- Reaction C: (heat) + reactants → products
- Since heat is required for the reaction to occur, it indicates that this reaction absorbs heat.
- **Conclusion**: Reaction C is endothermic.

5. **Analyze Reaction D**:
- Reaction D: C + 2S → CS2 + 91.9 kJ
- Similar to Reaction B, the "+ 91.9 kJ" indicates that this reaction releases heat.
- **Conclusion**: Reaction D is exothermic.

6. **Final Conclusion**:
- From our analysis, we find that both Reaction B and Reaction D are exothermic reactions.

### Answer:
The exothermic reactions are:
- Option B: N2 + 3H2(g) → 2NH3(g) + 90 kJ
- Option D: C + 2S → CS2 + 91.9 kJ

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