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 an exothermic reaction, we need to analyze each option based on the heat change associated with the reaction. An exothermic reaction is characterized by the release of heat, which is indicated by a negative value for the enthalpy change (ΔH).

### Step-by-Step Solution:

1. **Understand Exothermic and Endothermic Reactions:**
- Exothermic reactions release heat to the surroundings, resulting in a negative enthalpy change (ΔH < 0).
- Endothermic reactions absorb heat from the surroundings, resulting in a positive enthalpy change (ΔH > 0).

2. **Analyze Option 1:**
- Reaction: Reactant + O2 → 2NO
- Given: The enthalpy change is positive (ΔH > 0).
- Conclusion: This is an endothermic reaction. **(Incorrect)**

3. **Analyze Option 2:**
- Reaction: NO2 + H2 → 2NH3
- Given: The enthalpy change is negative (ΔH = -90 kJ).
- Conclusion: This is an exothermic reaction. **(Correct)**

4. **Analyze Option 3:**
- Reaction: C + H2O → CO + H2
- Given: The enthalpy change is negative (ΔH < 0).
- Conclusion: This is also an exothermic reaction. **(Correct)**

5. **Analyze Option 4:**
- Reaction: Carbon graphite + Sulfur → CH2
- Given: The enthalpy change is not specified, but if it were exothermic, it would typically be indicated by a negative value.
- Conclusion: Without specific information, we cannot definitively classify this reaction.

### Final Answer:
- The exothermic reactions from the options provided are **Option 2 (NO2 + H2 → 2NH3)** and **Option 3 (C + H2O → CO + H2)**.