All of the following reactions are exothermic EXCEPT:
Correct answer: C. N₂(g) + O₂(g) → 2NO(g)
- A. C(s) + O₂(g) → CO₂(g)
- B. 2H₂(g) + O₂(g) → 2H₂O(l)
- C. N₂(g) + O₂(g) → 2NO(g)
- D. CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Explanation
The formation of nitric oxide (NO) from nitrogen and oxygen is a highly endothermic reaction, meaning it requires a significant input of energy to proceed. This is why it typically only occurs at very high temperatures, such as in lightning strikes or internal combustion engines. All other options are exothermic.
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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