Which one of the following chemical equations represents an exothermic reaction?

• A. CaCO_3 + ext{energy} ightarrow CaO + CO_2
• B. 2H_2 + O_2 ightarrow 2H_2O + ext{energy}
• C. N_2 + O_2 + ext{energy} ightarrow 2NO
• D. 6CO_2 + 6H_2O + ext{energy} ightarrow C_6H_{12}O_6 + 6O_2

Answer: (B)

Verified step by step guidance

1. Understand the difference between exothermic and endothermic reactions: Exothermic reactions release energy to the surroundings, often shown by energy being a product in the chemical equation, while endothermic reactions absorb energy, shown by energy as a reactant.
2. Examine each given chemical equation and identify where the energy term appears. If energy is on the product side, the reaction is exothermic; if energy is on the reactant side, the reaction is endothermic.
3. Look at the first equation: \(\mathrm{CaCO_3 + \text{energy} \rightarrow CaO + CO_2}\). Since energy is a reactant, this is an endothermic reaction.
4. Check the second equation: \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O + \text{energy}}\). Energy is a product here, indicating that this reaction releases energy and is exothermic.
5. Verify the remaining equations to confirm they have energy as a reactant, meaning they are endothermic, and conclude that the second equation represents the exothermic reaction.