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

• A. N_2(g) + 3H_2(g) ightarrow 2NH_3(g) + heat
• B. 2H_2(g) + O_2(g) ightarrow 2H_2O(l) + heat
• C. CH_4(g) + 2O_2(g) ightarrow CO_2(g) + 2H_2O(g) + heat
• D. CaCO_3(s) + heat ightarrow CaO(s) + CO_2(g)

Answer: (D)

Verified step by step guidance

1. Understand the difference between exothermic and endothermic reactions: Exothermic reactions release heat to the surroundings, while endothermic reactions absorb heat from the surroundings.
2. Look at the chemical equations and identify where heat appears in the reaction: If heat is written on the product side, the reaction releases heat (exothermic). If heat is written on the reactant side, the reaction absorbs heat (endothermic).
3. Analyze the given options: The first three reactions have '+ heat' on the product side, indicating they release heat and are exothermic.
4. Examine the last reaction: 'CaCO_3(s) + heat \rightarrow CaO(s) + CO_2(g)' has heat on the reactant side, meaning heat is absorbed to drive the reaction forward, which characterizes an endothermic process.
5. Conclude that the reaction with heat as a reactant represents an endothermic reaction because it requires energy input to proceed.