Which of the following reactions is best represented by a diagram showing a decrease in enthalpy (exothermic process)?

• A. 2H_2(g) + O_2(g) → 2H_2O(l)
• B. N_2(g) + O_2(g) → 2NO(g)
• C. CaCO_3(s) → CaO(s) + CO_2(g)
• D. Ba(OH)_2·8H_2O(s) + 2NH_4Cl(s) → BaCl_2(s) + 2NH_3(g) + 10H_2O(l)

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that an exothermic process is one where energy is released to the surroundings, resulting in a decrease in enthalpy (ΔH < 0). This means the products have lower enthalpy than the reactants.
2. Step 2: Analyze each reaction to determine whether it is exothermic or endothermic by considering the nature of the reactants and products, and common knowledge about these reactions.
3. Step 3: For the reaction \$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$, recognize that the formation of liquid water from hydrogen and oxygen gases releases a significant amount of energy, making it strongly exothermic.
4. Step 4: For the reaction \(N_2(g) + O_2(g) \rightarrow 2NO(g)\), note that this reaction requires energy input to break the strong triple bond in nitrogen, so it is endothermic (ΔH > 0).
5. Step 5: For the decomposition reaction \(CaCO_3(s) \rightarrow CaO(s) + CO_2(g)\), understand that breaking down calcium carbonate requires heat (thermal decomposition), so it is endothermic. Similarly, the reaction involving \(Ba(OH)_2 \cdot 8H_2O\) and \(NH_4Cl\) is also endothermic due to the energy needed to break bonds and form gases.