Which of the following chemical equations represents an exothermic process?

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

Answer: (C)

Verified step by step guidance

1. Understand that an exothermic process is one that releases heat to the surroundings, meaning the enthalpy change (\$\Delta H\$) is negative.
2. Analyze each given chemical equation and recall or look up whether the reaction is known to release or absorb heat.
3. For the decomposition of calcium carbonate (\$\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}\$), recognize this is a thermal decomposition that requires heat input, so it is endothermic (absorbs heat).
4. For the formation of nitric oxide (\$\mathrm{N_2(g) + O_2(g) \rightarrow 2NO(g)}\$), note that this reaction requires high energy input (like a spark), so it is endothermic.
5. For the formation of liquid water from hydrogen and oxygen gases (\$\mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(l)}\$), this is a combustion reaction that releases a significant amount of heat, making it exothermic.