Which of the following equations represents energy being absorbed as a bond is broken?

• A. CH_4(g) + 2O_2(g) ightarrow CO_2(g) + 2H_2O(g) ext{; } riangle H < 0
• B. NaCl(s) ightarrow Na^+(aq) + Cl^-(aq) ext{; } riangle H < 0
• C. 2H(g) ightarrow H_2(g) ext{; } riangle H < 0
• D. H_2(g) ightarrow 2H(g) ext{; } riangle H > 0

Answer: (D)

Verified step by step guidance

1. Understand that breaking chemical bonds requires energy input, so the enthalpy change (\triangle H) for bond breaking is positive (\triangle H > 0).
2. Look at each given reaction and identify whether bonds are being formed or broken by analyzing the reactants and products.
3. For the reaction \( \mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)} \) with \( \triangle H < 0 \), note that the negative enthalpy indicates bond formation (energy released), not bond breaking.
4. For the reaction \( \mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)} \) with \( \triangle H < 0 \), the negative enthalpy suggests energy is released, indicating bond formation or solvation, not bond breaking.
5. For the reaction \( \mathrm{H_2(g) \rightarrow 2H(g)} \) with \( \triangle H > 0 \), recognize that the bond in \( \mathrm{H_2} \) is being broken into atoms, which requires energy absorption, matching the condition for bond breaking.