Based on the given bond energies, which of the following reactions is expected to be most exothermic?

• A. C(s) + O_2(g) → CO_2(g)
• B. CH_4(g) + 2O_2(g) → CO_2(g) + 2H_2O(g)
• C. N_2(g) + 3H_2(g) → 2NH_3(g)
• D. H_2(g) + Cl_2(g) → 2HCl(g)

Answer: (D)

Verified step by step guidance

1. Step 1: Understand that the enthalpy change (ΔH) of a reaction can be estimated using bond energies by applying the formula: \[\Delta H = \sum \text{Bond Energies of bonds broken} - \sum \text{Bond Energies of bonds formed}\] This means you subtract the total energy released when new bonds form from the total energy required to break the original bonds.
2. Step 2: For each reaction, identify all the bonds broken in the reactants and all the bonds formed in the products. For example, in the reaction \[\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}\], you would consider breaking the bonds in \[\mathrm{O_2}\] and forming the bonds in \[\mathrm{CO_2}\].
3. Step 3: Use the given bond energies to calculate the total energy required to break all bonds in the reactants and the total energy released when new bonds form in the products. Remember to multiply bond energies by the number of such bonds involved in the reaction.
4. Step 4: Calculate the enthalpy change (ΔH) for each reaction by subtracting the total bond energy of bonds formed from the total bond energy of bonds broken, as per the formula in Step 1.
5. Step 5: Compare the calculated ΔH values for all reactions. The reaction with the most negative ΔH (largest energy release) is the most exothermic. According to the problem, the reaction \[\mathrm{H_2(g) + Cl_2(g) \rightarrow 2HCl(g)}\] is the most exothermic, which means it has the greatest net release of energy based on bond energies.