Two gas-phase reactions, labeled Reaction A and Reaction B, are written below. No numerical bond-energy data are provided.
Reaction A: N2(g) + 3 H2(g) → 2 NH3(g)
Reaction B: C2H4(g) + H2(g) → C2H6(g)
Nitrogen–nitrogen triple bonds are among the strongest known covalent bonds, while carbon–carbon double bonds and carbon–hydrogen single bonds are appreciably weaker. Hydrogen–hydrogen single bonds are of intermediate strength.
Answer the following questions. Base your reasoning strictly on the relative strengths of the bonds that must be broken and the bonds that will be formed; do not perform any numerical calculations.
(a) For Reaction A, describe whether the reaction is likely to be exothermic or endothermic. Provide a clear explanation that references the relative strengths of the bonds broken and the bonds formed.
(b) For Reaction B, describe whether the reaction is likely to be exothermic or endothermic. Provide a clear explanation that references the relative strengths of the bonds broken and the bonds formed.
(c) Between Reaction A and Reaction B, identify which is expected to release the greater magnitude of energy per mole of reaction progress. Justify your choice in terms of the bonds involved.