Two analogous radical halogenation reactions of methane are examined.
Reaction 1: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
Reaction 2: CH₄(g) + Br₂(g) → CH₃Br(g) + HBr(g)
Average bond enthalpies (kJ mol⁻¹):
• C–H 413 • Cl–Cl 243 • Br–Br 193
• H–Cl 431 • H–Br 366
• C–Cl 328 • C–Br 276
(a) For each reaction, list the specific bonds that must be broken and the bonds that must be formed in the elementary step that converts reactants to products.
(b) Using the data provided, estimate the enthalpy change ΔH_rxn (kJ mol⁻¹) for each reaction.
(c) Identify which reaction is more exothermic and quantify the difference based on your calculations.
(d) Using the relative magnitudes of the bonds broken and formed, predict which reaction is expected to have the larger activation energy. Justify your answer qualitatively in terms of bond strengths and the extent of bond breaking in the transition state.