Chlorination of methane can be represented by the equation
CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
Average bond enthalpies (in kJ·mol⁻¹) are provided in the table.
C–H 413 C–Cl 328 H–Cl 431 Cl–Cl 243 H–Br 366 Br–Br 193 C–Br 285
Students are asked to evaluate whether alternative halogenation steps could make the overall process nearly thermoneutral.
(a) Calculate the enthalpy change, ΔH_rxn, for the chlorination reaction given above. Show all work, including how each bond contributes.
(b) Two single-step alternatives are proposed:
(i) CH₄(g) + Br₂(g) → CH₃Br(g) + HBr(g)
(ii) CH₄(g) + ½ Cl₂(g) + ½ Br₂(g) → CH₃Cl₀․₅Br₀․₅(g) + ½ HCl(g) + ½ HBr(g)
Assuming average bond energies for mixed C–Cl₀․₅Br₀․₅ bonds equal the arithmetic mean of C–Cl and C–Br values and making similar assumptions for H–Cl₀․₅Br₀․₅, determine ΔH_rxn for each alternative.
(c) Identify which of the three reactions (original plus the two alternatives) yields an enthalpy change whose magnitude is smallest. Briefly justify the selection based on your calculations.