Two halogenation reactions of methane are shown below

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Two halogenation reactions of methane are shown below. Reaction 1: CH4+Cl2→CH3Cl+HCl\mathrm{CH_4+Cl_2\rightarrow CH_3Cl+HCl} Reaction 2: CH4+Br2→CH3Br+HBr\mathrm{CH_4+Br_2\rightarrow CH_3Br+HBr} Selected average bond energies in kJ mol−1\mathrm{kJ\,mol^{-1}}: C−H\mathrm{C-H}: 413413; H−Cl\mathrm{H-Cl}: 431431; H−Br\mathrm{H-Br}: 366366; Cl−Cl\mathrm{Cl-Cl}: 243243; Br−Br\mathrm{Br-Br}: 192192; C−Cl\mathrm{C-Cl}: 328328; C−Br\mathrm{C-Br}: 276276. Assume all bonds in the reactants are completely broken and all bonds in the products are completely formed. (a) Without performing a full numerical calculation, identify which reaction is expected to be more exothermic per mole of CH4\mathrm{CH_4} consumed. State the specific comparative facts about the bond energies that lead to your conclusion. (b) Describe, in terms of bond breaking and bond making, why the difference in halogen-halogen bond energy is more important than the difference in carbon-halogen bond energy for determining the overall enthalpy change of these reactions. (c) The experimentally measured ΔH\Delta H for Reaction 1 is −104 kJ mol−1-104\mathrm{\,kJ\,mol^{-1}}. Explain qualitatively why the calculated value based on average bond energies might differ from the experimental value.

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