Chlorination of methane can be carried out in the gas phase according to the balanced equation
CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
A calorimetric study at 298 K found ΔH°rxn = −104 kJ for the reaction as written. The table below lists average bond energies (all in kJ mol⁻¹) for the species involved, except for the H—Cl bond.
C—H 413
Cl—Cl 243
C—Cl 328
H—Cl ?
(a) List each distinct bond type that is broken and each bond type that is formed when one mole of reaction occurs, and state how many of each bond are involved.
(b) Using the information in part (a), write an algebraic expression for ΔH°rxn in terms of the unknown average bond energy of H—Cl, B(H—Cl).
(c) Calculate B(H—Cl). Include the appropriate units in your answer.
(d) The calculated B(H—Cl) differs from the tabulated average value of 431 kJ mol⁻¹. Give one reason, grounded in chemical principles, that could account for the difference between the tabulated value and your calculated value.