The gaseous reaction below produces an industrial precursor.
A + B → C + D
The only covalent bonds present in the reactants and products are summarized in Table 3.
Table 3. Bonds Broken and Formed per Mole of Reaction
Broken: 1 A≡B bond 2 C–D bonds
Formed: 3 A–C bonds 1 B–D bond
Additional data:
• Average bond energies (kJ mol−1): A≡B 930 A–C 375 C–D 350 B–D ? (unknown)
• The experimentally measured enthalpy change at 298 K is ΔH = –280 kJ mol−1 (exothermic).
Assume that all other energetic contributions (phase changes, lattice energies, etc.) are negligible.
(a) Derive an algebraic expression for ΔH in terms of the four bond energies listed in Table 3. Clearly indicate the sign conventions for bond breaking and bond forming.
(b) Using the experimental ΔH value, calculate the average bond energy for the unknown B–D bond. Express your answer to three significant figures.
(c) If a chemist could synthesize a molecule containing the B–D bond calculated in part (b) and subsequently break that bond in an endothermic step, estimate the minimum energy (in kJ) required to break 0.250 mol of B–D bonds.
(d) Critically evaluate two assumptions made in your calculation that could introduce error when comparing the deduced bond energy to literature values obtained from spectroscopic measurements.