Table 1 lists average bond energies (all in kJ mol−1).
Table 1. Selected Average Bond Energies
C–H 413 C≡C 839 C=C 614 C–C 348 H–H 436 O=O 498 C=O (in CO2) 799 O–H 463
Consider the three combustion reactions written below in which the reactant quantities are scaled to produce exactly 2 mol of CO2 in each reaction.
Reaction A: C2H2 + (5⁄2) O2 → 2 CO2 + H2O
Reaction B: C2H4 + 3 O2 → 2 CO2 + 2 H2O
Reaction C: C2H6 + (7⁄2) O2 → 2 CO2 + 3 H2O
(a) For each reaction, calculate the approximate enthalpy change ΔH (in kJ) using the data in Table 1. Show your work clearly.
(b) Rank the three reactions from most exothermic to least exothermic based on your calculated ΔH values.
(c) Identify one limitation of using average bond energies for comparing the combustion enthalpies of these hydrocarbons and briefly discuss its potential impact on your ranking.