Three different gaseous hydrocarbons undergo complete combustion according to the general equation CxHy(g) + O2(g) → CO2(g) + H2O(g). The balanced equations for the three reactions are shown below.
Reaction 1: C2H4(g) + 3 O2(g) → 2 CO2(g) + 2 H2O(g)
Reaction 2: C2H6(g) + 3.5 O2(g) → 2 CO2(g) + 3 H2O(g)
Reaction 3: C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
Average bond energies (in kJ mol⁻¹):
C–C 348 C=C 614 C–H 413 O=O 498 C=O (in CO2) 799 O–H 463
Use the bond-energy data to answer the questions that follow.
(a) For each reaction, tabulate (i) the bonds broken in the reactants and (ii) the bonds formed in the products. Indicate the number of each bond type per mole of reaction.
(b) Calculate the estimated enthalpy change ΔH for each reaction.
(c) Rank the three reactions from most exothermic to most endothermic. Provide the numerical ΔH values as evidence for your ranking.
(d) Propane (Reaction 3) is often considered a better fuel than ethene (Reaction 1) for portable stoves. Use the bond-energy calculations and your ranking in part (c) to justify this preference.