Combustion of hydrocarbons is an important source of thermal energy. Consider the combustion of ethane at 298K and 1atm:
C₂H₆(g) + 27 O₂(g) → 2 CO₂(g) + 3 H₂O(l)
Selected standard enthalpies of formation, ΔH_f° (kJ mol⁻¹):
• C₂H₆(g) = −84.7 • CO₂(g) = −393.5 • H₂O(l) = −285.8 • O₂(g) = 0.0
Use the data to analyze the thermochemistry of the reaction.
(a) Without performing a full numerical calculation, predict the sign of ΔH° for the combustion of ethane. Cite specific features of the ΔH_f° data that support your prediction.
(b) Describe, in molecular-level terms, why combustion reactions of hydrocarbons such as ethane generally release heat to the surroundings.