The combustion of propane can be represented by the following thermochemical equation at 1bar pressure:
C₃H₈(g) + 5 O₂(g) → 3 CO₂(g) + 4 H₂O(l) ΔH∘=−2044kJmol−1 (per mole of propane)
Consider a demonstration in which a small quantity of propane is burned in an open flame to heat a laboratory beaker of water.
(a) Indicate whether the reaction is exothermic or endothermic and justify your choice with reference to the value of ΔH∘.
(b) Describe the direction of heat flow between the chemical system and its surroundings during the combustion.
(c) Sketch (in words) a qualitative potential-energy diagram for the reaction, indicating the relative energy levels of reactants and products and labeling the arrow that represents ΔH. Explain the meaning of the arrow’s direction.
(d) Using bond-energy reasoning, explain why the overall enthalpy change for propane combustion is negative even though bond breaking is an endothermic process.