Final answer:
The enthalpy change for the reaction O₃(g) → O₂(g) is exothermic.
Step-by-step explanation:
The enthalpy change for a reaction can be determined by examining the change in enthalpy per mole of substances involved in the reaction. In this case, the reaction is O₃(g) → O₂(g). Since the reaction involves the formation of O₂(g) from O₃(g), it can be considered as the reverse of the formation reaction of O₃(g) from O₂(g).
The enthalpy change for the formation of O₃(g) from O₂(g) is positive, indicating that it is an endothermic reaction. Therefore, the enthalpy change for the given reaction, O₃(g) → O₂(g), would have the opposite sign, making it exothermic.