Ozone, O3 (g), is a form of elemental oxygen that is important in the absorption of ultraviolet radiation in the stratosphere. It decomposes to O2 (g) at room temperature and pressure according to the following reaction: (a) What is the enthalpy change for this reaction per mole of O3 (g) ? (b) Which has the higher enthalpy under these conditions, 2 O3 (g) or 3 O2 (g) ?

-142.3 per mole, b) 2 O₃ (g)

Explanation:

2O₃ (g) → 3O₂ (g) ΔH = - 284.6 KJ

According to the equation 2 mole of ozone yields 3 mole of oxygen gas and the enthalpy change = - 284.6 KJ

a) enthalpy change per mole = - 284.6 / 2 mole = - 142.3 KJ per mole

b) The reaction is exothermic since the enthalpy change is negative meaning that the heat of reactant is greater than heat of product. If the heat of product is greater than heat of reactant meaning enthalpy change is positive; the reaction is endothermic. 2 O₃ (g) has higher enthalpy compared to 3 O₂ (g) since the enthalpy change is negative