Ozone in the stratosphere can be converted to oxygen gas by a mechanism that proceeds both in the absence and in the presence of chlorine radicals (Cl·). One overall reaction for this process is
2 O3(g) → 3 O2(g) ΔH = –285 kJ mol−1
Laboratory studies show that introduction of a trace amount of Cl· provides an alternative reaction pathway having a lower activation energy, while the enthalpy change remains the same.
Energy (kJ) versus reaction progress is the coordinate system assumed for any diagrams you construct.
(a) Describe qualitatively how the presence of Cl· affects (i) the activation energy and (ii) the value of ΔH for the overall reaction.
(b) On a single set of appropriately labeled axes, sketch both the uncatalyzed and catalyzed potential-energy curves. Label all of the following on your drawing: reactants, products, transition state for each pathway, the activation energy (Ea) for each pathway, and ΔH.
(c) Provide a brief molecular-level explanation for why the catalyst lowers the activation energy yet leaves ΔH unchanged.