A research team is studying a two-step catalytic mechanism for the conversion of gaseous substrate S to product P over a metal surface M.
Step 1 S + M ⇌ S–M (fast equilibrium)
Step 2 S–M → P + M (slow, rate-limiting)
From calorimetric measurements, the following energies relative to separated S(g) and M(s) are obtained:
• S–M adsorbed complex: –60 kJ mol⁻¹
• Transition state for Step 2: +25 kJ mol⁻¹
• Product P(g): –40 kJ mol⁻¹
(a) Construct a qualitative potential-energy diagram for the overall catalytic pathway, labeling all minima and maxima clearly.
(b) Using the energies provided, calculate the activation energy for the slow step and the overall enthalpy change for conversion of S to P.
(c) Propose how the presence of a competitive inhibitor I that also binds to M might alter the shape of the energy profile you drew in part (a). Explain your reasoning.
(d) Evaluate the statement: “Because the catalyst participates in an intermediate step, it must change the equilibrium position of the reaction S ⇌ P.” Provide a detailed argument supporting or refuting this claim.
(e) Suggest one modification to the metal surface that could further increase the catalytic rate and justify your suggestion in terms of activation energy or transition-state stabilization.