An uncatalyzed decomposition of substance D is represented by a single-step energy diagram with the following characteristics:
• Reactant energy is defined as 0 kJ mol⁻¹.
• Transition-state energy is +160 kJ mol⁻¹.
• Product energy is –40 kJ mol⁻¹.
A heterogeneous catalyst provides an alternative two-step pathway involving an intermediate. Relative to the reactants, the energies for the catalyzed pathway are:
• First transition state: +75 kJ mol⁻¹
• Intermediate: +20 kJ mol⁻¹
• Second transition state: +85 kJ mol⁻¹
• Products: –40 kJ mol⁻¹
Respond to the following prompts.
(a) Sketch qualitative potential-energy diagrams (reactants → products) for both the uncatalyzed and catalyzed pathways on the same set of axes. Clearly label all key features, including transition states and the intermediate.
(b) Identify the rate-determining step for the catalyzed pathway and justify your choice based on the diagram.
(c) Calculate the percentage decrease in the overall activation energy when the catalyst is present. Use the highest-energy transition state on each pathway to represent Ea.
(d) Discuss whether the presence of the catalyst affects the value of the equilibrium constant for the decomposition of substance D. Justify your answer with reference to thermodynamic quantities visible on the energy diagram.