A reaction converts gas-phase reactant X into product Z through two elementary steps that proceed in sequence: X → Y → Z.
The uncatalyzed pathway has the following energetic characteristics, measured relative to the energy of X:
• Transition state of step 1: +95 kJ mol⁻¹
• Intermediate Y: +35 kJ mol⁻¹
• Transition state of step 2: +82 kJ mol⁻¹
• Product Z: –18 kJ mol⁻¹
When a solid catalyst is introduced, both transition states are lowered in energy. The transition state of step 1 is decreased by 25 kJ mol⁻¹, and the transition state of step 2 is decreased by 40 kJ mol⁻¹. The energies of X, Y, and Z are unchanged.
Answer the following questions about the catalyzed and uncatalyzed pathways.
(a) On a single set of labeled axes (reaction progress on the horizontal axis, potential energy on the vertical axis), sketch qualitative energy profiles for the catalyzed and uncatalyzed pathways. Clearly label all transition states, the intermediate, the reactants, and the products.
(b) Describe how the catalyst alters the activation energy for each elementary step and identify which step is rate determining in each pathway.
(c) Explain why the overall enthalpy change of the reaction is identical for both the catalyzed and uncatalyzed pathways, even though individual activation energies differ.
(d) Based on your sketches and the data provided, predict whether the catalyst will have a greater effect on the rate of step 1 or step 2, and justify your answer.