A research team studies the gas-phase conversion of reactant S by two alternative one-step pathways that share the same reactants and products.
Pathway I (uncatalyzed): Activation energy = 90kJ⋅mol−1, ΔH=−20kJ⋅mol−1
Pathway II (catalyzed): Activation energy = 55kJ⋅mol−1, ΔH=−20kJ⋅mol−1
The team gathers the following additional information:
• At 300 K, 0.020 mol of S is converted per minute via Pathway II, whereas conversion via Pathway I is negligible over the same interval.
• Both pathways are elementary and involve the same molecularity.
Using this information, address the tasks below.
(a) Sketch qualitative energy profile diagrams for both pathways on the same set of axes, clearly labeling reactants, products, transition states, activation energies, and overall enthalpy change.
(b) At 300 K, estimate the minimum order of magnitude (e.g., 100, 101, 102, \ldots) by which the rate constant for Pathway II exceeds that for Pathway I. Base your estimate on the observed conversion data and appropriate qualitative kinetic reasoning.
(c) The researchers raise the temperature to 340 K. Predict how the ratio kII/kI changes—does it increase, decrease, or remain nearly the same? Provide a detailed explanation grounded in activation energy considerations and the distribution of molecular kinetic energies.
(d) Propose one additional experimental change (other than temperature or the existing catalyst) that could further favor Pathway II over Pathway I. Justify your proposal mechanistically.