Compound Q is converted to product S by the three-step mechanism shown below.
Step 1 Q → A
Step 2 A → B
Step 3 B → S
Laboratory measurements at 298 K give the following activation energies for each elementary step (assume similar pre-exponential factors).
Step Activation energy, Ea (kJ mol⁻¹)
1 55
2 95
3 40
Answer the questions that follow.
(a) Which step is expected to control the overall rate of formation of S at 298 K? Provide a brief justification.
(b) Arrange the three steps in order of decreasing rate (fastest to slowest) at 298 K. Explain how the data support the order you provide.
(c) Sketch a qualitative reaction energy profile for the uncatalyzed process, labeling all intermediates, transition states, and the relative heights of the activation barriers consistent with the information above. (A precise scale is not required.)