The conversion of reactant M to product Q follows the three-step mechanism summarized in Table 3. Energies are in kJ mol⁻¹ relative to M = 0 kJ mol⁻¹.
Table 3 – Partial energetic data for the mechanism
Step
Ea (kJ mol⁻¹)
ΔH (kJ mol⁻¹)
1
65
–15
2
85
???
3
50
–30
The experimentally determined overall enthalpy change for the conversion of M to Q is –40 kJ mol⁻¹.
(a) Determine the enthalpy change, ΔH, for Step 2. Show your reasoning.
(b) Using all available numerical information, calculate the energy of each intermediate relative to M.
(c) Sketch a quantitatively accurate reaction energy profile for the mechanism on labeled axes, including all transition states and intermediates.
(d) Identify the step that is most likely to be rate determining and explain your reasoning with reference to the profile you have drawn.