A research group reports the following verbal description of the energy profile for the decomposition of a dimer D₂ into two monomer units D:
• Reactants: 0 kJ mol⁻¹
• Transition state 1 (TS₁): +120 kJ mol⁻¹
• Intermediate I: +10 kJ mol⁻¹
• Transition state 2 (TS₂): +90 kJ mol⁻¹
• Products: –40 kJ mol⁻¹
The same group measures the enthalpy change for the overall reaction to be –40 ±2 kJ mol⁻¹.
Use this information to analyze the reaction pathway.
(a) Calculate the enthalpy change ΔH for each individual elementary step in the mechanism implied by the profile.
(b) Identify which elementary step is endothermic and which is exothermic. Provide numerical support for your answer.
(c) Determine which elementary step is expected to be rate-determining at 298 K, and explain your reasoning using the quantitative data from the profile.
(d) A colleague proposes a three-step mechanism for the same reaction that would involve two intermediates. Explain why the reported energy profile argues against that alternative mechanism.