A student investigates a reaction that converts species X into product Z through an unknown multistep pathway. A reaction energy profile obtained calorimetrically is described as follows:
• The potential-energy curve begins at 0 kJ mol⁻¹ for the reactants.
• The curve rises to a first maximum at +75 kJ mol⁻¹, descends to a local minimum at –15 kJ mol⁻¹, rises again to a second maximum at +45 kJ mol⁻¹, and finally falls to a minimum at –60 kJ mol⁻¹ for the products.
Answer the following questions about the information that can be extracted from this profile.
(a) State the number of transition states and the number of reaction intermediates indicated by the profile.
(b) Explain why the positions of the local minima correspond to reaction intermediates rather than transition states.
(c) Describe how the overall enthalpy change of the reaction can be determined from the profile, and calculate its value.
(d) Based solely on the shape of the profile, predict whether the first or the second elementary step is expected to be slower at a moderate temperature. Justify your answer qualitatively in terms of molecular kinetic energy distribution.