Consider the hypothetical two-step reaction mechanism below, carried out at constant pressure. All energies are expressed per mole of the species involved.
Step 1 A → B ΔH1=+25kJ, Activation energy Ea1=45kJ
Step 2 B → C ΔH2=−60kJ, Activation energy Ea2=20kJ
Species A is the initial reactant and species C is the final product.
(a) Determine the overall enthalpy change ΔHoverall for the conversion of A to C. State whether the overall process is endothermic or exothermic.
(b) Construct a complete multi-step energy diagram for the mechanism. Label the energies of A, the intermediate B, the transition states for both steps, C, each step’s activation energy, and the overall ΔH.
(c) Identify which elementary step is rate-limiting if kinetics are controlled solely by the magnitude of the activation energies given. Justify your answer based on features of the energy diagram.
(d) A chemist proposes adding a catalyst that selectively lowers Ea1 to 25 kJ but leaves Ea2 unchanged and does not affect enthalpy changes. Redraw only the portion of your diagram that changes and discuss how (i) the rate-limiting step and (ii) the overall thermochemistry of the reaction would be affected.