Two unrelated chemical processes, Process X and Process Y, are each described by a single-step energy diagram. Both diagrams share the same vertical potential-energy scale.
• In Process X, the reactants begin at 120 kJ, the transition state peaks at 170 kJ, and the products end at 80 kJ.
• In Process Y, the reactants begin at 60 kJ, the transition state peaks at 140 kJ, and the products end at 100 kJ.
Assume that each energy value is referenced to the same arbitrary zero and that no external work other than pdV work is performed.
(a) Calculate ΔH for each process and classify each as endothermic or exothermic.
(b) Compute the activation energy for the forward direction of each process and compare their kinetic barriers. Which process would you predict to be faster at the same temperature, all else being equal? Provide a reasoned argument.
(c) The two processes are carried out sequentially in a coupled reaction system where the heat released or absorbed by the first reaction is used to drive the second. Propose which ordering of the two processes (X followed by Y or Y followed by X) would minimize the external heating or cooling required. Support your proposal with quantitative reasoning based on the enthalpy changes you calculated.
(d) For the ordering you selected in part (c), qualitatively sketch a combined energy diagram that shows the overall enthalpy change from the initial reactants of the first process to the final products of the second. Label the two transition states and all reactant and product energy levels.