A three-step gas-phase mechanism converts A(g) to D(g) through two observable intermediates, B(g) and C(g). A researcher constructs an energy diagram whose salient numerical features are listed below (all energies are relative to A(g), which is assigned 0 kJ mol⁻¹).
Step 1: A(g) → B(g) Transition-state energy = 48 kJ mol⁻¹ Energy of B(g) = 14 kJ mol⁻¹
Step 2: B(g) → C(g) Transition-state energy = 66 kJ mol⁻¹ Energy of C(g) = –5 kJ mol⁻¹
Step 3: C(g) → D(g) Transition-state energy = 42 kJ mol⁻¹ Energy of D(g) = –32 kJ mol⁻¹
Use these data to answer the following.
(a) Calculate ΔH for each of the three individual steps.
(b) Determine the overall enthalpy change, ΔHoverall, for conversion of A(g) to D(g).
(c) Identify the rate-determining step and justify your answer quantitatively.
(d) A catalyst lowers the highest transition-state energy by 12 kJ mol⁻¹ without affecting any other energy values. Predict which step’s rate will be most strongly affected and explain your reasoning.