A multi-step mechanism converts reactant R to product P in three sequential elementary steps. A reaction energy profile for the uncatalyzed process is described verbally below.
• The potential energy of the reactants is defined as 0 kJ mol⁻¹.
• Transition state 1 lies at +45 kJ mol⁻¹.
• Intermediate I₁ lies at +10 kJ mol⁻¹.
• Transition state 2 lies at +80 kJ mol⁻¹.
• Intermediate I₂ lies at –20 kJ mol⁻¹.
• Transition state 3 lies at +30 kJ mol⁻¹.
• The products lie at –60 kJ mol⁻¹ relative to the reactants.
Use this information to answer the following questions.
(a) Identify which elementary step is expected to be the slowest in the uncatalyzed mechanism. Justify your answer using the information provided about the energetics of the transition states.
(b) Predict qualitatively how increasing the temperature from 298 K to 318 K would affect the relative rates of the three elementary steps. Would the identity of the slowest step change? Explain your reasoning.
(c) A heterogeneous catalyst lowers the energy of transition state 2 by 50 kJ mol⁻¹ while leaving the energies of the other transition states and intermediates unchanged. Predict the new rate-determining step for the catalyzed pathway and justify your answer.