Chemistry

Problem

A multistep mechanism converts reactant R to product P through two sequential elementary steps. Table 1 – Energetic data for the uncatalyzed pathway (energies are given relative to

R = 0\,\text{kJ mol}^{-1}

)

Step	$E_a$ ( $\text{kJ mol}^{-1}$ )	$\Delta H$ ( $\text{kJ mol}^{-1}$ )
1	75	$-20$
2	60	$-35$

Answer the following questions. In every written description of a graph, specify all axis labels, approximate numerical values for key points, and clearly indicate transition states and reaction intermediates. (a) Determine the overall enthalpy change,

\Delta H_{\text{overall}}

, for the conversion of R to P. Show your calculation. (b) Draw a fully labeled reaction energy profile for the mechanism on appropriately scaled axes. Indicate the energy of each transition state, the energy of the intermediate, and the overall energy change. (c) Identify the rate-determining step for the uncatalyzed mechanism. Justify your answer using numerical values from Table 1.

Intuitive explanation

This problem tests your understanding of reaction mechanisms and energy diagrams—how to visualize a two-step pathway and identify which step controls the overall rate. The key insight is that the rate-determining step is the one with the highest activation energy, and intermediates appear as local minima between steps. Sketch the energy profile carefully, marking both transition states and the short-lived intermediate formed after step 1.

Intuitive explanation

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