A four-step mechanism for the gas-phase oxidation of hydrocarbon X is summarized in Table 2. Energies are expressed in kJ mol⁻¹ relative to X = 0 kJ mol⁻¹.
Table 2 – Energetic data for uncatalyzed versus catalyzed pathways
Step
Ea_uncat
ΔH_step
Ea_cat (with solid Pt catalyst)
1
90
+10
45
2
80
–35
40
3
95
–10
55
4
70
–60
35
Answer the prompts that follow. Your graphs should contain two curves plotted on the same set of axes—one for the uncatalyzed reaction and one for the catalyzed reaction.
(a) Using the uncatalyzed data, compute the cumulative energy (relative to X) of each intermediate formed after Steps 1, 2, and 3.
(b) Construct a fully labeled energy profile for the uncatalyzed reaction, indicating all intermediates and transition states with quantitative energies.
(c) On the same axes, draw the catalyzed profile using the Ea_cat values. Assume that each ΔH_step is unchanged by the catalyst. Clearly distinguish the two pathways in your sketch.
(d) Calculate the percentage decrease in the highest activation energy when the catalyst is present. Identify which elementary step experiences the greatest catalytic effect and justify your choice.