A generic exothermic reaction between gaseous reactant A and gaseous reactant B produces a single product AB.
The uncatalyzed reaction proceeds by a single elementary step whose potential-energy profile can be verbally described as follows:
• At reaction progress = 0, the potential energy of the reactants is defined as 0 kJ mol⁻¹.
• The reaction coordinate rises smoothly to a single maximum of +180 kJ mol⁻¹ at 40 % reaction progress.
• The curve then falls to a final plateau at –75 kJ mol⁻¹, representing the products at 100 % reaction progress.
A chemist discovers a solid surface catalyst that enables the same overall reaction to occur by *two* elementary steps. The catalyzed profile can be described qualitatively as having two maxima, each lower than the uncatalyzed maximum.
Answer the following related questions.
(a) Describe how you would annotate (label) the transition state and the enthalpy change, ΔH, on the uncatalyzed energy-profile diagram as verbally described above.
(b) Qualitatively sketch (no numerical scale required) the potential-energy curve for the catalyzed pathway on the same set of axes as the uncatalyzed curve. Your sketch must clearly show:
• two distinct maxima,
• at least one intermediate, and
• the relative activation energies compared with the uncatalyzed reaction.
(c) Using your sketch, explain how the presence of the catalyst affects (i) the activation energy of the overall reaction and (ii) the number of elementary steps in the mechanism.