A chemist is investigating a gas-phase elementary reaction that converts reactant R to product P. When no catalyst is present, the reaction proceeds through a single transition state at an energy of 145 kJ mol⁻¹ relative to the reactants. When a heterogeneous solid catalyst is introduced, an alternative pathway becomes available whose transition state lies at 105 kJ mol⁻¹ relative to the reactants. For both pathways, the reactants and products have energies of 0 kJ mol⁻¹ and –38 kJ mol⁻¹, respectively.
(a) Describe how the catalyst changes the potential-energy profile of this reaction. In your answer, reference the relative positions of the reactants, products, transition states, and activation energies for both the catalyzed and uncatalyzed pathways.
(b) Explain why the enthalpy change ΔH for the reaction is unaffected by the presence of the catalyst, even though the activation energy is lower.
(c) The catalyst is later removed from the reaction vessel after equilibrium is established. Predict how (if at all) the concentrations of R and P at equilibrium will change after catalyst removal. Justify your prediction.
(d) Under identical conditions of temperature and pressure, explain qualitatively how the rates of the forward and reverse reactions are affected by adding the catalyst. Include a discussion of whether one direction is favored kinetically over the other.