Potential energy diagrams Knowledge-Based Practice

Questions

1. 1.

   A potential energy diagram for a multi-step reaction displays three distinct peaks separated by two valleys between the initial reactants and final products. Which statement correctly identifies the number of elementary steps, transition states, and reaction intermediates involved in this mechanism?

   • A) It consists of two elementary steps, two transition states, and three reaction intermediates.
   • B) It consists of three elementary steps, two transition states, and three reaction intermediates.
   • C) It consists of three elementary steps, three transition states, and two reaction intermediates.
   • D) It consists of four elementary steps, three transition states, and two reaction intermediates.
2. 2.

   A chemical reaction proceeds by a two-step mechanism in which an endothermic first step forms an intermediate in rapid equilibrium, followed by a slow, rate-determining second step that forms the final product. Which statement correctly describes the relative energy levels and features on the potential energy diagram for this reaction?

   • A) The potential energy of the intermediate is lower than that of the reactants, and the first peak represents the highest maximum on the diagram.
   • B) The potential energy of the intermediate is lower than that of the reactants, and the transition state of the second step has a lower energy than the final product.
   • C) The potential energy of the intermediate is higher than that of the reactants, and the highest potential energy maximum corresponds to the transition state of the second step.
   • D) The potential energy of the intermediate is higher than that of the reactants, and the highest potential energy maximum corresponds to the transition state of the first step.
3. 3.

   For an elementary reaction represented on a potential energy diagram, the forward activation energy is $E_{a,\text{fwd}}$ and the enthalpy change of the reaction is $\Delta H$. Which equation correctly expresses the activation energy of the reverse reaction ($E_{a,\text{rev}}$)?

   • A) $E_{a,\text{rev}} = \Delta H - E_{a,\text{fwd}}$
   • B) $E_{a,\text{rev}} = E_{a,\text{fwd}} + \Delta H$
   • C) $E_{a,\text{rev}} = E_{a,\text{fwd}} - \Delta H$
   • D) $E_{a,\text{rev}} = -(E_{a,\text{fwd}} + \Delta H)$
4. 4.

   How does the addition of a catalyst alter the potential energy diagram of a chemical reaction?

   • A) It decreases the potential energy of the products while keeping the activation energy of the forward reaction unchanged.
   • B) It increases the potential energy of the reactants, thereby reducing the net enthalpy change of the reaction.
   • C) It provides an alternative reaction pathway with lower activation energy barriers without altering the energies of the reactants and products.
   • D) It eliminates all transition states from the profile, allowing reactants to transform directly into products with zero activation energy.
5. 5.

   A reaction proceeds by a two-step mechanism where the activation energy of the first elementary step is significantly greater than that of the second step. Which statement correctly identifies the feature corresponding to the local potential energy minimum between the two peaks and the rate-determining step?

   • A) The local minimum corresponds to a transition state, and the second step is the rate-determining step.
   • B) The local minimum corresponds to a catalyst, and the first step is the rate-determining step.
   • C) The local minimum corresponds to a reaction intermediate, and the first step is the rate-determining step.
   • D) The local minimum corresponds to the final product, and the second step is the rate-determining step.

Answers and step-by-step solutions appear after you finish the test.