Reaction intermediates Knowledge-Based Practice

Questions

1. 1.

   In chemical kinetics, what fundamental condition is assumed for a reactive reaction intermediate under the steady-state approximation?

   • A) Its concentration remains significantly higher than the concentrations of the reactants throughout the entire course of the reaction.
   • B) Its activation energy for decomposition back into initial reactants is zero, making the initial step completely irreversible.
   • C) It continuously accumulates in the reaction vessel because its rate of consumption is negligible relative to its rate of production.
   • D) Its rate of consumption equals its rate of formation, meaning its net change in concentration over the main course of the reaction is approximately zero.
2. 2.

   When deriving an experimentally testable rate law from a proposed multistep reaction mechanism, why must the concentration of a reaction intermediate be eliminated and replaced with terms involving reactants?

   • A) Because reaction intermediates are typically short-lived species whose concentrations cannot be directly measured or externally controlled.
   • B) Because reaction intermediates do not physically participate in the rate-determining step of any multistep mechanism.
   • C) Because the actual concentration of a reaction intermediate remains exactly zero throughout the reaction.
   • D) Because reaction intermediates are regenerated in subsequent steps and therefore act as inert spectators.
3. 3.

   Which of the following statements correctly describes the role and behavior of a reaction intermediate in a multistep chemical reaction?

   • A) It is formed in an earlier elementary step and consumed in a subsequent elementary step, so it does not appear in the overall balanced chemical equation.
   • B) It is consumed in an earlier elementary step and regenerated in a subsequent elementary step, so it does not appear in the overall balanced chemical equation.
   • C) It exists only at a potential energy maximum with partially formed bonds, so it cannot possess a measurable chemical lifetime.
   • D) It is present among the initial reactants and accelerates the reaction by lowering the activation energy without being chemically transformed overall.
4. 4.

   On a potential energy profile for a multi-step reaction mechanism, which of the following features corresponds to a reaction intermediate?

   • A) An energy maximum representing an activated complex of an elementary step
   • B) A local energy minimum located between two consecutive transition states
   • C) The highest energy peak that defines the overall rate-determining step
   • D) The baseline energy level corresponding to the initial stable reactants
5. 5.

   A proposed two-step reaction mechanism is given below: Step 1: $2\,\text{NO}_2\text{(g)} \rightarrow \text{NO}_3\text{(g)} + \text{NO}\text{(g)}$ Step 2: $\text{NO}_3\text{(g)} + \text{CO}\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{CO}_2\text{(g)}$ Which species acts as a reaction intermediate in this process?

   • A) $\text{NO}_2\text{(g)}$
   • B) $\text{CO}\text{(g)}$
   • C) $\text{NO}_3\text{(g)}$
   • D) $\text{NO}\text{(g)}$

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