Reaction intermediates Knowledge-Based Download PDF

Questions

1. 1.

   In chemical kinetics, what fundamental assumption is made regarding a reactive reaction intermediate under the steady-state approximation?

   • A) Its net rate of change of concentration is approximately zero because its rate of formation equals its rate of consumption.
   • B) Its net rate of change of concentration is constant because the intermediate is regenerated at the end of the overall reaction.
   • C) Its rate of formation is significantly greater than its rate of consumption, causing it to accumulate as a major product.
   • D) Its concentration decreases continuously from an initial non-zero maximum to zero as the reaction proceeds.
2. 2.

   When the rate-determining step of a multistep reaction mechanism contains a reaction intermediate, how must that intermediate be handled to formulate an acceptable overall rate law?

   • A) It is directly retained in the final rate law because the molecularity of the rate-determining step dictates the rate expression.
   • B) It must be eliminated and expressed in terms of reactant concentrations using a pre-equilibrium step or the steady-state approximation.
   • C) It is omitted entirely from the expression because it cancels out of the overall balanced stoichiometric equation.
   • D) It is assigned a constant value of one because its transient nature prevents it from affecting the reaction rate.
3. 3.

   On a potential energy profile for a multistep reaction mechanism, how is a reaction intermediate represented?

   • A) As the highest potential energy peak representing an activated complex
   • B) As an unstable species located at the summit of the first activation barrier
   • C) As a local potential energy minimum situated between two transition states
   • D) As a constant horizontal plateau matching the potential energy of the reactants
4. 4.

   A proposed mechanism for the reaction between nitrogen dioxide and carbon monoxide consists of the following elementary steps: Step 1: $\text{NO}_2(g) + \text{NO}_2(g) \rightarrow \text{NO}_3(g) + \text{NO}(g)$ Step 2: $\text{NO}_3(g) + \text{CO}(g) \rightarrow \text{NO}_2(g) + \text{CO}_2(g)$ Which species acts as a reaction intermediate in this mechanism?

   • A) $\text{NO}_3(g)$
   • B) $\text{NO}(g)$
   • C) $\text{CO}(g)$
   • D) $\text{CO}_2(g)$
5. 5.

   Which of the following statements correctly describes a reaction intermediate in a multistep reaction mechanism?

   • A) It is present at the beginning of the reaction, accelerates the rate, and is regenerated in a later elementary step.
   • B) It represents the highest potential energy state along the reaction coordinate and cannot possess finite lifetimes.
   • C) It appears as a stable component on the product side of the overall stoichiometric chemical equation.
   • D) It is formed in an earlier elementary step and consumed in a subsequent step, occupying a local potential energy minimum.