AP Chemistry: 5.7 Introduction to Reaction Mechanisms – Exam Style questions with Answer- MCQ

Question

Which of the following best describes the elementary step(s) in the reaction mechanism represented in the diagram above?
A     One step: \(2X(g)+Y(g)→X_2Y(g)\)
B     One step: \(X_2(g)+Y(g)→X_2Y(g)\)
C   Two steps: Step 1: \(X(g)+Y(g)→XY(g)\)
      Step 2: \(X(g)+XY(g)→X_2Y(g)\)
D   Two steps: Step 1: \(2X(g)→X_2(g)\)
      Step 2: \(X_2(g)+Y(g)→X_2Y(g)\)

▶️Answer/Explanation

Ans:D

From the first diagram to the second diagram, two X atoms joined to form the intermediate \(X_2\). From the second diagram to the third diagram, the \(X_2\) joined with a Y to form the final product \(X_2Y\).

Question

The diagram above shows the progress of the chemical reaction for the synthesis of ammonia from its elements. The adsorption of the N2 molecules on the surface of Ru weakens the triple bond between the two N atoms. Based on the diagram, what is the role of Ru

in this process?
A Ru is a catalyst.
B Ru is a reactant.
C Ru is a product.
D Ru is an intermediate.

▶️Answer/Explanation

Ans:A

The amount of Ru atoms does not change. Ru is a catalyst that provides an alternate path to the process of breaking the triple bond between the two N atoms, which is the rate-limiting step in the synthesis of \(NH_3\).

Question

 

The proposed mechanism for a reaction involves the three elementary steps represented by the particle models shown above. On the basis of this information, which of the following models represents an intermediate in the overall reaction?

▶️Answer/Explanation

Ans: D

\( N_2O\) is a reaction intermediate. \( N_2O\) is generated in step 2 and is consumed in step 3. Intermediates, such as \( N_2O\) and \( N_2O_2\), do not appear in the overall chemical equation of the reaction, which is \(2NO+2H_2→N_2+2H_2O\).

Question

            \(O_3(g)+O(g)→2O_2(g)\)

The decomposition of \(O_3\) occurs according to the balanced equation above. In the presence of NO , the decomposition proceeds in two elementary steps, as represented by the following mechanism.
Step 1:          \( O_3+NO→NO_2+O_2\)          (slow)
Step 2:          \(NO_2+O→NO+O_2\)                (fast)

Based on the information, which of the following is the rate law?
A Rate=\(k[O_3][NO]\)

B Rate=\(k[NO_2][O_2]\)

C Rate=\(k[NO_2][O]\)

D Rate=\(k[NO][O]\)

▶️Answer/Explanation

Ans: A

Rate=\(k[O_3][NO]\) is consistent with step 1 being the rate-determining step. When the first step of a multistep reaction is the slowest step, the rate of reaction depends on the concentration and stoichiometry of the reacting species in this elementary reaction.

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