Question 1

A reaction mechanism is proposed for the decomposition of hydrogen peroxide ($$2\,H_2O_2 \rightarrow 2\,H_2O + O_2$) as follows:

Step 1 (fast equilibrium): H_2O_2$$ \rightleftharpoons H_2O + $$O_2$$^-$$

Step 2 (slow): H_2O_2$$ + $$O_2$$^- \rightarrow H_2O + $$O_2 + OH$$^-$$

Which of the following is the correct rate law for this mechanism?

Accepted Answer

$$	ext{rate} = k[H_2O_2]^2$$

Question 2

In a proposed mechanism for a reaction, the following steps are given:

Step 1 (fast): $$A + B ightleftharpoons C$$

Step 2 (slow): $$C + D ightarrow E

If $$the experimental rate law is $$	ext{rate} = k[A][B][D]$$, what does this suggest about the mechanism?

Accepted Answer

The mechanism is consistent with the experimental rate law after substituting for the intermediate $$C$$ using the pre-equilibrium assumption.

Question 3

Which of the following statements best describes an intermediate in a reaction mechanism?

Accepted Answer

A species that is formed in one step and consumed in a subsequent step, and does not appear in the overall reaction.

Question 4

A reaction mechanism is as follows:

Step 1 (slow): $$NO_2 + CO$$ ightarrow NO + $$CO_2$$

Step 2 (fast): $$NO_2 + NO$$ ightarrow N_2O_3$$

Given that the experimental rate law is $$	ext{rate} = k[$$NO_2$$][CO]$$, what can you conclude?

Accepted Answer

The rate law is consistent with the slow step being the rate-determining step.

Question 5

Which of the following is the correct sequence for determining the rate law from a proposed mechanism according to the flowchart?

Accepted Answer

Find the slow step, write the slow-step rate law, remove intermediates, use equilibrium substitution if needed, ensure only reactants/catalysts remain.

Question 6

A catalyst in a reaction mechanism is best described as:

Accepted Answer

A reactant that is regenerated at the end of the reaction.

Question 7

Consider the following mechanism:

Step 1 (fast equilibrium): $$A ightleftharpoons B$$

Step 2 (slow): $$B + C ightarrow D

If $$the experimental rate law is $$	ext{rate} = k[A][C]$$, what must be done to derive the correct rate law from the mechanism?

Accepted Answer

Express $$[B] in $$terms of $$[A]$$ using the equilibrium from step 1.

Question 8

Which of the following is NOT a step in the master decision tree for mechanism analysis?

Accepted Answer

Write the overall balanced equation for the reaction.

Question 9

A student proposes the following mechanism for a reaction:

Step 1 (fast): $$X + Y ightleftharpoons Z$$

Step 2 (slow): $$Z ightarrow P

If $$the experimental rate law is $$	ext{rate} = k[X][Y]$$, what is the role of $$Z in $$the mechanism?

Accepted Answer

$$Z is an $$intermediate and should not appear in the final rate law.

Question 10

In a reaction mechanism, which of the following would indicate the need to use a pre-equilibrium assumption?

Accepted Answer

The slow step contains an intermediate not present in the overall reaction, and the experimental rate law includes only reactants.

Question 11

A reaction mechanism is:

Step 1 (fast equilibrium): $$E ightleftharpoons F$$

Step 2 (slow): $$F + G ightarrow H

If $$the experimental rate law is $$	ext{rate} = k[E][G]$$, what is the correct approach to derive the rate law?

Accepted Answer

Express $$[F] in $$terms of $$[E]$$ using the equilibrium constant, then substitute into the slow-step rate law.

Question 12

Which of the following best describes the rate-determining step (RDS) in a reaction mechanism?

Accepted Answer

The slowest step in the mechanism, which limits the overall reaction rate.

Question 13

A reaction has the following mechanism:

Step 1 (slow): $$A + B ightarrow C$$

Step 2 (fast): $$C + D ightarrow E

If $$the experimental rate law is $$	ext{rate} = k[A][B]$$, what does this indicate?

Accepted Answer

The rate law is determined by the slow step and matches the experimental data.

Question 14

Which of the following is a correct statement about removing intermediates from the rate law?

Accepted Answer

Intermediates must be replaced using equilibrium expressions or other steps so that the final rate law contains only reactants and catalysts.

Question 15

A reaction mechanism is:

Step 1 (fast): $$M + N ightleftharpoons O$$

Step 2 (slow): $$O + P ightarrow Q

If $$the experimental rate law is $$	ext{rate} = k[M][N][P]$$, what is the correct rate law derived from the mechanism?

Accepted Answer

$$	ext{rate} = k[M][N][P]$$

Question 16

Which of the following is a key step in the quick exam checklist for mechanism analysis?

Accepted Answer

Ensure the final rate law contains only reactants and catalysts.

Question 17

A reaction mechanism is:

Step 1 (fast equilibrium): $$S ightleftharpoons T$$

Step 2 (slow): $$T + U ightarrow V

If $$the experimental rate law is $$	ext{rate} = k[S][U]$$, what is the correct process to derive the rate law?

Accepted Answer

Express $$[T] in $$terms of $$[S]$$ using the equilibrium, then substitute into the slow-step rate law.

Question 18

Which of the following best describes the purpose of comparing the experimental rate law to the mechanism?

Accepted Answer

To verify that the proposed mechanism is consistent with observed kinetics.

Question 19

A reaction mechanism is:

Step 1 (fast): $$A + B ightleftharpoons C$$

Step 2 (slow): $$C + D ightarrow E

If $$the experimental rate law is $$	ext{rate} = k[A][B][D]$$, which of the following is true?

Accepted Answer

The rate law is derived by substituting $$[C] in $$terms of $$[A]$$ and $$[B]$$ using the equilibrium from step 1.

Mechanism Identification and Rate Law Determination in Organic Chemistry

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