Question 1

Which of the following statements about reaction intermediates is correct?

Accepted Answer

Intermediates are produced in one elementary step and consumed in a later step.

Question 2

Given the following mechanism for the reaction $$2NO_2 + CO \rightarrow 2NO + CO_2$:

Step 1: NO_2$ + NO_2$ \rightarrow NO_3$ + NO ($$slow)

Step 2: $$NO_3 + CO$$ \rightarrow $$NO_2$$ + $$CO_2 ($$fast)

What is the correct rate law for this reaction?

Accepted Answer

Rate = $$k[NO_2$]^2$$

Question 3

Which of the following is NOT a requirement for a plausible reaction mechanism?

Accepted Answer

The mechanism must be proven by direct observation of all intermediates.

Question 4

For the elementary step $$2A + B \rightarrow C$$, what is the rate law?

Accepted Answer

Rate = $$k[A]^2[B]$$

Question 5

A catalyst in a reaction mechanism is best described as:

Accepted Answer

A substance that is consumed in an early step and regenerated in a later step.

Question 6

Which of the following would increase the rate constant $$k$$ for a reaction?

Accepted Answer

Adding a catalyst.

Question 7

Consider the following mechanism:

Step 1: $$SO_2$$ + $$SO_2$$ \rightarrow S_2O + $$O_3 ($$slow)

Step 2: $$SO_3$$ + $$O_3$$ \rightarrow 2O_2 + $$SO_2 ($$fast)

What is the intermediate in this mechanism?

Accepted Answer

$$O_3$$

Question 8

Why are elementary steps involving more than three molecules colliding at once considered unlikely?

Accepted Answer

Because the probability of four or more molecules colliding simultaneously with the correct orientation and energy is extremely low.

Question 9

In a reaction mechanism, which step determines the overall rate law?

Accepted Answer

The slowest (rate-determining) step.

Question 10

For the reaction $$A + B \rightarrow C$$, the experimentally determined rate law is $$Rate = k[A]^2. $$Which of the following mechanisms is plausible?

Accepted Answer

Step 1: $$A + A \rightarrow D ($$slow); Step 2: $$D + B \rightarrow C ($$fast)

Question 11

Which of the following best describes the effect of a catalyst on a reaction?

Accepted Answer

It increases the rate constant $$k by $$lowering the activation energy.

Question 12

Which of the following is true about the rate law for an elementary step?

Accepted Answer

The rate law for an elementary step is determined by the molecularity of the step.

Question 13

A reaction mechanism is proposed for a reaction, but the sum of the elementary steps does not match the overall balanced equation. What does this indicate?

Accepted Answer

The mechanism is not plausible.

Question 14

Which of the following is a correct rate law for the elementary step $$A + B \rightarrow Products$$?

Accepted Answer

Rate = $$k[A][B]$$

Question 15

In the reaction $$O_3 + O$$ \rightarrow 2O_2$$, the following mechanism is proposed:

Step 1: Cl$$ + $$O_3$$ \rightarrow ClO + $$O_2$$

Step 2: $$ClO + O \rightarrow Cl + O_2$

What role does $$ClO$$ play in this mechanism?

Accepted Answer

$$ClO is an $$intermediate.

Question 16

Which of the following would NOT increase the rate of a chemical reaction?

Accepted Answer

Lowering the temperature.

Question 17

A student proposes the following mechanism for a reaction:

Step 1: $$A + B + C \rightarrow D ($$slow)

Step 2: $$D \rightarrow E ($$fast)

Which of the following is a valid criticism of this mechanism?

Accepted Answer

The slow step involves three molecules colliding, which is highly unlikely.

Question 18

Which of the following statements about catalysts is FALSE?

Accepted Answer

Catalysts are consumed in the reaction and not regenerated.

Question 19

If the rate law for a reaction is $$Rate = k[A][B]$$, which of the following could be the slow (rate-determining) step in the mechanism?

Accepted Answer

$$A + B \rightarrow C ($$slow)

Question 20

Which of the following best explains why the rate law for a reaction mechanism cannot include intermediates?

Accepted Answer

Intermediates are not present at the start of the reaction and their concentrations are not easily measurable.

CH 12

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