Question 1

Which of the following best describes a reaction mechanism?

Accepted Answer

A series of elementary steps by which a chemical reaction occurs.

Question 2

In the reaction mechanism for the formation of NO from NO$$_2$$ and CO, which species is a reaction intermediate?

Accepted Answer

NO$$_3(g)$$

Question 3

Which step in a reaction mechanism determines the overall rate of the reaction?

Accepted Answer

The slowest elementary step

Question 4

Given the following mechanism:

NO$$_2(g) + NO$_2(g$$) $$\rightarrow NO$_3(g) + NO(g) ($$slow)
NO$$_3(g) + CO(g$$) $$\rightarrow NO$_2(g) + CO$_2(g) ($$fast)

What is the overall balanced equation for the reaction?

Accepted Answer

$$\mathrm{NO_2(g) + CO(g) \rightarrow NO(g) + CO_2(g)}$$

Question 5

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

Accepted Answer

The mechanism must have the lowest possible activation energy.

Question 6

Why are elementary steps involving three reactant molecules rare?

Accepted Answer

The probability of three entities colliding simultaneously is very low.

Question 7

Given the following elementary step: $$\mathrm{A + B \rightarrow Products}$$, what is the rate law for this step?

Accepted Answer

$$\mathrm{rate = k[A][B]}$$

Question 8

A rocket launch uses nitryl fluoride, NO$$_2F$$, as a propellant. The reaction is:

2 NO$$_2(g) + F$_2(g$$) $$\rightarrow 2 NO$_2F(g)

If $$the experimentally determined rate law is $$\mathrm{rate = k[NO_2][F_2]}$$, which step is most likely the rate-determining step in a plausible mechanism?

Accepted Answer

NO$$_2(g) + F$_2(g$$) $$\rightarrow NO$_2F(g) + F(g)$$

Question 9

Which of the following statements about reaction intermediates is correct?

Accepted Answer

They are formed and consumed during the reaction sequence.

Question 10

A student claims that the rate law for a reaction can always be written directly from the overall balanced equation. Is this correct?

Accepted Answer

No, because the rate law must be determined experimentally or from the rate-determining step of the mechanism.

Question 11

Given the following mechanism:

O$$_3(g$$) $$\rightarrow O$_2(g) + O(g) ($$fast)
O$$_3(g) + O(g$$) $$\rightarrow 2 O$_2(g) ($$slow)

What is the expected rate law for the overall reaction?

Accepted Answer

$$\mathrm{rate = k[O_3][O]}$$

Question 12

Which analogy best explains the concept of a rate-determining step in a reaction mechanism?

Accepted Answer

The slowest part of a journey, such as waiting for a bus, determines the total travel time.

Question 13

Which of the following is a correct rate law for the elementary step: $$\mathrm{O_3(g) + O(g) \rightarrow 2 O_2(g)}$$?

Accepted Answer

$$\mathrm{rate = k[O_3][O]}$$

Question 14

Which of the following best describes why the overall reaction rate is controlled by the rate-determining step?

Accepted Answer

Because products can only be formed as fast as the slowest step occurs.

Question 15

Which of the following is NOT a reaction intermediate?

Accepted Answer

A reactant that appears in the overall balanced equation.

Question 16

If the experimentally determined rate law for a reaction is $$\mathrm{rate = k[NO_2]^2}$$, which of the following elementary steps is most likely the rate-determining step?

Accepted Answer

$$\mathrm{NO_2 + NO_2 \rightarrow products}$$

Question 17

Which of the following is a requirement for a plausible reaction mechanism?

Accepted Answer

The mechanism must agree with the experimentally determined rate law.

Question 18

A rocket launch (as shown in the image) uses a chemical reaction for thrust. If the reaction mechanism involves a slow step followed by a fast step, what determines the rate at which thrust is produced?

Accepted Answer

The slowest step in the reaction mechanism

Lesson 6.6: Reaction Mechanisms in Chemical Kinetics

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