Q: Which of the following would NOT be a plausible elementary step in a reaction mechanism?

$$A + B + C + D \rightarrow \,\text{products}$$

Question 1

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

Accepted Answer

A molecule that is produced in one elementary step and consumed in a subsequent step.

Question 2

Given the reaction mechanism:

Step 1: $$2NO_2 \rightarrow NO_3$ + NO ($$slow)
Step 2: $$NO_3 + CO$$ \rightarrow $$NO_2$$ + $$CO_2 ($$fast)

What is the correct rate law for the overall reaction?

Accepted Answer

$$	ext{rate} = k[NO_2$]^2$$

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 $$A + B ightarrow \,	ext{products}$$, what is the rate law?

Accepted Answer

$$	ext{rate} = k[A][B]$$

Question 5

Why are elementary steps involving three or more molecules colliding simultaneously considered unlikely?

Accepted Answer

Because the probability of three molecules colliding at the same time is very low.

Question 6

In the reaction 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)

Which species is an intermediate?

Accepted Answer

$$O_3$$

Question 7

A catalyst in a reaction mechanism is characterized by which of the following?

Accepted Answer

It is consumed first and then produced in a later step.

Question 8

Which of the following statements about catalysts is TRUE?

Accepted Answer

Catalysts increase the rate constant $$k by $$lowering the activation energy.

Question 9

For the elementary step $$2A ightarrow \,	ext{products}$$, what is the rate law?

Accepted Answer

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

Question 10

Which of the following would most likely increase the rate of a chemical reaction?

Accepted Answer

Adding a catalyst

Question 11

In a reaction mechanism, why can't intermediates be included in the rate law for the overall reaction?

Accepted Answer

Because intermediates are not present at the start of the reaction.

Question 12

Which of the following is the correct rate law for the elementary step $$2A + B ightarrow \,	ext{products}$$?

Accepted Answer

$$	ext{rate} = k[A]^2[B]$$

Question 13

A student proposes a reaction mechanism for $$A + B ightarrow C$$ with the following steps:

Step 1: $$A + A ightarrow D ($$slow)
Step 2: $$D + B ightarrow C ($$fast)

If experimental evidence shows the rate law is $$	ext{rate} = k[A]^2$$, is this mechanism plausible?

Accepted Answer

Yes, because the slow step matches the experimental rate law.

Question 14

Which of the following is a characteristic of a heterogeneous catalyst?

Accepted Answer

It is in a different phase than the reactants.

Question 15

Consider the overall reaction $$NO + CO \rightarrow NO + CO_2$. Which of the following is true about the mechanism if NO_3$ is an intermediate?

Accepted Answer

$$NO_3 is $$produced in one step and consumed in another.

Question 16

Which of the following would NOT be a plausible elementary step in a reaction mechanism?

Accepted Answer

$$A + B + C + D ightarrow \,	ext{products}$$

Question 17

If the slow step in a reaction mechanism is $$A + B \rightarrow C$$, and the fast step is $$C + D \rightarrow E$$, what is the rate law for the overall reaction?

Accepted Answer

$$	ext{rate} = k[A][B]$$

Question 18

Which of the following best explains why the rate law for a reaction matches the slowest step in the mechanism?

Accepted Answer

Because the slowest step determines the overall rate of product formation.

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