Question 1

Which of the following statements best explains why not all collisions between reactant molecules lead to a chemical reaction, as illustrated in the first image?

Accepted Answer

The orientation of the molecules during collision must be correct for bonds to break and form.

Question 2

Refer to the second image. What does the peak of the energy diagram represent in the context of a chemical reaction?

Accepted Answer

The transition state (activated complex) of the reaction.

Question 3

In the third image, what does the symbol $$E_a$$ represent?

Accepted Answer

The activation energy required for the reaction to proceed.

Question 4

Based on the fourth image, how does increasing temperature affect the fraction of molecules with sufficient energy to react?

Accepted Answer

It increases the fraction of molecules with energy greater than $$E_a.$$

Question 5

In the fifth image, what does the valley between the two peaks represent in a multi-step reaction energy diagram?

Accepted Answer

An intermediate species formed during the reaction.

Question 6

The sixth image shows a plot of $$\ln k$$ versus $$1/T. $$According to the Arrhenius equation, what does the slope of this line represent?

Accepted Answer

The negative activation energy divided by the gas constant, $$-E_a/R.$$

Question 7

Refer to the seventh image. Why is the orientation of reactant molecules critical in forming the activated complex?

Accepted Answer

Only certain orientations allow for the correct atoms to interact and form new bonds.

Question 8

In the context of catalysis, as shown in the eighth image, what is the role of a catalyst in a chemical reaction?

Accepted Answer

It provides an alternative pathway with a lower activation energy.

Question 9

Based on the ninth image, which step determines the overall rate of a multi-step reaction?

Accepted Answer

The slowest step, also known as the rate-determining step.

Question 10

Refer to the tenth image. If $$E_{a,1} \u003e E_{a,2}$$, which step is the rate-limiting step and why?

Accepted Answer

Step 1, because it has the higher activation energy and thus a smaller rate constant.

Question 11

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

Accepted Answer

A substance that is produced in one step and consumed in a later step.

Question 12

In the context of surface catalysis (see the image with ethylene and Ni surface), what is the first step in the catalytic process?

Accepted Answer

Adsorption of the reactant onto the catalyst surface.

Question 13

Which of the following is true about the effect of a catalyst on the energy profile of a reaction?

Accepted Answer

It lowers the activation energy, providing a new pathway for the reaction.

Question 14

Given a reaction with the observed rate law $$r = k[NO_2$][CO]$$, which of the following mechanisms is consistent with this rate law?

Accepted Answer

A single-step mechanism: $$NO_2 + CO$$ \rightarrow NO + $$CO_2$$

Question 15

Which of the following best describes the difference between a catalyst and an intermediate?

Accepted Answer

A catalyst is consumed and then produced; an intermediate is produced and then consumed.

Question 16

According to the Arrhenius equation $$k = Ae$$^{-E_a/RT}$$, what happens to the rate constant k$ as the activation energy E_a$ increases, assuming temperature is constant?

Accepted Answer

The rate constant $$k$$ decreases.

Question 17

A reaction is observed to be first order in $$A$$ and zero order in $$B. $$Which of the following rate laws is consistent with this observation?

Accepted Answer

$$r = k[A]$$

Question 18

Which of the following statements about the rate-determining step is correct?

Accepted Answer

It is the slowest step and controls the overall reaction rate.

Question 19

In a two-step reaction mechanism, if the first step is fast and reversible and the second step is slow, how is the rate law typically derived?

Accepted Answer

By using the rate law for the slow step and substituting the equilibrium expression for any intermediates.

Question 20

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

Accepted Answer

It is in the same phase as the reactants, often as a dissolved particle or gas.

L22 Reaction Mechanisms and Catalysis: Study Notes for General Chemistry

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