$$\text{rate} = k[CH_3Br][OH^-]$$

Question 1

Which of the following best describes the role of light in the chlorination of methane?

Accepted Answer

Light provides the activation energy needed for the homolytic cleavage of the Cl-Cl bond, generating chlorine radicals.

Question 2

In the free-radical chain reaction mechanism for methane chlorination, which step is responsible for the formation of the methyl radical?

Accepted Answer

First propagation step

Question 3

Which of the following statements about reactive intermediates in free-radical reactions is correct?

Accepted Answer

Reactive intermediates are short-lived and react as quickly as they are formed.

Question 4

Which equation correctly relates the equilibrium constant $$K_{eq} to $$the standard free energy change $$\Delta G^\circ$$?

Accepted Answer

$$\Delta G^\circ = -RT \ln K_{eq}$$

Question 5

During the propagation steps of methane chlorination, which bond is broken and which bond is formed in the first propagation step?

Accepted Answer

A C-H bond is broken and an H-Cl bond is formed.

Question 6

Which of the following best explains why the concentration of free radicals remains low during a chain reaction?

Accepted Answer

Free radicals are consumed in propagation steps as quickly as they are formed.

Question 7

What is the effect of a catalyst on the activation energy and the energies of reactants and products in a chemical reaction?

Accepted Answer

A catalyst lowers the activation energy but does not change the energies of reactants and products.

Question 8

Which of the following is the correct rate law for a reaction that is first order in $$[CH_3Br]$$ and first order in $$[OH^-]$$?

Accepted Answer

$$	ext{rate} = k[CH_3Br][OH^-]$$

Question 9

Which of the following best describes homolytic cleavage?

Accepted Answer

Each atom receives one electron from the bond, forming two radicals.

Question 10

In the reaction-energy diagram for an endothermic reaction, which of the following is true?

Accepted Answer

The products are higher in energy than the reactants.

Question 11

Which of the following is a termination step in the free-radical chlorination of methane?

Accepted Answer

Two chlorine radicals combine to form Cl$$_2.$$

Question 12

Which of the following factors does NOT affect the rate of a chemical reaction?

Accepted Answer

The color of the reactants

Question 13

Which of the following best explains why free-radical chlorination of hexane gives poor yields of 1-chlorohexane compared to cyclohexane?

Accepted Answer

Hexane has more secondary hydrogens, leading to a mixture of products.

Question 14

Which of the following is true about the quantum yield in the light-initiated chlorination of methane?

Accepted Answer

Many molecules of product are formed for each photon of light absorbed.

Question 15

Which of the following is the correct expression for the overall enthalpy change $$\Delta H^\circ of a $$reaction using bond-dissociation enthalpies?

Accepted Answer

$$\Delta H^\circ = \sum 	ext{(BDE of bonds broken)} - \sum 	ext{(BDE of bonds formed)}$$

Question 16

Which of the following best describes the transition state in a chemical reaction?

Accepted Answer

It is a high-energy, transient configuration where bonds are partially broken and formed.

Question 17

If the activation energy for a reaction is increased, what is the expected effect on the rate of the reaction?

Accepted Answer

The rate of the reaction decreases.

Question 18

Which of the following is a correct statement about the rate-limiting step in a multistep reaction?

Accepted Answer

It determines the overall rate of the reaction.

Question 19

Which of the following is true about the effect of temperature on the rate of a chemical reaction?

Accepted Answer

Increasing temperature increases the number of molecules with enough energy to overcome the activation barrier.

Mechanisms, Thermodynamics, and Kinetics of Free-Radical Halogenation: Chlorination of Methane

Study Guide - Practice Questions