Q: A reaction has $$\Delta G^\circ = -5.00$$ kcal/mol at $$25^\circC. $$What is the equilibrium constant $$K_{eq} at $$this temperature? ($$R = 1.987$$ cal/mol·K)

$$K_{eq} \approx 5.2 \times 10^3$

Question 1

Which of the following statements best describes the initiation step in the free-radical chlorination of methane?

Accepted Answer

It involves the homolytic cleavage of a chlorine molecule to generate two chlorine atoms.

Question 2

Which of the following is NOT a possible termination step in the free radical chlorination of methane?

Accepted Answer

$$\mathrm{CH_3$^\cdot + Cl_2$ \rightarrow CH_3Cl + Cl^\cdot}$$

Question 3

For the reaction $$\mathrm{CH_4$ + Cl_2$ ightleftharpoons CH_3Cl + HCl}$$, the equilibrium constant $$K_{eq} is$$ $$1.1 	imes 10$$^{19}$$. What does this indicate about the reaction?

Accepted Answer

The reaction strongly favors the formation of products.

Question 4

Which of the following best explains why tertiary hydrogens are more reactive than primary hydrogens in free radical chlorination?

Accepted Answer

Tertiary free radicals are more stable due to greater alkyl substitution.

Question 5

Given the following bond dissociation energies (in kcal/mol): $$A-B = 63$$, $$C-D = 88$$, $$A-C = 47$$, $$B-D = 96$$, calculate $$\Delta H$$ for the reaction $$A-B + C-D \rightarrow A-C + B-D$$ and state whether it is endothermic or exothermic.

Accepted Answer

$$+8$$ kcal/mol, endothermic

Question 6

Which of the following best describes the effect of increasing temperature on the rate constant $$k$$ according to the Arrhenius equation?

Accepted Answer

Increasing temperature increases $$k$$ because more molecules have energy greater than $$E_a.$$

Question 7

Which of the following is a propagation step in the free radical chlorination of dichloromethane ($$\mathrm{CH_2Cl_2}$$)?

Accepted Answer

$$\mathrm{$$CHCl_2$$^\cdot + Cl_2$ \rightarrow $$CHCl_3$$ + Cl^\cdot}$$

Question 8

A reaction has $$\Delta G^\circ = -5.00$$ kcal/mol at $$25^\circC. $$What is the equilibrium constant $$K_{eq} at $$this temperature? ($$R = 1.987$$ cal/mol·K)

Accepted Answer

$$K_{eq} \approx 5.2 	imes 10^3$

Question 9

Which of the following best describes the rate-determining step in a multi-step reaction?

Accepted Answer

It is the step with the highest activation energy ($$E_a).$$

Question 10

Which of the following statements about the Hammond Postulate is correct?

Accepted Answer

The structure of the transition state resembles the species (reactant or product) to which it is closer in energy.

Question 11

Which of the following reactive intermediates is stabilized by resonance when adjacent to a double bond?

Accepted Answer

Both carbocations and free radicals

Question 12

Which of the following best explains why free radical inhibitors are added to foods?

Accepted Answer

They combine with free radicals to form stable molecules, preventing spoilage.

Question 13

Which of the following species is most stable?

Accepted Answer

Tertiary carbocation

Question 14

Which of the following statements about carbanions is correct?

Accepted Answer

Carbanions are more stable when less substituted.

Question 15

Which of the following best describes the hybridization of the carbon atom in a methyl carbanion ($$\mathrm{CH_3$^-}$$)?

Accepted Answer

$$sp^3$$

Question 16

Which of the following is true for the termination step of a free radical chlorination reaction?

Accepted Answer

$$\Delta H^\circ \u003c 0$$ and $$\Delta S^\circ \u003c 0$$

Question 17

Which of the following best describes the effect of a free radical inhibitor on a chain reaction?

Accepted Answer

It prevents propagation by removing free radicals from the reaction mixture.

Question 18

Which of the following is the correct order of stability for free radicals?

Accepted Answer

Tertiary $$\u003e$$ Secondary $$\u003e$$ Primary $$\u003e$$ Methyl

Question 19

Which of the following best describes the hybridization of the carbon atom in a carbene?

Accepted Answer

$$sp^2$$

Question 20

Which of the following is a real-life application of the free radical chain reaction mechanism?

Accepted Answer

Ozone depletion in the atmosphere by chlorofluorocarbons (CFCs)

Chapter 4: The Study of Chemical Reactions – Mechanisms, Thermodynamics, and Kinetics

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