Question 1

Which of the following statements best describes the role of a catalyst in a chemical reaction?

Accepted Answer

A catalyst lowers the activation energy $$E_a$$ without affecting the energies of reactants or products.

Question 2

During the chlorination of methane, which step is responsible for generating the first reactive intermediate?

Accepted Answer

Initiation step

Question 3

Given the following bond dissociation energies: $$	ext{CH}_3	ext{–H} = 104$$ kcal/mol, $$	ext{Cl–Cl} = 58$$ kcal/mol, $$	ext{CH}_3	ext{–Cl} = 84$$ kcal/mol, $$	ext{H–Cl} = 103$$ kcal/mol, calculate the enthalpy change $$\Delta H^\circ$$ for the reaction $$	ext{CH}_4 + 	ext{Cl}_2 ightarrow 	ext{CH}_3	ext{Cl} + 	ext{HCl}.$$

Accepted Answer

$$-25$$ kcal/mol

Question 4

Which of the following best explains why bromination of propane is more selective than chlorination?

Accepted Answer

Bromination is endothermic and has a product-like (late) transition state, making it more selective.

Question 5

Which of the following is the correct expression for the equilibrium constant $$K_{eq}$$ for the reaction $$A + B \rightleftharpoons C + D$$?

Accepted Answer

$$K_{eq} = \dfrac{[C][D]}{[A][B]}$$

Question 6

If the standard Gibbs free energy change $$\Delta G^\circ$$ for a reaction is negative, what does this imply about the reaction?

Accepted Answer

The reaction is thermodynamically favored and will proceed spontaneously.

Question 7

Which of the following radicals is the most stable?

Accepted Answer

Tertiary (3°) radical

Question 8

In the free-radical chlorination of propane, why are secondary hydrogens abstracted more often than primary hydrogens?

Accepted Answer

The secondary radical and its transition state are lower in energy than the primary radical and its transition state.

Question 9

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

Accepted Answer

The slowest step, which has the highest energy barrier

Question 10

Which of the following is the correct formula for calculating the standard Gibbs free energy change $$\Delta G^\circ$$ from the equilibrium constant $$K_{eq}$$?

Accepted Answer

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

Question 11

Which of the following statements about carbocations is TRUE?

Accepted Answer

Carbocations are stabilized by electron-donating groups.

Question 12

Which of the following is a correct description of the transition state in a chemical reaction?

Accepted Answer

It is the highest-energy state along the reaction coordinate and cannot be isolated.

Question 13

Which of the following best explains why the chlorination of methane is mostly driven by enthalpy change?

Accepted Answer

The enthalpy change $$\Delta H^\circ is $$large and negative, while the entropy change $$T\Delta S^\circ is $$small.

Question 14

Which of the following is the correct rate law for the reaction $$	ext{CH}_3	ext{Br} + 	ext{OH}^- ightarrow 	ext{CH}_3	ext{OH} + 	ext{Br}^-$$, given that it is first order in each reactant?

Accepted Answer

$$	ext{rate} = k[	ext{CH}_3	ext{Br}][	ext{OH}^-]$$

Question 15

Which of the following statements about free radicals is FALSE?

Accepted Answer

Free radicals are strong nucleophiles.

Question 16

Which of the following is the correct formula for calculating the enthalpy change $$\Delta H^\circ of a $$reaction using bond dissociation energies?

Accepted Answer

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

Question 17

Which of the following is a real-world product formed by a free radical polymerization reaction?

Accepted Answer

Polyethylene plastic bags

Question 18

Which of the following statements about carbanions is TRUE?

Accepted Answer

Carbanions are stabilized by electron-withdrawing groups.

Question 19

Which of the following is the correct order of stability for carbocations?

Accepted Answer

3° \u003e 2° \u003e 1° \u003e methyl

Chapter 4: Chemical Reactions – Mechanisms, Thermodynamics, and Kinetics in Organic Chemistry

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