Q: The overall enthalpy change ($$\Delta H^\circ_{\text{rxn}}) $$for a reaction can be calculated using bond dissociation energies (BDE) as follows:

$$\Delta H^\circ_{\text{rxn}} = \sum \text{BDE of bonds broken} - \sum \text{BDE of bonds formed}$$

Question 1

Which of the following statements best describes the initiation step in the free-radical halogenation of alkanes?

Accepted Answer

It is the step where a halogen molecule absorbs energy and splits into two halogen radicals.

Question 2

Given the following reaction: $$\mathrm{CH_4$ + Cl_2$ \xrightarrow{hv} CH_3Cl + HCl}$$, what is the role of light ($$hv) in $$this reaction?

Accepted Answer

It provides energy to break the Cl-Cl bond homolytically, generating chlorine radicals.

Question 3

Which of the following best explains why tertiary free radicals are more stable than primary free radicals?

Accepted Answer

Tertiary radicals have more hyperconjugation and alkyl group stabilization.

Question 4

The overall enthalpy change ($$\Delta H^\circ_{	ext{rxn}}) $$for a reaction can be calculated using bond dissociation energies (BDE) as follows:

Accepted Answer

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

Question 5

Which of the following is true regarding the relationship between the equilibrium constant ($$K_{eq}) $$and the standard free energy change ($$\Delta G^\circ$$)?

Accepted Answer

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

Question 6

In the free-radical halogenation of propane, which hydrogen is most likely to be abstracted and why?

Accepted Answer

A secondary hydrogen, because secondary radicals are more stable than primary radicals.

Question 7

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

Accepted Answer

It is the step with the highest activation energy and thus the slowest step.

Question 8

According to the Hammond Postulate, the structure of the transition state in an exothermic reaction most closely resembles:

Accepted Answer

The reactants, because the transition state is closer in energy and structure to the reactants.

Question 9

Which of the following statements about activation energy ($$E_a) is $$correct?

Accepted Answer

$$E_a is $$the minimum kinetic energy required for molecules to react and reach the transition state.

Question 10

A catalyst increases the rate of a chemical reaction by:

Accepted Answer

Lowering the activation energy ($$E_a) of $$the reaction.

Question 11

Which of the following is the correct rate law for the reaction $$\mathrm{CH_3Br + OH^- \rightarrow CH_3OH + Br^-}$$, if the reaction is first order in both $$\mathrm{CH_3Br}$$ and $$\mathrm{OH^-}$$?

Accepted Answer

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

Question 12

Which of the following best describes the propagation step in the free-radical halogenation of alkanes?

Accepted Answer

It is the step where a radical reacts with a stable molecule to produce a new radical and a new molecule.

Question 13

If the standard enthalpy change ($$\Delta H^\circ) $$for a reaction is negative and the standard entropy change ($$\Delta S^\circ) is $$also negative, under what conditions will the reaction be spontaneous?

Accepted Answer

At low temperatures, because the negative $$\Delta H^\circ$$ will dominate over the $$-T\Delta S^\circ$$ term.

Question 14

Which of the following statements about bond dissociation energy (BDE) is correct?

Accepted Answer

BDE is the amount of energy required to break a bond homolytically.

Question 15

In the free-radical halogenation of ethane, how many different mono-halogenated products are formed?

Accepted Answer

One, because all hydrogens in ethane are equivalent.

Question 16

Which of the following is the correct expression for the change in free energy ($$\Delta G^\circ) in $$terms of enthalpy and entropy?

Accepted Answer

$$\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$$

Question 17

Which of the following best explains why reactions favor products with the greatest entropy?

Accepted Answer

Because systems tend to move toward greater disorder, increasing the number of possible microstates.

Question 18

A reaction has $$\Delta H^\circ = -100\ \mathrm{kJ/mol}$$ and $$\Delta S^\circ = -200\ \mathrm{J/(mol\cdot K)}. At $$what temperature does the reaction become non-spontaneous?

Accepted Answer

$$T \u003e 500\ \mathrm{K}$$

Question 19

Which of the following is a correct statement about the termination step in a free-radical chain reaction?

Accepted Answer

It involves the combination of two radicals to form a stable molecule, ending the chain reaction.

Study of Chemical Reactions: Free-Radical Halogenation, Thermodynamics, Kinetics, and Free Radical Stability

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