Question 1

Which of the following statements best describes the difference between homolytic and heterolytic bond cleavage?

Accepted Answer

Homolytic cleavage results in each atom taking one electron from the bond, forming radicals; heterolytic cleavage results in one atom taking both electrons, forming ions.

Question 2

Given the following bond dissociation energies: $$\mathrm{BDE_{C-H}} = 397\ \mathrm{kJ/mol}$$, $$\mathrm{BDE_{Br-Br}} = 193\ \mathrm{kJ/mol}$$, $$\mathrm{BDE_{C-Br}} = 285\ \mathrm{kJ/mol}$$, $$\mathrm{BDE_{H-Br}} = 368\ \mathrm{kJ/mol}$$, calculate $$\Delta H$$ for the reaction: $$\mathrm{CH_4$} + \mathrm{Br_2$} \rightarrow \mathrm{CH_3Br} + \mathrm{HBr}.$$

Accepted Answer

$$\Delta H = -63\ \mathrm{kJ/mol}$$

Question 3

Which of the following is true for an exothermic reaction?

Accepted Answer

The enthalpy change $$\Delta H is $$negative.

Question 4

A reaction has $$\Delta H = +50\ \mathrm{kJ/mol}$$ and $$\Delta S = +100\ \mathrm{J/(mol\cdot K)}. At $$what temperature will the reaction become spontaneous?

Accepted Answer

At temperatures above $$500\ \mathrm{K}$$

Question 5

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

Accepted Answer

Concentration of reactants

Question 6

For the reaction $$A + B \rightarrow C + D$$, the rate law is $$\mathrm{Rate} = k[A][B]. If $$the concentration of $$A is $$doubled and $$B is $$tripled, by what factor does the rate increase?

Accepted Answer

6

Question 7

Which statement best explains why a spontaneous reaction may occur very slowly?

Accepted Answer

The activation energy is high, so few molecules have enough energy to react.

Question 8

Which of the following is a 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 9

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

Accepted Answer

The reactants

Question 10

Which of the following species is most likely to act as a nucleophile in a polar reaction?

Accepted Answer

$$\mathrm{Cl^-}$$

Question 11

Which of the following is a correct description of a carbocation rearrangement?

Accepted Answer

A carbocation rearranges to a more stable carbocation via a 1,2-hydride or 1,2-methyl shift.

Question 12

Which of the following is true regarding the use of curved arrows in reaction mechanisms?

Accepted Answer

The tail of the arrow starts on a pair of electrons and the head ends on a nucleus.

Question 13

Which of the following processes is always reversible under typical conditions?

Accepted Answer

Loss of a leaving group

Question 14

Which of the following best describes the effect of a catalyst on a chemical reaction?

Accepted Answer

It lowers the activation energy, increasing the reaction rate.

Question 15

A reaction has $$\Delta G = -17\ \mathrm{kJ/mol}. $$What is the approximate equilibrium constant $$K_{eq}$$ for this reaction at $$298\ \mathrm{K}$$?

Accepted Answer

$$K_{eq} \approx 10^6$

Question 16

Which of the following is NOT a factor that affects the rate of a chemical reaction?

Accepted Answer

Gibbs free energy change ($$\Delta G)$$

Question 17

Which of the following statements about nucleophilic attack is correct?

Accepted Answer

The nucleophile donates a pair of electrons to an electrophile.

Question 18

Which of the following is true about the entropy change ($$\Delta S) in a $$chemical reaction?

Accepted Answer

Entropy always increases when a cyclic compound becomes acyclic.

Question 19

Which of the following is a correct statement about the relationship between kinetics and thermodynamics?

Accepted Answer

Kinetics describes the rate of reaction, while thermodynamics describes the equilibrium position.

Question 20

Which of the following is most likely to be an irreversible process?

Accepted Answer

Carbocation rearrangement to a more stable carbocation

Chemical Reactivity and Mechanisms – Enthalpy, Bond Dissociation, and Energy Diagrams

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