Deck 4: The Study of Chemical Reactions

Question 1

In the reaction of Cl₂ with ethane and UV light, which of the following reactions would be a chain termination event(s)?
I. Cl∙ + CH₃-CH₃ → CH₃-CH₂-Cl + H∙
II. Cl∙ + CH₃-CH₃ → CH₃-H₂C∙ + HCl
III. Cl∙ + CH₃-H₂C∙ → CH₃-CH₂-Cl
IV. Cl₂ + CH₃-H₂C∙ → CH₃-CH₂-Cl + Cl∙
V. Cl_{2 + UV light} → C l∙ + Cl∙

A) reaction V
B) reactions I and IV
C) reactions III and IV
D) reactions I and II
E) reaction III

Answer 1

reaction III

Question 2

Which of the following species is $\underline{\text{ not }}$ formed through a termination reaction in the chlorination of methane?

A) CH₃Cl
B) HCl
C) H₂
D) CH₃CH₃

Answer 2

H₂

Question 3

Explain how the termination step in a free-radical chain reaction stops the chain.

Answer 3

In order for a free-radical chain to propagate, each mechanistic step must yield a free-radical species as one of its products. A mechanistic step that does not yield a free-radical stops the chain. Such a step is known as a termination step.

Question 4

In the reaction of Cl₂ with ethane and UV light, which of the following reactions would be a propagation event(s)?
I. Cl∙ + CH₃-CH₃ → CH₃-CH₂-Cl + H∙
II. Cl∙ + CH₃-CH₃ → CH₃-H₂C∙ + HCl
III. Cl∙ + CH₃-H₂C∙ → CH₃-CH₂-Cl
IV. Cl₂ + CH₃-H₂C∙ → CH₃-CH₂-Cl + Cl∙
V. Cl_{2 + UV light} → C l∙ + Cl∙

A) reactions I and V
B) reactions II, III and IV
C) reactions I and IV
D) reactions II and IV
E) reactions I, II and IV

Question 5

________ is the study of reaction rates.

Question 6

Species with unpaired electrons are called ________.

Question 7

When the reaction between methane and chlorine is photochemically initiated, which of the following compounds $\underline{\text{ cannot }}$ be formed through a termination reaction?

A) CH₃Cl
B) HCl
C) CH₃CH₃
D) Cl₂

Question 8

What is meant by the $\underline{\text{ mechanism }}$ of a chemical reaction?

Question 9

Chlorination of methane can result in a mixture of chlorinated products. What experimental conditions should be used to favor the production of chloromethane over the other chlorinated products?

Question 10

Write a detailed, stepwise mechanism for the following reaction.

Question 11

Write an equation to describe the initiation step in the chlorination of methane.

Question 12

Provide the two propagation steps in the free-radical chlorination of ethane.

Question 13

Within the visible spectrum, it has been experimentally determined that blue light is the most effective in initiating the chlorination of methane. What is the mechanistic significance of this observation?

Question 14

When light is shined on a mixture of chlorine and chloromethane, carbon tetrachloride is one of the components of the final reaction mixture. Propose a series of mechanistic steps which explain this observation.

Question 15

Which of the following is $\underline{\text{ not }}$ a possible termination step in the free radical chlorination of methane?

A) ∙CH₃ + Cl₂ → CH₃Cl + Cl∙
B) ∙CH₃ + Cl∙ → CH₃Cl
C) ∙CH₃ + ∙CH₃ → CH₃CH₃
D) ∙CH₃ + wall → CH₃-wall
E) Cl∙ + wall → Cl-wall

Question 16

The chlorination of methane is characterized by a high quantum yield. Explain what this means.

Question 17

In the first propagation step of the free radical chlorination of methane, which of the following occurs?

A) Cl₂ dissociates.
B) A chlorine radical abstracts a hydrogen.
C) A carbon radical reacts with Cl₂.
D) A carbon radical reacts with a chlorine radical.
E) Two chlorine radicals combine.

Question 18

For a given reaction, if ΔG° is greater than zero, then:

A) K_eq < 0.
B) K_eq = 0.
C) 0 < K_eq < 1.
D) K_eq = 1.
E) K_eq > 1.

Question 19

Which of the following is a propagation step in the free radical chlorination of dichloromethane?

A) ∙ CHCl₂ + Cl₂ → CHCl₃ + Cl∙
B) ∙ CHCl₂ + Cl∙ → CHCl₃
C) CH₂Cl₂ + Cl∙ → CHCl₃ + H∙
D) Cl₂ + UV light → 2 Cl∙
E) ∙ CHCl₂ + ∙ CHCl₂ → CHCl₂CHCl₂

Question 20

If ΔG° for a given reaction at 25°C is less than zero, which of the following statements also correctly describes this reaction at this temperature?

A) The reaction must be exothermic.
B) The reaction must be endothermic.
C) K_eq is greater than zero.
D) Both A and C are true.
E) Both B and C are true.

Question 21

Given a K of 0.45 at 25°C, calculate the corresponding DG° in kJ/mol. [R = 8.314 J/K∙ mol]

Question 22

Which is a measure of the randomness of a system?

A) entropy
B) enthalpy
C) free energy
D) halogenation
E) stoichiometry

Question 23

Given a K of 2.2 at 25°C, calculate the corresponding DG° in kJ/mol. [R = 8.314 J/K∙ mol]

Question 24

Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is +0.5 kcal/mol, the K_eq is ________ and the % conversion is ________.

A) 0.18, 15%
B) 0.43, 30%
C) 1.0, 50%
D) 2.3, 70%
E) 5.4, 84%

Question 25

Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is the K_eq is ________ and the % conversion is ________.

A) 0.18, 15%
B) 0.43, 30%
C) 1.0, 50%
D) 2.3, 70%
E) 5.4, 84%

Question 26

If the equilibrium constant (K_eq) of a reaction is 0.5 then which of the following that must be true?

A) The reaction will have an early transition state.
B) Reaction equilibrium will favor the products.
C) Gibbs free energy (G) is positive.
D) Gibbs free energy (G) is negative.

Question 27

For the reaction A + B → C + D, ΔG° = -5.00 kcal/mol. What is the corresponding equilibrium constant at 25°C? R = 1.987 cal/mol∙K.

Question 28

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

A) ΔH° > 0 and ΔS° > 0
B) ΔH° > 0 and ΔS° < 0
C) ΔH° < 0 and ΔS° > 0
D) ΔH° < 0 and ΔS° < 0
E) ΔH° = 0 and ΔS° = 0

Question 29

Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is -0.5 kcal/mol, the K_eq is ________ and the % conversion is ________.

A) 0.18, 15%
B) 0.43, 30%
C) 1.0, 50%
D) 2.3, 70%
E) 5.4, 84%

Question 30

Provided the following pK_a values for the two acids below, draw an energy diagram
for the acid-base reaction. Be sure to label each axis correctly.

Question 31

If a reaction is exothermic, then:

A) ΔS° < 0.
B) ΔS° > 0.
C) ΔH° < 0.
D) ΔH° > 0.
E) both B and D

Question 32

Consider the transformation of A to B (i.e., A →
B). If at equilibrium at 25°C the concentration of A is 20% of the initial concentration of A, determine the value of ΔG° (in kcal/mol) for this reaction.
R = 1.987 cal/mol∙K.

Question 33

Which of the following correctly expresses the standard Gibbs free energy change of a reaction in terms of the changes in enthalpy and entropy?

A) ΔG° = ΔH° - TΔS°
B) ΔG° = ΔH° + TΔS°
C) ΔG° = ΔS° - TΔH°
D) ΔG° = ΔS° + TΔH°
E) none of the above

Question 34

Given a DG° of 0.8 kJ/mol at 25°C for the equilibrium shown below, calculate the percentage of the axial conformer at 25°C. [R = 8.314 J/K∙ mol]

Question 35

Given a reaction in which reactant A is converted only to product B at 25°C, what K_eq results if at equilibrium 80% of A has become B?

Question 36

Which of the following statements correctly describes the contribution of ΔS° to ΔG°?

A) The entropy term makes a greater contribution to ΔG° at low temperatures.
B) The entropy term makes a greater contribution to ΔG° at high temperatures.
C) The entropy term makes a greater contribution to ΔG° in exothermic reactions.
D) The entropy term makes a greater contribution to ΔG° in endothermic reactions.
E) The entropy term always makes a more significant contribution to ΔG° than does the enthalpy term.

Question 37

In an exothermic reaction, are stronger bonds broken and weaker bonds formed or are weaker bonds broken and stronger bonds formed?

Question 38

Consider the following substitution reaction with a ΔG° value of -91.1 kJ/mole.
HO- + CH₃Cl ↔ CH₃OH + Cl-
Given this information which of the following statements must be true?
(R = 8.315 J/mole K)

A) The Keq at 25°C for this reaction is very large, in other words this reaction proceeds to near completion as written, left to right under standard conditions.
B) The Keq at 25°C for this reaction is very small (<1), in other words this reaction does not proceed from left to right but rather is favored from right to left under standard conditions .
C) At 250°C the equilibrium concentration is shifted right in favor of the products (CH₃OH and Cl-). In other words there is more product than at 25°C.
D) At 250°C the equilibrium concentration of products and reactants is nearly the same.
E) Both A and C are correct.

Question 39

Given a DG° of -8.0 kJ/mol at 25°C, calculate the corresponding K. [R = 8.314 J/K∙ mol]

Question 40

Assume the reaction A + B → C + D proceeds to equilibrium. Calculate the equilibrium concentration of D at 25°C, given that the starting concentrations of A and B are 2M and that ΔG° for the reaction is 1.0 kcal/mol. R = 1.987 cal/mol∙K.

A) 0.40M
B) 0.60M
C) 1.00M
D) 1.40M
E) 1.60M

Question 41

Do you expect the initiation step in the free radical chlorination of 2,2-dimethylpropane to have a positive or negative DS? Explain briefly.

Question 42

Energy is ________ when bonds are formed and is ________ when bonds are broken; therefore, bond dissociation energies are always ________.

A) released / consumed / exothermic
B) released / consumed / endothermic
C) consumed / released / exothermic
D) consumed / released / endothermic
E) consumed / released / isothermic

Question 43

If stronger bonds are formed and weaker bonds are broken, then the reaction is ________.

Question 44

Does one expect ΔS° in a propagation step of the free-radical chlorination of methane to be greater than zero, less than zero, or approximately equal to zero? Briefly explain your choice.

Question 45

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

A) ΔH° > 0 and ΔS° > 0
B) ΔH° > 0 and ΔS° < 0
C) ΔH° < 0 and ΔS° > 0
D) ΔH° < 0 and ΔS° < 0
E) ΔH° = 0 and ΔS° = 0

Question 46

Consider the bond dissociation energies listed below in kcal/mol.
These data show that the carbon-bromine bond is weakest when bromine is bound to a ________.

A) methyl carbon
B) primary carbon
C) secondary carbon
D) tertiary carbon
E) quaternary carbon

Question 47

Consider the elementary step in the solvolysis of isopropyl chloride shown below and write the rate equation for this step.
(CH₃)₂CHCl → (CH₃)₂CH⁺ + Cl^-

Question 48

What reactive species is produced in the initiation step of the free radical chlorination of 2,2-dimethylpropane?

A) a chlorine atom
B) a chlorine radical anion
C) a carbon radical
D) a carbocation

Question 49

Predict the sign of DS° in the combustion of propane.

Question 50

Which of the presented mechanisms would be the most energetically favorable and thus the most likely mechanism to actually occur for the following free radical chain reaction? (bond dissociation energies -- H-H = 104 kcal/mol; Cl-Cl = 58 kcal/mol; H-Cl = 103 kcal/mol)
H₂ + Cl₂
2 HCl

A) H₂
H∙ + H∙
H∙ + Cl₂ → Cl∙ + HCl
H∙ + Cl∙ → HCl
B) Cl₂
Cl∙ + Cl∙
Cl∙ + H₂ → H∙ + HCl
H∙ + Cl₂ → HCl + Cl∙
C) H₂
H∙ + H∙
H∙ + Cl₂ → Cl∙ + HCl
Cl∙ + H₂ → HCl + H∙
D) Cl₂ Cl∙ + Cl∙
Cl∙ + H₂ → H∙ + HCl
H∙ + Cl∙ → HCl

Question 51

Predict the enthalpy (ΔH) value for the theoretical reaction below, and indicate whether it is endothermic or exothermic. The bond dissociation energy for each bond in Kcal/mol is shown below each reactant and product.

A) +8 Kcal/mol, endothermic
B) -8 Kcal/mol, exothermic
C) +16 Kcal/mol, endothermic
D) +8 Kcal/mol, exothermic

Question 52

The hydrogenation of acetylene to produce ethane is shown below. Is ΔS° for this reaction positive, negative, or impossible to predict? Explain your reasoning.
C₂H₂ (g) + 2H₂ (g) → C₂H₆ (g)

Question 53

Which compound has the smaller bond dissociation energy for its carbon-chlorine bond, CH₃Cl or (CH₃)₃CCl? Explain your reasoning.

Question 54

Of the two C-H bonds shown, which has the smaller bond dissociation energy? Explain your choice.
(CH₃)₂CH-H vs. CH₃CH₂-H

Question 55

Given the bond dissociation energies below (in kcal/mol), calculate the overall ΔH° for the following reaction:
(CH₃)₃CH + Br₂ → (CH₃)₃CBr + HBr
(CH₃)₃C-H 91
(CH₃)₃C-Br 65
Br-Br 46
H-Br 88
CH₃-Br 70

Question 56

Predict the signs of DH° and DS° in the reaction of cyclohexene with H₂ to form cyclohexane.

Question 57

Given the bond dissociation energies below (in kcal/mol), estimate the ΔH° for the propagation step

CH₃CH₂CH₂-H 98
(CH₃)₂CH-H 95
Cl-Cl 58
H-Cl 103
CH₃CH₂CH₂-Cl 81
(CH₃)₂CH-Cl 80

A) -22 kcal/mol
B) +22 kcal/mol
C) -40 kcal/mol
D) +45 kcal/mol

Question 58

Do you expect the initiation step in the free radical chlorination of 2,2-dimethylpropane to be endo- or exothermic? Explain briefly, and comment on the sign of DH.

Question 59

The bond dissociation energy is the amount of energy required to break a bond ________.

A) homolytically
B) heterolytically
C) so as to produce the more stable pair of ions
D) via hydrogenation
E) none of the above

Question 60

Given the chlorination of acetone shown below, choose the correct rate law. CH₃COCH₃ + Cl₂ → CH₃COCH₂Cl + HCl

A) rate = [CH₃COCH₃]
B) rate = [Cl₂]
C) rate = [CH₃COCH₃][Cl₂]
D) rate = [CH₃COCH₃][Cl₂]^1/2
E) cannot be determined from stoichiometry; must be determined experimentally

Question 61

Explain the significance of the exponential factor e^-Ea/RT in the Arrhenius equation.

Question 62

The difference in energy between reactants and the transition state is known as ________.

Question 63

Consider the one-step conversion of F to G. Given that the reaction is endothermic by 5 kcal/mol and that the energy difference between G and the transition state for the process is 15 kcal/mol, sketch a reaction-energy diagram for this reaction. Make sure to show how the given energy differences are consistent with your sketch.

Question 64

Explain the significance of the frequency factor A in the Arrhenius equation.

Question 65

In the hydrocarbon shown below, how many tertiary hydrogens are present?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 66

Consider the conversion of C to D via a one-step mechanism. The activation energy of this conversion is 3 kcal/mol. The energy difference between D and the transition state of the reaction is 7 kcal/mol. Estimate ΔH° for the reaction C → D.

Question 67

Consider the conversion of X to Z through the sole intermediate Y. Given the reaction-energy diagram shown below, which step is the rate-limiting step? Explain your reasoning.

Question 68

Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below.

A → B E_a = 15 kcal/mol, ΔH° = 13 kcal/mol
B → C E_a = 10 kcal/mol, ΔH° = -6 kcal/mol
C → D E_a = 2 kcal/mol, ΔH° = -20 kcal/mol
Which of the three steps is rate-limiting?

A) The reaction of A to B.
B) The reaction of B to C.
C) The reaction of C to D.
D) All three steps occur at the same rate; there is no rate-limiting step.
E) The most exothermic step is rate-limiting.

Question 69

Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below.

A → B E_a = 15 kcal/mol, ΔH° = 13 kcal/mol
B → C E_a = 10 kcal/mol, ΔH° = -6 kcal/mol
C → D E_a = 2 kcal/mol, ΔH° = -20 kcal/mol
What's the enthalpy difference between reactant A and intermediate C?

Question 70

Which of the following correctly expresses the standard Gibbs free energy change of a reaction in terms of the reaction temperature (T) and equilibrium constant (K)?

A) ΔG° = e^-K/RT
B) ΔG° = e^K/RT
C) ΔG° = RTlnK
D) ΔG° = -RTlnK
E) none of the above

Question 71

Consider the reaction: CH₃CH₂∙ + Br₂ → CH₃CH₂Br + Br∙ .
Given that this reaction has an activation energy of +6 kcal/mol and a ΔH° of -22 kcal/mol, sketch a reaction-energy diagram for this reaction. Label the axes and show E_a and ΔH° on your drawing.

Question 72

Consider the reaction (CH₃)₃CBr + CH₃CH₂OH → (CH₃) ₃COCH₂CH₃ + HBr. Experimentally one finds that if the concentration of (CH₃)₃CBr is tripled, the rate of the reaction triples. One also finds that if the concentration of CH₃CH₂OH is doubled, the rate of the reaction is unchanged. Which of the following correctly describes the kinetics of this reaction?

A) The reaction is third order in (CH₃)₃CBr.
B) The reaction is first order in CH₃CH₂OH.
C) The reaction is second order overall.
D) The reaction is first order overall.
E) none of the above

Question 73

What term describes the highest-energy structure in a molecular collision which leads to reaction?

Question 74

Given that the theoretical reaction below was found to be second order and bimolecular, provide a rate equation for the reaction.
A-B + C-D → A-C + B-D

Question 75

Given an activation energy of 15 kcal/mol, use the Arrhenius equation to estimate how much faster the reaction will occur if the temperature is increased from 100°C to 120°C. R = 1.987 cal/mol∙K.

Question 76

________ is the minimum kinetic energy reacting molecules must possess to overcome the repulsions between their electron clouds when they collide.

Question 77

The Arrhenius equation mathematically models which of the following statements?

A) The rate of a chemical reaction increases exponentially with increasing concentration of reactants.
B) The rate of a chemical reaction is directly related to the Ea and that increasing the temperature will alter the Ea for that reaction.
C) Increasing the temperature of a chemical reaction increases the number of particles in the reaction that have the minimum energy required to meet the Ea.
D) The rate of a chemical reaction is exponentially related to the Ea and relatively small differences in the Ea can dramatically affect the reaction rates of similar reactions at the same temperature.
E) both C and D

Question 78

The rate of a reaction typically increases as the temperature increases because:

A) the A term in the Arrhenius equation increases.
B) the fraction of molecules with kinetic energy greater than E_a increases.
C) the activation energy decreases.
D) the activation energy increases.
E) the molecules make more collisions with the wall of the reaction vessel.

Question 79

Provide the structure of the transition state in the first propagation step of the free radical chlorination of ethane.

Question 80

The following reaction occurs readily: Experimentally one finds that if the concentration of I^- is doubled, the rate doubles. Also if the concentration of CH₃Br is halved, the rate is halved. What is the rate equation for this reaction?