Chapter 4 Flashcards
Which of the following species is not formed through a termination reaction in the chlorination of methane? A) CH3Cl B) HCl C) H2 D) CH3CH3
C) H2
Which of the following is 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
A) ∙CH3 + Cl2 → CH3Cl + Cl∙
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.
B) A chlorine radical abstracts a hydrogen.
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) Cl2 + 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
E) reaction III
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) Cl2 + 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
D) reactions II and IV
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
A) ∙ CHCl2 + Cl2 → CHCl3 + Cl∙
When the reaction between methane and chlorine is photochemically initiated, which of the following compounds cannot be formed through a termination reaction? A) CH3Cl B) HCl C) CH3CH3 D) Cl2
B) HCl
For a given reaction, if ΔG° is greater than zero, then: A) Keq < 0. B) Keq = 0. C) 0 < Keq < 1. D) Keq = 1. E) Keq > 1.
C) 0 < Keq < 1.
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) Keq is greater than zero. D) Both A and C are true. E) Both B and C are true.
C) Keq is greater than zero.
If the equilibrium constant (Keq) 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.
C) Gibbs free energy (G) is positive.
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 (
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.
Which is a measure of the randomness of a system? A) entropy B) enthalpy C) free energy D) halogenation E) stoichiometry
A) entropy
In an _____ reaction, weaker bonds are broken and stronger bonds are
exothermic
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.
B) The entropy term makes a greater contribution to ΔG° at high temperatures.
If a reaction is exothermic, then: A) ΔS° < 0. B) ΔS° > 0. C) ΔH° < 0. D) ΔH° > 0. E) both B and D
C) ΔH° < 0.
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
A) ΔG° = ΔH° - TΔS°
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
D) ΔH°
A) ΔH° > 0 and ΔS° > 0
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
D) ΔH°
D) ΔH°
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
A) homolytically
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
A) a chlorine atom
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
B) released / consumed / endothermic
Consider the bond dissociation energies listed below in kcal/mol.
CH3-Br 70
CH3CH2-Br 68
(CH3)2CH-Br 68
(CH3)3C-Br 65
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
D) tertiary carbon
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
E) cannot be determined from stoichiometry; must be determined experimentally
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
D) The reaction is first order overall.
________ is the minimum kinetic energy reacting molecules must possess to overcome the repulsions between their electron clouds when they collide.
Ea
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
E) both C and D
What term describes the highest-energy structure in a molecular collision which leads to reaction?
transition state
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.
-4 kcal/mol
Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below.
A → B Ea = 15 kcal/mol, ΔH° = 13 kcal/mol
B → C Ea = 10 kcal/mol, ΔH° = -6 kcal/mol
C → D Ea = 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.
A) The reaction of A to B.
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 Ea increases.
C) the activation energy decreases.
D) the activation energy increases.
E) the molecules make more collisions with the wall of the reaction vessel.
B) the fraction of molecules with kinetic energy greater than Ea increases.
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° = eK/RT C) ΔG° = RTlnK D) ΔG° = -RTlnK E) none of the above
D) ΔG° = -RTlnK
How many distinct dichlorination products can result when isobutane is subjected to free radical chlorination? A) 1 B) 2 C) 3 D) 4 E) 6
C) 3
How many distinct monochlorinated products can result when isobutane is subjected to free radical chlorination? A) 1 B) 2 C) 3 D) 4 E) 5
B) 2
How many distinct monochlorinated products can result when cyclopentane is subjected to free radical chlorination? A) 1 B) 2 C) 3 D) 4 E) 5
A) 1
The major monobrominated product which results when ethylcyclohexane is subjected to free radical bromination is: A) a primary bromide. B) a secondary bromide. C) a tertiary bromide. D) a quaternary bromide. E) bromomethane.
C) a tertiary bromide.
Which of the halogens below undergoes free radical halogenation with ethane most rapidly? A) fluorine B) chlorine C) iodine D) bromine E) pyridine
A) fluorine
Which of the following statements is the best statement of the Hammond Postulate?
A) In an endothermic reaction, the transition state is closer to the reactants in structure.
B) In an exothermic reaction, the transition state is closer in energy to the products.
C) Related species that are similar in energy are also similar in structure.
D) The structure of the transition state in an organic reaction is always modeled on the structure of the reactants leading to that transition state.
E) Transition states are molecular species of finite lifetime whose properties can be probed through free radical reactions.
C) Related species that are similar in energy are also similar in structure.
Which of the following is a carbene? A) CH2CHO- B) CH3CH2∙ C) :CCl2 D) CH3CH2+ E) NCO-
C) :CCl2
Which of the following reactive intermediates can best be described as both nucleophilic and strongly basic? A) carbanions B) carbocations C) carbenes D) free radicals E) alkanes
A) carbanions
How do alkyl substituents stabilize a carbocationic center to which they are attached?
A) through an inductive donation of electron density to the cationic center
B) through an inductive removal of electron density from the cationic center
C) through hyperconjugation
D) both A and C
E) both B and C
D) both A and C
Which of the following reactive intermediate species maintains sp3 hybridization? A) methyl carbanion B) dibromocarbene C) tertiary carbocation D) secondary alkyl radical E) both B and C
A) methyl carbanion
When two carbenes collide, they immediately dimerize to give \_\_\_\_\_\_\_\_. A) an alkane B) an alkene C) an alkyne D) an aromatic ring E) a carbanion
B) an alkene
What reactive intermediate is formed when CHBr3 is treated with hydroxide or when diazomethane is irradiated? A) carbene B) carbanion C) carbocation D) carbon radical
A) carbene
