CHEM 243- Chapter 4

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

What is the name of the major monobrominated product which results when methylcyclohexane is subjected to Br2/hν conditions?

1-bromo-1-methylcyclohexane

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

4

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.

4648

Rank the free radicals (I-III) shown below in order of decreasing stability (i.e., from most stable to least stable). CH3CH2C(CH3)2 I CH2CH2CH(CH3)2 II CH3CHCH(CH3)2 III A) I > III > II B) II > III > I C) I > II > III D) II > I > III E) III > II > I

A) I > III > II

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.

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

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

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

Describe the hybridization of the cationic center and predict the CCC bond angle in (CH3)3C+.

Cationic center is sp2 hybridized, therefore the CCC bond angle is 120°.

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

D) ΔH° < 0 and ΔS° < 0

Given that tertiary H atoms react with a chlorine radical about 5.5 times faster than primary ones, estimate the ratio of the two monochlorinated products that result when 2,3-dimethylbutane undergoes free radical chlorination.

1-chloro-2,3-dimethylbutane, 52%; 2-chloro-2,3-dimethylbutane, 48%

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

25

What is the name of the major monobrominated product which results when 3-methylpentane is subjected to Br2/hν conditions?

3-bromo-3-methylpentane

What is meant by the mechanism of a chemical reaction?

complete, step-by-step description of which bonds break, which bonds form, and the order in which these events occur in the transformation of reactants into products.

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

exothermic

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

Many molecules of product are formed for every photon of light which is absorbed by the reaction mixture.

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

Rate = kr[AB][CD] where kr is the rate constant.

What is the relative reactivity of 2° vs 1° hydrogens in the free radical bromination of n-butane if the ratio of 1-bromobutane to 2-bromobutane formed is 7:93?

The 2° hydrogens are 20 times more reactive than the 1° ones.

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

The activation energy or Ea

Draw an energy diagram for a two step reaction where the structure of the transition state of the rate determining step most closely resembles the starting material and the overall reaction is exothermic.

check test bank for image

Provide the major organic product that results when 2,2,4-trimethylpentane is subjected to free radical bromination.

check test bank for image

When 1,1,3,3-tetramethylcyclobutane is brominated at 125°C, the relative reactivity of the 1°: 2°: 3° hydrogens is approximately 1:82:1600. Estimate the amount of each monobromination product.

primary bromide, 3.5%; secondary bromide, 96.5%

Species with unpaired electrons are called __________.

radicals or free radicals.

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-

rate = k[(CH3)2CHCl]

When compound I, C7H16, was treated with chlorine and light it yielded 3 monochlorination products that could be separated by chromatography. Two of the products were primary alkyl halides and the other was a secondary alkyl halide. Provide a possible structure for compound I.

check test bank for image

Write the structures of all of the monobromination products of 1,1,3,3-tetramethylcyclobutane.

check test bank for image

Add a lone pair of electrons to the following structure to create the most reactive (least stable) carbanion intermediate.

check test bank for images

Predict the major monobromination product in the following reaction. Answer:

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Provide the structure of the carbene that results when diazomethane (shown below) decomposes.

check test bank for images

When acetaldehyde (CH3CHO) is deprotonated, the resulting anion is stabilized by resonance. Draw the major resonance contributing forms of this anion.

check test bank for images

The following reaction occurs readily: CH3Br + I- CH3I + Br-. 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?

rate = k[CH3Br][I-]

_________ is the study of reaction rates

Kinetics

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

(CH3)2CH-H has the smaller bond dissociation energy. This homolytic bond cleavage yields a more stable secondary radical.

Predict the major monobromination product in the following reaction. (CH3)3CCH2CH3 + Br2 --->(hv light)-->

(CH3)3CCHBrCH3

What C5H12 isomer will give only a single monochlorination product?

(CH3)4C or neopentane or 2,2-dimethylpropane

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 What's the enthalpy difference between reactant A and intermediate C?

+7 kcal/mol

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

-16 kcal/mol

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-B+C-D →A-C+B-D 63 88 47 96 A) +8 Kcal/mol, endothermic B) -8 Kcal/mol, exothermic C) +16 Kcal/mol, endothermic D) +8 Kcal/mol, exothermic

A) +8 Kcal/mol, endothermic

Given the bond dissociation energies below (in kcal/mol), estimate the ΔH° for the propagation step (CH3)2CH· + Cl2 (CH3)2CHCl + Cl·. 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

A) -22 kcal/mol

Consider the reaction of A being converted into B at 25°C. If the ΔG° of this reaction is +1.0 kcal/mol, the Keq 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%

A) 0.18, 15%

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

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.

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

Which is a measure of the randomness of a system? A) entropy B) enthalpy C) free energy D) halogenation E) stoichiometry

A) entropy

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

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

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

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

A) ΔH° > 0 and ΔS° > 0

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∙

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∙

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.

Answer: y-axis energy x-axis reaction coordinate starts close to the bottom right with F and increases, until it reaches a peak, then decreases to G. G is higher than F by 5 kcal/mol. The space between G and the peak is 15 kcal/mol.

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 Ea and ΔH° on your drawing. Answer:

Answer: graph starts at top, increases a little to account for activation energy, then goes down.

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.

Answer: -0.82 kcal/mol

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

Answer: -2.0 kJ/mol

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

Answer: 2.0 kJ/mol

Given a ΔG° 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] (use the test bank for image)

Answer: 42%

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

Answer: CH3CH3 + Cl∙ → CH3CH2∙ + H-Cl CH3CH2∙ + Cl-Cl → CH3CH2Cl + Cl∙

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.

Answer: CH3Cl + Cl∙ → ∙ CH2Cl + HCl ∙ CH2Cl + Cl-Cl → CH2Cl2 + Cl∙ CH2Cl2 + Cl∙ → ∙ CHCl2 + HCl ∙ CHCl2 + Cl-Cl → CHCl3 + Cl∙ CHCl3 + Cl∙ → ∙ CCl3 + HCl ∙ CCl3 + Cl-Cl → CCl4 + Cl∙

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

Answer: Cl-Cl + photon (hν) → 2 Cl∙

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

Answer: 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.

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?

Answer: Make sure the molar ratio of methane to chlorine is relatively large.

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. x-axis energy y-axis reaction coordinate graph has 2 peaks and 3 valleys (x, y and z). peak between y and z is higher than peak between x and y.

Answer: The conversion of Y to Z has a higher Ea than does the conversion of X to Y. The conversion of Y to Z is therefore rate-limiting.

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

B) 0.43, 30%

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

B) 0.60M

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 secondary hydrogens are present in the hydrocarbon below? A) 2 B) 6 C) 7 D) 8 E) 16

B) 6 check test bank for image of molecule

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

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 -->(hv light)-->2HCl A) H2--> (hv light)-->H∙ + H∙ H∙ + Cl2 → Cl∙ + HCl H∙ + Cl∙ → HCl B) Cl2-->(hv light)-->Cl∙ + Cl∙ Cl∙ + H2 → H∙ + HCl H∙ + Cl2 → HCl + Cl∙ C) H2-->(hv light)-->H∙ + H∙ H∙ + Cl2 → Cl∙ + HCl Cl∙ + H2 → HCl + H∙ D) Cl2-->(hv light)-->Cl∙ +Cl∙ Cl∙ + H2 → H∙ + HCl H∙ + Cl∙ → HCl

B) Cl2-->(hv light)-->Cl∙ + Cl∙ Cl∙ + H2 → H∙ + HCl H∙ + Cl2 → HCl + 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

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.

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.

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?

Blue light is effectively absorbed by chlorine but not by methane. Thus the initiating step is most likely the production of two chlorine atoms from molecular chlorine.

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

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

Which of the following is a carbene? A) CH2=CHO- B) CH3CH2∙ C) :CCl2 D) CH3CH2+ E) NCO-

C) :CCl2

Which H atom in the molecule shown will be most readily abstracted by a bromine radical? A) Ha B) Hb C) Hc D) Hd E) He

C) Hc check test bank for image of molecule

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.

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.

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

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

When Br radical reacts with 1-butene (CH3CH2CH=CH2), the hydrogen atom which is preferentially abstracted is the one which produces a resonance stabilized radical. Draw the major resonance contributing forms of this radical.

CH3 HCH=CH2 ↔ CH3CH=CH H2

List the following radicals in order of increasing stability (i.e., from least stable to most stable). (CH3)3C∙, CH2=CHCH2∙, CH3CH2∙, CH3∙, (CH3)2CH∙

CH3∙ < CH3CH2∙ < (CH3)2CH∙ < (CH3)3C∙ < CH2=CHCH2∙

The relative reactivity of the 1°: 2°: 3° hydrogens of (CH3)3CCH2CH3 in free radical chlorination is 1: 3.8: 5.0. Provide the structure of each monochlorination product, and estimate the relative amount of each in the mixture of monochlorinated products.

ClCH2C(CH3)2CH2CH3, 45.9%; (CH3)3CCHClCH3, 38.8%; (CH3)3CCH2CH2Cl, 15.3%

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

D) 2.3, 70%

In the hydrocarbon shown below, how many tertiary hydrogens are present? A) 0 B) 1 C) 2 D) 3 E) 4

D) 3

A certain free radical chlorination reaction of 2,3-dimethylbutane leads to the production of two major monochlorinated products 1-chloro-2,3-dimethylbutane and 2-chloro-2,3-dimethylbutane. When run through a Gas chromatograph, it was determined that the product distribution was 51.7% 1-chloro-2,3-dimethylbutane and 48.3% 2-chloro-2,3-dimethylbutane. If we assign the primary H's a reactivity of 1.0, what is the relative reactivity of the tertiary H's. A) 5.0 times more reactive B) 5.1 times more reactive C) 4.6 times more reactive D) 5.6 times more reactive E) 3.0 times more reactive

D) 5.6 times more reactive

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.

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

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

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

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

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

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

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

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

Free radical bromination of pentane results in poor yields of 1-bromopentane, while cyclopentane can be readily brominated under similar conditions to yield bromocyclopentane. Offer an explanation.

In the bromination of pentane, the lowest energy reaction pathways go through secondary free radical intermediates to produce secondary alkyl bromides (2-bromopentane and 3-bromopentane). Thus 1-bromopentane is a very minor product. All of the hydrogen atom abstractions of cyclopentane lead to the same secondary radical which eventually leads to bromocyclopentane.

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

The carbon-chlorine bond of (CH3)3CCl has the smaller bond dissociation energy. The carbon radical that results upon bond cleavage is a tertiary radical. Tertiary alkyl radicals are more stable than primary due to hyerconjugative or inductive stabilization.

The hydrogen atom abstraction step in the free radical bromination of methane is endothermic. Use the Hammond Postulate to speculate on the extent of bond formation and bond cleavage in the transition state.

The endothermic nature of this step means that the energy of the transition state is closer to that of the step's products than to its reactants. Using the Hammond Postulate this means the transition state more closely resembles a carbon radical than a bromine radical. The carbon-hydrogen bond is almost completely broken and the hydrogen-bromine bond completely formed in the transition state.

The hydrogen atom abstraction step in the free radical chlorine of methane is exothermic. Use the Hammond Postulate to speculate on the extent of bond formation and bond cleavage in the transition state.

The exothermic nature of this step means that the energy of the transition state is closer to that of the step's reactants than to its products. Using the Hammond Postulate this means the transition state more closely resembles a bromine radical than a carbon radical. The carbon-hydrogen bond has broken very little and the hydrogen-chlorine bond has formed very little in the transition state.

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

The exponential factor e-Ea/RT in the Arrhenius equation corresponds to the fraction of collisions in which the reacting molecules have the appropriate activation energy to react at a given temperature.

Use the Hammond Postulate to explain why free radical brominations are more selective than free radical chlorinations.

The first propagation step in free radical bromination is endothermic while the analogous step in free radical chlorination is exothermic. From the Hammond Postulate, this means that the transition state for the bromination is product-like (ie, radical-like) while the transition state for the chlorination is reactant-like. The product-like transition state for bromination has the C-H bond nearly broken and a great deal of radical character on the carbon atom. The energy of this transition state reflects most of the energy difference of the radical products. This is not true in the chlorination case where the transition state possesses little radical character.

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

The frequency factor accounts for the number of collisions between reacting molecules and the fraction of these collisions with the proper orientation for the reaction to happen. There is a linear relationship between the number of these collisions and the rate constant of the reaction. For a given reaction A is a constant.

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.

The propagation steps of the free-radical chlorination of methane are shown below. CH4 + Cl∙ → CH3∙ + H-Cl CH3∙ + Cl-Cl → CH3Cl + Cl∙ In each of the steps, two reactant molecules generate two product molecules. This similarity in the number of molecular species means that the disorder in the reaction is neither greatly increased nor diminished. Therefore, one expects ΔS° in either propagation step to be approximately equal to zero.

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.

The reaction will occur about 2.8 times faster.

What accounts for the relatively high stability of the benzyl radical?

The unpaired electron is delocalized through a conjugated pi system.

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

[CH3CH2--------H-----Cl]∙ ++

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

the activation energy or Ea

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

transition state

Predict the signs of ΔG° and ΔS° in the reaction of cyclohexene with H2 to form cyclohexane.

ΔH° < 0 and ΔS° < 0

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)

ΔS° < 0. The three reactant molecules have significantly more freedom of motion, and randomness, than does the single molecule of product into which they are converted.

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

ΔS°>0