SECTION L: KINETICS Documents/ACS Bank... · Web viewThe Arrhenius equation, k = Ae-E/RT expresses the relationship between the reaction rate constant, k, and the energy of activation,

L–1

SECTION L: KINETICS

INDEX: L001 LEVEL: (U) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (FR)

The Arrhenius equation, k = Ae-E/RT expresses the relationship between the reaction rate constant, k, and the energy of activation, E. How does increasing activation energy affect the probability that colliding molecules will react?

The Arrhenius equation, k = Ae-E/RT expresses the relationship between the reaction rate constant, k, and the energy of activation, E. The probability that colliding molecules will react

(A) increases with increasing energy of activation.

(B) depends only on the empirical constant, A.

(C) increases with decreasing temperature.

(D) decreases with increasing energy of activation.


Omit responses.

A change in temperature from 10 °C to 20 °C is found to double the rate of a given chemical reaction. How did this change affect the reacting molecules?

(A) It doubled their average velocity.

(B) It doubled their average energy.

(C) It doubled the number of collisions per second.

(D) It doubled the proportion of molecules possessing at least the minimum energy required for the reaction.

INDEX: L003 LEVEL: (A) ANSWER: (B)KEYWORDS: equilibriumALTERNATIVE FORMAT: (FR)

What experimental information is required to determine the rate equation for a chemical reaction?

The rate equation for a chemical reaction is determined by

(A) theoretical calculations.

(B) measuring reaction rate as a function of concentration of reacting species.

(C) determining the equilibrium constant for the reaction.

(D) measuring reaction rates as a function of temperature.

INDEX: L004 LEVEL: (A) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (FR)

The rate of a reaction in the absence of a catalyst has been measured. The rate of the same reaction when catalyzed is 106 times faster. How does the activation energy for the catalyzed reaction compare to the activation energy for the uncatalyzed reaction?

The rate of a reaction in the absence of a catalyst has been measured. The rate of the same reaction when catalyzed is 106 times faster. The activation energy for this reaction

(A) is 6/RT kcal/moIe.

(B) can be calculated from the information above if the temperatures of the reactions are given.

(C) is a concept that cannot be applied to chemical reactions.

(D) is different in the two cases.

(SA) = short answer; (FR) = free response; (TF) = true/false (U) = understanding; (A) = analysis; (G) = generalization

L–2 Kinetics

INDEX: L005 LEVEL: (U) ANSWER: (B)KEYWORDS:ALTERNATIVE FORMAT: (TF)

The value of the rate constant of a reaction can generally be expected to . Use each response in a separate question.

The value of the rate constant of a reaction can generally be expected to

(A) be independent of temperature.

(B) increase with increasing temperature.

(C) decrease with increasing temperature.

(D) decrease with increasing temperature only if the reaction is exothermic.

INDEX: L006 LEVEL: (U) ANSWER: (A)KEYWORDS: equilibriumALTERNATIVE FORMAT: (FR)

Sketch a graph of energy vs. a reaction coordinate for an exothermic reation. Label the segment that represents the activation energy for the forward reaction. Also label the segment that represents the activation energy for the reverse reaction.

Which line in the diagram represents the activation energy for a forward reaction?


Kinetics L–3

A

B C

D↑E

reaction coordinate →


L–4 Kinetics

INDEX: L007 LEVEL: (U) ANSWER: (B)KEYWORDS: ALTERNATIVE FORMAT: (SA)

The rate law for the reaction

A + B C + D

is first order in [A] and second order in [B]. If [A] is halved and [B] is doubled, how will the rate of the reaction be affected?

The rate law for the reaction

A + B C + D

is first order in [A] and second order in [B]. If [A] is halved and [B] is doubled, the rate of the reaction will

(A) remain the same.

(B) be increased by a factor of 2.

(C) be increased by a factor of 4.

(D) be increased by a factor of 8.

INDEX: L008 LEVEL: (U) ANSWER: (B)KEYWORDS: ALTERNATIVE FORMAT: (TF)

Some chemical reactions proceed at a rate that is proportional to the concentration of a single reactant. Such reactions . Use each response in a separate question.

Some chemical reactions proceed at a rate that is proportional to the concentration of a single reactant. Such reactions

(A) are called zero order reactions.

(B) are called first order reactions.

(C) are called second order reactions.

(D) do not occur. For a reaction to occur, at least two molecules (or ions) must collide; in this case, however, there is only one reactant.

INDEX: L009 LEVEL: (A) ANSWER: (A)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (TF)

The Arrhenius equation expresses the relationship between the rate constant, k, of a reaction, and the energy of activation Ea.

k = Ae–Ea/RT

In this equation, A is an empirical constant, R is the ideal gas constant, e is the base of natural logarithms, and T is the absolute temperature. According to the Arrhenius equation, . Use each response in a separate question.

The Arrhenius equation expresses the relationship between the rate constant, k, of a reaction, and the energy of activation Ea.

k = Ae–Ea/RT

In this equation, A is an empirical constant, R is the ideal gas constant, e is the base of natural logarithms, and T is the absolute temperature. According to the Arrhenius equation,

(A) at constant temperature, reactions with lower activation energies proceed more rapidly.

(B) at constant temperature, reactions with lower activation energies proceed less rapidly.

(C) at constant energy of activation, reactions at lower temperatures proceed more rapidly.

(D) at constant energy of activation, reactions with smaller values of A proceed more rapidly.


Kinetics L–5

INDEX: L010 LEVEL: (A) ANSWER: (C)KEYWORDS: organic chemistryALTERNATIVE FORMAT: (FR)

Give at least three procedures that will increase the rate of reaction of chlorine gas with a liquid hydrocarbon.

All but one of these procedures will increase the rate of reaction of chlorine gas with a certain liquid hydrocarbon. Which one would be ineffective?

(A) Use light to activate the Cl2.

(B) Introduce more Cl2 gas, keeping volume constant.

(C) Double the pressure by introducing N2 gas.

(D) Divide the liquid into small droplets.

(E) Increase the temperature, keeping pressure constant.


Why do bimolecular reactions generally exhibit much slower rates than one would predict from the rate of collision between molecules, as calculated from the gas kinetic theory?

Bimolecular reactions generally occur at much slower rates than one would predict from the rate of collision between molecules as calculated from the gas kinetic theory. The discrepancy can be explained in terms of

(A) intermolecular repulsions.

(B) nonspherical molecular shapes.

(C) erroneous estimates of molecular size.

(D) activation energies of reaction.

(E) the uncertainty principle.

INDEX: L012 LEVEL: (U) ANSWER: (E)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (TF)

In a chemical reaction involving the formation of an intermediate activated complex, the step must always be exothermic. Use each response in a separate question.

In a chemical reaction involving the formation of an intermediate activated complex, which step must always be exothermic?

(A) Reactants products

(B) Products reactants

(C) Reactants activated complex

(D) Products activated complex

(E) Activated complex products

INDEX: L013 LEVEL: (A) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (SA)

Omit responses.

The gas–phase reaction, A2 + B2 2AB, proceeds by bimolecular collisions between A2 and B2 molecules. If the concentrations of both A2 and B2 are doubled, the reaction rate will be changed by a factor of

(A) 1/2 (B) 2 (C) 3 (D) 4

(E)


L–6 Kinetics

INDEX: L014 LEVEL: (A) ANSWER: (E)KEYWORDS: ALTERNATIVE FORMAT: (SA)

Omit responses.

A change in temperature from 10 °C to 20 °C is found to double the rate of a given chemical reaction. How did this change affect the reacting molecules?

(A) It doubled their average velocity.

(B) It doubled their average energy.

(C) It doubled the number of collisions per second.

(D) It doubled the pressure inside the reaction vessel.

(E) It doubled the proportion of molecules possessing at least the minimum energy required for the reaction.

INDEX: L015 LEVEL: (A) ANSWER: (D)KEYWORDS: bonding, organic chemistryALTERNATIVE FORMAT: (TF)

For a chemical chain reaction to continue it is essential that . Use each response in a separate question.

For a chemical chain reaction to continue it is essential that

(A) the chain of molecules break into smaller pieces.

(B) two activated species unite to form an inactive substance.

(C) long molecular chains form by linkage through covalent bonding.

(D) an activated species be formed in each step of the reaction.

(E) a catalyst be constantly present to activate the reactants.

INDEX: L016 LEVEL: (U) ANSWER: (C)KEYWORDS: ALTERNATIVE FORMAT: (SA)

Omit responses.

If a reaction proceeding by the mechanism

A + B C + D

occurs at a rate x, and if the concentrations of A and B are both doubled, what will be the new rate of reaction?

(A) x (B) 2x (C) 4x (D) 8x

(E) 16x

INDEX: L017 LEVEL: (A) ANSWER: (D)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (TF)

The best experimental evidence for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that . Use each response in a separate question.

The best experimental evidence for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that

(A) all chemical reactions are speeded up by a rise in temperature.

(B) at the same temperature light molecules have a higher average velocity than heavy molecules.

(C) collision between molecules will not result in reaction unless enough energy is available to activate the particles.


Kinetics L–7

(D) a small rise in temperature causes a rather large increase in the velocity of most chemical reactions.

(E) a large rise in the average kinetic energy of molecules is caused by a small rise in temperature.

INDEX: L018 LEVEL: (A) ANSWER: (C)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (TF)

The best theoretical explanation for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that . Use each response in a separate question.

The best theoretical explanation for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that


(B) at the same temperature light molecules have a higher average velocity than heavy molecules.




INDEX: L019 LEVEL: (U) ANSWER: (A)KEYWORDS: equilibrium, oxidation and reductionALTERNATIVE FORMAT: (TF)

Given that1

2SO2(g) +O2(g) 2SO3(g)2

The forward reaction (1) is proceeding at a certain rate at some temperature and pressure; when the pressure is increased, we may expect for the forward reaction (1) . Use each response in a separate question.

ALTERNATIVE FORMAT: (FR)Given that

12SO2(g) +O2(g) 2SO3(g)

2The forward reaction (1) is proceeding at a certain rate at some temperature and pressure; when the pressure is increased, what may we expect to happen to the rate for the forward reaction?

Given that1

2SO2(g) +O2(g) 2SO3(g)2

The forward reaction (1) is proceeding at a certain rate at some temperature and pressure; when the pressure is increased, we may expect for the forward reaction (1)

(A) a greater rate of reaction and a greater yield of SO3 at equilibrium.

(B) a greater rate of reaction and the same yield of SO3 at equilibrium.

(C) a lesser rate of reaction and a lesser yield of SO3 at equilibrium.

(D) a lesser rate of reaction and a greater yield of SO3 at equilibrium.

(E) no change in rate or yield.

INDEX: L020 LEVEL: (A) ANSWER: (D)KEYWORDS: laboratoryALTERNATIVE FORMAT: (TF)

The best experimental evidence for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that . Use each

The best experimental evidence for the assertion that molecules of higher-than-average kinetic energy are involved in chemical reactions is that



L–8 Kinetics

response in a separate question.(B) at the same temperature light molecules have a higher average velocity than heavy molecules.




INDEX: L021 LEVEL: (U) ANSWER: (A)KEYWORDS: bonding, laboratoryALTERNATIVE FORMAT: (TF)

Ionic substances are characterized by the fact that . Use each response in a separate question.

Which is not a characteristic of ionic substances?

(A) Their reactions are generally extremely slow.

(B) They conduct an electric current when fused.

(C) Those having a common ion exhibit some similar chemical properties.

(D) They lower the vapor pressure of water when dissolved in it.

(E) They raise the boiling point of water when dissolved in it.

INDEX: L022 LEVEL: (U) ANSWER: (C)KEYWORDS: introductory conceptsALTERNATIVE FORMAT: (FR)

How does addition of a catalyst in a chemical reaction affect the activation energy for the reaction?

The addition of a catalyst in a chemical reaction

(A) increases the concentration of products at equilibrium.

(B) increases the fraction of reactant molecules with a given kinetic energy.

(C) provides an alternate path with a different activation energy.

(D) lowers the enthalpy change in the overall reaction.

INDEX: L023 LEVEL: (U) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (TF)

will lower the activation energy for a particular reaction. Use each response in a separate question.

Which procedure will lower the activation energy for a particular reaction?

(A)subdividing the reactants

(B)increasing the reactant concentrations

(C)increasing the temperature

(D) adding a catalyst

INDEX: L024 LEVEL: (U) ANSWER: (C)KEYWORDS: equilibrium, thermochemistry

Which statement most accurately describes the behavior of a catalyst?


Kinetics L–9

ALTERNATIVE FORMAT: (TF)Use each response as a separate question. (A) A catalyst increases the G of a reaction and hence the

forward rate.

(B) A catalyst reduces the H of a reaction and hence the temperature needed to produce products.

(C) A catalyst reduces the activation energy for a reaction and increases the rate of a reaction.

(D) A catalyst increases the equilibrium constant and final product concentrations.

INDEX: L025 LEVEL: (U) ANSWER: (A)KEYWORDS: ALTERNATIVE FORMAT: (TF)

A catalyst will . Use each response in a separate question.

A catalyst will

(A) alter the pathway (mechanism) of a chemical reaction.

(B) increase H for the reaction.

(C) decrease H for the reaction.

(D) decrease Ea for the forward reaction only.

INDEX: L026 LEVEL: (U) ANSWER: (A)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (TF)

Consider the reaction diagram. . Use each response in a separate question.


L–10 Kinetics

. .


no catalyst present

catalyst present

A + B

C + D. .


no catalyst present

catalyst present

A + B

C + D


Kinetics L–11

INDEX: L027 LEVEL: (A) ANSWER: (C)KEYWORDS: equilibriumALTERNATIVE FORMAT: (TF)

The role of a catalyst at equilibrium is to . Use each response, slightly modified, in a separate question.

Which is necessarily true of the role of a catalyst in equilibrium?

(A) It affects the equilibrium constant.

(B) It shifts equilibrium to the products.

(C) It decreases the time interval necessary for establishment of equilibrium.

(D) It has a greater effect on the forward reaction rate than on the reverse reaction rate.

INDEX: L028 LEVEL: (U) ANSWER: (C)KEYWORDS: ALTERNATIVE FORMAT: (SA)

Two reaction pathways are indicated: one for the uncatalyzed reaction and one for the reaction in the presence of a catalyst. Which line represents the activation energy?

Two reaction pathways are indicated: one for the uncatalyzed reaction and one for the reaction in the presence of a catalyst.


L–12 Kinetics

A

B

CD

E

reaction coordinate

E

X + YZ → XZ + Y


Kinetics L–13

INDEX: L029 LEVEL: (A) ANSWER: (A)KEYWORDS: organic chemistryALTERNATIVE FORMAT: (SA)

Omit responses.

The following mechanism has been proposed for the formation of ethylbenzene:

CH3CH2Br + AlBr3 AlBr4– + CH3CH2

+

CH3CH2+ + C6H6 C6H6CH2CH3

+

C6H6CH2CH3 + AlBr4– AlBr3 + HBr + C6H5CH2CH3

Which substance serves as the catalyst?

(A) AlBr3 (C) CH3CH2+

(B) AlBr4– (D) C6H6CH2CH3

+

INDEX: L030 LEVEL: (A) ANSWER: (D)KEYWORDS: oxidation and reductionALTERNATIVE FORMAT: (SA)

Omit responses.

The table presents data for the reaction:2H2(g) + 2NO(g)

The temperature of the reaction is constant. The initial rate is in arbitrary units.

Initial Concentration (mol·L–1) InitialExp. [NO] 10–3 [H2] 10–3 RateI 6.0 1.0 18II 6.0 2.0 36III 1.0 6.0 3IV 2.0 6.0 12

What is the rate law for this reaction?

(A) rate = k1 [H2] [NO] (C) rate = k1 [H2]2 [NO]

(B) rate = k1 [H2]2 [NO]2 (D) rate = k1 [H2] [NO]2

INDEX: L031 LEVEL: (A) ANSWER: (D)KEYWORDS: oxidation and reductionALTERNATIVE FORMAT: (FR)

For the reaction between gaseous chlorine and nitric oxide:

2NO(g) + Cl2(g) 2NOCl(g)

doubling the concentration of chlorine doubles the rate of reaction. Doubling the concentrations of both reactants increases the rate by a factor of eight. Write the rate law for the reaction.

For the reaction between gaseous chlorine and nitric oxide:

2NO(g) + Cl2(g) 2NOCl(g)

doubling the concentration of chlorine doubles the rate of reaction. Doubling the concentrations of both reactants increases the rate by a factor of eight. The reaction is

(A) first order in NO, first order in Cl2.

(B) first order in NO, second order in Cl2.

(C) second order in NO, second order in Cl2.

(D) second order in NO, first order in Cl2.


L–14 Kinetics

INDEX: L032 LEVEL: (U) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (SA)

For the reaction A + B C these data were obtained:

Initial Initial Initial Rate ofExpt. Conc. A Conc. B formation of C

I 0.10 M 0.10 M 0.030 M·h–1

II 0.10 M 0.20 M 0.12 M·h–1

Write the rate law for this reaction.

For the reaction A + B C these data were obtained:

Initial Initial Initial Rate ofExpt. Conc. A Conc. B formation of C

I 0.10 M 0.10 M 0.030 M·h–1

II 0.10 M 0.20 M 0.12 M·h–1

III 0.20 M 0.20 M 0.12 M·h–1

The rate law for this reaction is

(A) rate = k [A] [B] (C) rate = k [A]2 [B]

(B) rate = k [A]2 (D) rate = k [B]2

INDEX: L033 LEVEL: (A) ANSWER: (D)KEYWORDS: equilibriumALTERNATIVE FORMAT: (SA)

The reaction 2A + 2B C + D proceeds by this mechanism:

2A A2 (equilibrium)A2 + B X + C (rate determining)X + B D (rapid)

Write the rate equation for the reaction.

The reaction 2A + 2B C + D proceeds by this mechanism:

2A A2 (equilibrium)A2 + B X + C (rate determining)X + B D (rapid)

The rate equation for the reaction is

(A) rate = k[A] [B] (C) rate = k

(B) rate = k[A]2 [B]2 (D) rate = k[A]2 [B]

INDEX: L034 LEVEL: (A) ANSWER: (C)KEYWORDS: ALTERNATIVE FORMAT: (SA)

Omit responses.

Substance A undergoes a first order reaction A B with a half life of 20 min at 25 °C. If the initial concentration of A in a sample is 1.6 M, what will be the concentration of A after 80 min?

(A) 0.40 M (B) 0.20 M (C) 0.10 M (D) 0.050 M


Write the general equation for the rate law for a first order reaction.

The rate law for a first order reaction has the form

(A) rate = k (C) rate = k [A]2

(B) rate = k [A] (D) rate = k [A] [B]


Write the general equation for the rate expression for a second order reaction.

The rate expression for a second order reaction is

(A) rate = k [A] (C) rate = k [A]2 [B]

(B) rate = k [A] [B] (D) rate == k [A]2 [B]2


Omit responses.

For the reaction A + 2B AB2, given this data:

[A] [B] Rate of Production ofExperiment (mol·L–1) (mol·L–1) AB2 (mol·L–1·min–1)


Kinetics L–15

l 0.10 0.10 0.00902 0.20 0.10 0.0363 0.10 0.20 0.018

What is the rate equation?

(A) rate = k [A] [B] (C) rate = k [A] [B]2

(B) rate = k [A]2 [B] (D) rate = k [A]2 [B]2

INDEX: L038 LEVEL: (A) ANSWER: (D)KEYWORDS: ALTERNATIVE FORMAT: (FR)

For the reaction

X2 + Y + Z XY + XZ

the mechanism was determined to be

l) X2 + Y XY + X (very slow)

2) X + Z XZ (very fast)

Write the rate law for this reaction.

For the reaction

X2 + Y + Z XY + XZ

the mechanism was determined to be

l) X2 + Y XY + X (very slow)

2) X + Z XZ (very fast)


(A) rate = k [X2] [Y] [Z] (C) rate = k [X] [Z]

(B) rate = k [X2] (D) rate = k [X2] [Y]

INDEX: L039 LEVEL: (U) ANSWER: (D)KEYWORDS: laboratoryALTERNATIVE FORMAT: (SA)

For the reaction

2H2O2 2H2O + O2

a plot of log against time produces a straight line. What is the reaction order?

For the reaction

2H2O2 2H2O + O2

which plot confirms that the rate is first order with respect to H2O2?

(A)


L–16 Kinetics

log [H2O2]

time

[H2O2]

time


Kinetics L–17

INDEX: L040 LEVEL: (A) ANSWER: (A)KEYWORDS: oxidation and reductionALTERNATIVE FORMAT: (SA)

Omit responses.

Initial rate data for the reaction

2N2O5(g) 4NO2(g) + O2(g)

are as follows:

Experiment [N2O5] [O2] Rate in M·s–1

1 0.15 M 0.30 M 462 0.20 M 0.60 M 613 0.20 M 0.30 M 61


(A) rate = k[N2O5] (C) rate = k[N2O5]1.3[O2]2

(B) rate = k[[N2O5]2 (D) rate = k[N2O5]2[O2]

INDEX: L041 LEVEL: (A) ANSWER: (E)KEYWORDS: thermochemistryALTERNATIVE FORMAT: (SA)

Omit responses.

At 10 °C, the reaction between CO and Cl2 occurs at a rate that converts 5 g of CO per hour. If the temperature is raised to 30 °C, the approximate number of grams of CO converted per hour should be

(A) 5 (B) 5.56 (C) 10 (D) 15

(E) 20

INDEX: L042 LEVEL: (A) ANSWER: (B)KEYWORDS: oxidation and reduction, metals & nonmetalsALTERNATIVE FORMAT: (FR)

The reaction H2 + Br2 2HBr proceeds by this chain mechanism:

Step I Br2 + hn (ultraviolet) 2Br•

Step II Br• + H2 HBr + H•

Step III H• + Br2 HBr + Br•

Write the equation for a reaction that would break the chain.

The reaction H2 + Br2 2HBr proceeds by this chain mechanism:

Step I Br2 + hn (ultraviolet) 2Br•

Step II Br• + H2 HBr + H•

Step III H• + Br2 HBr + Br•

Which would break the chain?

(A) Br• + H2 HBr + H•

(B) Br• + Br• Br2

(C) H• + Br2 HBr + Br•

(D) Br2 + H2 2HBr

INDEX: L043 LEVEL: (A) ANSWER: (A)KEYWORDS: ALTERNATIVE FORMAT: (FR)

The decomposition of hydrogen peroxide in the presence of iodide ion is believed to occur via the mechanism

The decomposition of hydrogen peroxide in the presence of iodide ion is believed to occur via the mechanism

H2O2(aq) + I–(aq) H2O(l) + IO–(aq)


L–18 Kinetics

H2O2(aq) + I–(aq) H2O(l) + IO–(aq)

H2O2(aq) + IO–(aq) H2O(l) + O2(g) + I–(aq)

What is the role of I–(aq) in the reaction?

H2O2(aq) + IO–(aq) H2O(l) + O2(g) + I–(aq)

In this mechanism, I–(aq) is

(A) a catalyst.

(B) a reactant in the overall reaction.

(C) the activated complex.

(D) a product of the overall reaction.

INDEX: L044 LEVEL: (A) ANSWER: (C)KEYWORDS: oxidation and reductionALTERNATIVE FORMAT: (SA)

Omit responses.

Consider the reaction:

2NO2(g) + F2(g) 2NO2F(g)

A proposed mechanism for this reaction is

NO2 + F2 NO2F + F (slow)

NO2 + F NO2F (fast)

What is the rate law for this mechanism?

(A) rate = k (C) rate = k [NO2] [F2]

(B) rate = k [NO2]2 [F2] (D) rate = k [NO2] [F]


Kinetics L–19
