Deck 12: Chemical Kinetics: Rates of Reactions

Question 1

Exhibit 12-1 The following question(s) relate to the reaction between water and a complex ion of Co²⁺; the rate varies with concentration as follows. (The rate has no dependence on [H₂O].)
Refer to Exhibit 12-1. The rate law for the reaction is rate =

A) k²[Co²⁺ complex].
B) k[Co²⁺ complex].
C) 2k[Co²⁺ complex].
D) k[Co²⁺ complex]/2.
E) k[Co²⁺ complex]².

Answer 1

k[Co²⁺ complex].

Question 2

Exhibit 12-1 The following question(s) relate to the reaction between water and a complex ion of Co²⁺; the rate varies with concentration as follows. (The rate has no dependence on [H₂O].)
Refer to Exhibit 12-1. The value and units of the rate constant are:

A) 3.6 × 10³ M min^-1.
B) 2.8 × 10^-4 M min^-1.
C) 3.6 × 10³ min^-1.
D) 2.8 × 10^-4 min^-1.
E) 3.6 × 10³ M^-1 min^-1.

Answer 2

2.8 × 10^-4 min^-1.

Question 3

Which of the following factors will affect the rate of a homogeneous reaction?

A) the presence of a catalyst, and, if one is present, its concentration
B) the properties (particularly, molecular structure and bonding) of reactants and products
C) the concentrations of the reactants and sometimes the products
D) the temperature at which the reaction occurs
E) all of these

Answer 3

all of these

Question 4

For many homogeneous reactions, the rate law has the general form

A) Rate = 1/k[A]^m[B]ⁿ
B) Rate = k[A]^m[B]ⁿ
C) Rate = k[A]^1/m[B]^1/n
D) Rate = k[1/A]^m[1/B]ⁿ
E) None of these

Question 5

Which unit is appropriate for describing reaction rates?

A) kJ mol^-1
B) ( ^$\circ$ C) min^-1
C) M min^-1
D) kJ g^-1
E) g L^-1

Question 6

The units for the rate constant k must always include

A) a volume unit
B) a concentration unit
C) a time unit
D) all of these
E) none of these

Question 7

Which statement about a rate constant is correct?

A) Its units are always M min^-1.
B) Its value generally decreases as temperature increases.
C) Its value depends on the concentration of the reactant(s).
D) Its value at a particular temperature depends on the reaction involved.
E) It is symbolized by "K".

Question 8

At a given temperature, the kinetics parameters for a new chemical reaction are determined in the following sequence from first to last

A) orders of reaction, k, rate law
B) k, orders of reaction, rate law
C) orders of reaction, rate law, k
D) k, rate law, orders of reaction
E) rate law, k, orders of reaction

Question 9

The rate of the chemical reaction involving two substances, A and B, is measured. It is found that if the initial concentration of A used is doubled, keeping the B concentration the same, the rate doubles. If the concentrations of both A and B are doubled, the rate is eight times that measured in the first experiment. The rate law for this reaction is rate =

A) k[A][B].
B) k[A]²[B].
C) k[A][B]².
D) 2k[A][B].
E) k[A][B]/2.

Question 10

A heterogeneous reaction mixture may contain

A) only a solid phase and a liquid phase
B) only a gas phase and a solid phase
C) only a gas phase and a liquid phase
D) a gas phase, a solid phase, and a liquid phase
E) any of these

Question 11

Which of the following could not be the units of a rate constant?

A) min^-1
B) M min^-1
C) M^-1 min^-1
D) M² min^-1
E) M^-2 min^-1

Question 12

In a reaction that is first-order with respect to reactant A and second-order with respect to reactant B, the rate of the reaction

A) doubles with a doubling of the concentration of either reactant
B) doubles with a doubling of the concentration of B
C) is halved by a doubling of the concentration of B
D) is tripled by any equal and simultaneous increase in the concentration of A and B
E) quadrupled by a doubling in the concentration of B while the concentration of A is held constant

Question 13

Consider the hypothetical reaction The rate of reaction is ____ times D[B]/Dt and ____ times D[C]/Dt.

A) -2; -3
B) -1/3; 1/2
C) 1/2; -1/3
D) -1/2; -1/3
E) -1; 2/3

Question 14

The rate law for a given reaction is rate = k[reactant]², with k = 2.64 × 10^-4 M^-1 min^-1. If the initial concentration is 0.0250 M, what is the initial rate, with the correct units?

A) 4.36 × 10^-11 M min^-1
B) 1.65 × 10^-7 min^-1
C) 6.60 × 10^-6 M min^-1
D) 1.65 × 10^-7 M min^-1
E) 6.60 × 10^-6 min^-1

Question 15

If a reaction is first-order with respect to each of its two reactants, then the rate of the reaction is

A) inversely proportional to the concentration of each reactant
B) directly proportional to the concentration of one reactant and inversely proportional to the concentration of the other reactant
C) directly proportional to the inverse of the concentration of each of the two reactants
D) unaffected by concentration as long as the concentrations of the two reactants are always equal to one another
E) directly proportional to the concentration of each reactant

Question 16

For a reaction that is zeroth-order with respect to reactant A, and nth-order overall

A) Rate = k[0]^m[B]ⁿ
B) Rate = k[B]ⁿ
C) Rate = k[A]^m[0]ⁿ
D) Rate = k[A]^m[B]⁰
E) Rate = k[A]⁰[B]⁰

Question 17

For a new chemical reaction involving several reactants, the order of reaction with respect to each reactant

A) is easily determined from reactions similar to the one in question
B) may be determined from the coefficient for the respective reactant in the balanced chemical equation
C) must be determined from the rate constant at time = 0
D) must be determined from the overall order of the reaction
E) must be determined experimentally using one or more methods

Question 18

Which of the following units may be used for describing reaction rates?

A) mol L^-1 s^-1
B) M min^-1
C) M h^-1
D) M s^-1
E) all of these

Question 19

Which of the following factors will affect the rate of a reaction only if it is heterogeneous?

A) the surface area of the reactants
B) the concentrations of reactants
C) the concentrations of the products
D) the temperature of the reaction system
E) the presence of catalysts

Question 20

The rate of a chemical reaction may be expressed as

A) the instantaneous rate at some moment in time after the reaction has begun
B) the average change in the concentration of a reactant or product over some unit of time
C) the instantaneous rate at the very beginning of the reaction when time = 0
D) any of these
E) none of these

Question 21

The decomposition of N₂O(g) to nitrogen and oxygen has the rate law where k = 1.1 × 10^-3 M^-1 s^-1 at 565 ^$\circ$ C. If the initial concentration of a sample of N₂O is 0.50 M, what is the concentration after one hour at 565 ^$\circ$ C?

A) 9.5 × 10^-3 M
B) 2.2 × 10^-3 M
C) 0.029 M
D) 0.17 M
E) 0.46 M

Question 22

In a bimolecular reaction, which term can be equated to the probability that a collision between molecules will have the correct relative orientation for chemical reaction to occur?

A) steric factor
B) Arrhenius Equation
C) molecularity
D) activation energy
E) frequency factor

Question 23

A certain reaction is studied at room temperature, and gives a straight line plot for 1/[reactant] versus time. Which one of the following statements is true?

A) The same reaction at a higher temperature would yield a similar plot parallel to the original graph, with a larger value for the y-intercept.
B) The same reaction at a higher temperature would yield a similar plot parallel to the original graph, with a smaller value for the y-intercept.
C) The reaction is second-order.
D) The same reaction at a higher temperature would yield a plot with a smaller slope than the original graph, but with the same y-intercept.
E) A graph of ln[reactant] versus 1/time will be a straight line at both temperatures.

Question 24

From the stoichiometry of the reaction the rate law

A) is predicted to be third-order overall.
B) is predicted to be first-order in I₂(s).
C) is predicted to be first-order in all reactants.
D) is predicted to be fifth-order overall.
E) cannot be predicted.

Question 25

The Arrhenius equation is

A)
B)
C)
D)
E)

Question 26

The half-life for the first-order conversion of A to B is 2.22 hr. What is the rate constant?

A) 0.312 hr^-1
B) 0.465 hr^-1
C) 1.54 hr^-1
D) 2.22 hr^-1
E) 3.20 hr^-1

Question 27

The species representing the combination of atoms or molecules at the top of the activation energy barrier is called the

A) frequency factor.
B) activated complex.
C) catalytic convertor.
D) reaction intermediate.
E) enthalpy of reaction.

Question 28

For a first-order reaction, which expression describes the relationship between the rate constant, k, and the half-life, t_1/2?

A) t_1/2 = 2 / k
B) t_1/2 = ln 2 / k
C) t_1/2 = k / 2
D) t_1/2 = ln 2 × k
E) t_1/2 = k / ln 2

Question 29

A reaction displays zero-order kinetics for its single reactant. It therefore follows that a plot of ____ versus time is linear, and that the slope of this plot = ____.

A) [reactant]; -k
B) [reactant]; k
C) 1/[reactant]; -k
D) 1/[reactant]; k
E) ln[reactant]; -k

Question 30

The frequency factor in the Arrhenius equation can best be described as

A) the probability that a collision between molecules will have the correct relative orientation for reaction to occur.
B) a numerical description of the amount of energy released by colliding reactant molecules when they form products.
C) a numerical description of how often molecules collide with the proper orientation to react at a specific concentration.
D) a numerical description of the amount of energy needed by colliding reactant molecules in order to form products.
E) a factor which corrects for the conversion between J and kJ.

Question 31

Consider the reaction with the mechanism shown: The first step is ____; the second step is ____; and the third step is ____.

A) unimolecular; unimolecular; unimolecular
B) unimolecular; bimolecular; bimolecular
C) bimolecular; unimolecular; bimolecular
D) bimolecular; bimolecular; unimolecular
E) bimolecular; bimolecular; bimolecular

Question 32

Consider the reaction Use the data shown to determine the rate law for the reaction.

A) rate = k[NO₂] [O₃]
B) rate = k[NO₂]² [O₃]
C) rate = k[NO₂] [O₃]²
D) rate = k[NO₂]
E) rate = k[NO₃] [O₃] / [NO₂] [O₂]

Question 33

The reaction is ____ order with respect to NO, ____order with respect to Cl₂, and ____ order overall.

A) first; second; third
B) first; second; second
C) first; first; second
D) second; second; third
E) second; first; third

Question 34

The activation energy in the Arrhenius equation can best be described as

A) a numerical description of the amount of energy released by colliding reactant molecules when they form products.
B) a factor which corrects for the conversion between J and kJ.
C) a numerical description of how often molecules collide with the proper orientation to react at a specific concentration.
D) the probability that a collision between molecules will have the correct relative orientation for reaction to occur.
E) a numerical description of the amount of energy needed by colliding reactant molecules in order to form products.

Question 35

If an elementary reaction is exothermic, then

A) The activation energy for the forward reaction is smaller than that for the reverse reaction.
B) The sum of activation energies gives the value of DH for the reaction.
C) The activation energy for the forward reaction is larger than that for the reverse reaction.
D) The activation energy for the forward reaction is positive, and for the reverse reaction, negative.
E) The activation energy for the forward reaction is negative, and for the reverse reaction, positive.

Question 36

The rate constant for the first-order conversion of A to B is 3.33 hr^-1. How much time will be required for the concentration of A to reach 75% of its original value?

A) 0.086 hr
B) 0.83 hr
C) 0.96 hr
D) 2.50 hr
E) 4.44 hr

Question 37

For a zero-, first- or second-order reaction the order of reaction with respect to reactant A may be determined

A) from the specific rate law, if it is already known
B) from the integrated rate law, if it is already known
C) from plots of [A]_t vs. t, ln[A]_t vs. t, and 1/[A]_t vs. t
D) any of these
E) none of these

Question 38

When two atoms, molecules or ions are involved in an elementary reaction, it is said to be a(n):

A) activated complexation.
B) unimolecular reaction.
C) first-order reaction.
D) bimolecular reaction.
E) Arrhenius reaction.

Question 39

For the reaction the integrated rate law for a particular order of reaction

A) relates concentration [A]_t to concentration [A]₀
B) allows the determination of k when [A]₀ and [A]_t are known
C) allows the calculation of [A]_t when k and [A]₀ are known
D) allows the calculation of [A]₀ when k and [A]_t are known
E) all of these

Question 40

The rate constant for the first order decomposition of SO₂Cl₂ is 1.37 × 10^-3 min^-1 at a certain temperature. If the concentration is initially 0.500 M, predict it after 300 min.

A) 6.85 × 10^-4 M
B) 0.331 M
C) 0.500 M
D) 0.754 M
E) 1.33 M

Question 41

Exhibit 12-2 For the following question(s), consider the following reaction mechanism:
Refer to Exhibit 12-2. The rate-limiting step in this reaction is Step ____, because ____.

A) 3; the last step is always the rate-limiting step
B) 2; it uses a product of the first step
C) 2; it is the slow step
D) 1; it is bimolecular
E) 1; the first step must occur before any others can occur

Question 42

The best description of the induced fit model of enzyme function is

A) a non substrate molecule must first bind to the active site, allowing the substrate to fit properly.
B) a molecule not directly involved in the catalysis interacts with a site on the protein far from the active site, changing the shape of the active site to make it work.
C) the active site, the substrate, or both change shape upon binding so that the activation energy of the desired change is decreased.
D) the active site has a definite, rigid shape that only fits one substrate molecule.
E) a short protein chain that blocks access to the active site is chemically removed as the substrate approaches.

Question 43

Which of the following best describes a typical mechanism for an enzyme-catalyzed reaction?

A) Slow formation of an enzyme-substrate complex, then fast formation of products and regeneration of enzyme
B) Fast formation of an enzyme-substrate complex, then slow formation of products and regeneration of enzyme
C) Slow binding of a cofactor to the substrate, then fast reaction with the enzyme to form an enzyme-substrate complex
D) Fast binding of a cofactor to the substrate, then slow reaction with the enzyme to form an enzyme-substrate complex
E) Slow formation of regenerated enzyme, then fast formation of products

Question 44

The function of an automobile catalytic converter is to convert

A) CO to CO₂ and NO to N₂.
B) C to CO and NO to N₂.
C) CO to CO₂ and O₂ to O₃.
D) CO to CO₂ and NO to N₂O.
E) CO to CO₂ and N₂ to NO.

Question 45

In a heterogeneous chemical reaction requiring the use of a solid catalyst, the rate of the reaction could not be increased by increasing the concentration of reactants, but was doubled by doubling the amount of solid catalyst added. This probably indicated that

A) a minimum amount of catalyst was required to make the catalyst active
B) the active sites on the initial amount of catalyst were fully saturated
C) catalysts should be considered as just another chemical reactant
D) the catalyst actually does not have any effect on the rate of this reaction at all
E) all of these

Question 46

Which of the following factors increases the rate of a chemical reaction by increasing the fraction of molecules with energy in excess of a given value?

A) increasing the concentration of products
B) increasing the concentration of reactants
C) increasing the temperature
D) adding a catalyst
E) increasing the surface area of a phase interface

Question 47

The substrate in an enzyme-controlled biological reaction is a

A) molecule serving as a rigid support for the enzyme.
B) molecule that is acted on by the catalyst.
C) species necessary for the proper functioning of the enzyme.
D) biological molecule used for storage of energy.
E) catalyst for that chemical reaction.

Question 48

The solid catalysts used in many industrial processes where the reactants are liquids or gases are referred to as

A) silicon catalysts.
B) zeolites.
C) homogeneous catalysts.
D) heterogeneous catalysts.
E) enzymes.

Question 49

An enzyme is a(n)

A) organic or inorganic molecule or ion necessary for proper functioning of biological catalysts.
B) catalyst for chemical reactions in a living system.
C) protein molecule whose primary function is transport of insoluble molecules in the bloodstream.
D) molecule that is acted on by a catalyst in a living system.
E) biological molecule used for storage of energy.

Question 50

Which statement is true about a catalyst?

A) It increases the activation energy involved in a reaction.
B) It can be the same phase as the reactants (heterogeneous) or a different phase (homogeneous).
C) It is formed during an early step in the reaction and consumed in a later step.
D) It does not participate in the reaction.
E) It does not appear in the overall equation for the reaction.

Question 51

Exhibit 12-2 For the following question(s), consider the following reaction mechanism:
Refer to Exhibit 12-2. The intermediates in this reaction are

A) Z and V.
B) X and Y.
C) Y only.
D) W and Y.
E) There are no intermediates in this reaction.

Question 52

If the rate law for an elementary reaction is second order, we can conclude that

A) no catalysts are involved in the reaction.
B) no intermediates are formed in the reaction.
C) additional experiments at different temperatures are needed to determine the molecularity of the reaction.
D) the elementary reaction involves a collision between two molecules.
E) the elementary reaction is unimolecular.

Question 53

The kinetics of a reaction are observed to be third-order. The least likely mechanism

A) involves three molecules reacting together in a single step.
B) involves two molecules reacting in one step, and the third in a subsequent step.
C) involves a single intermediate.
D) involves more than one intermediate.
E) involves a fast step followed by a slow step.

Question 54

A reaction intermediate is a species which

A) speeds up a reaction by lowering the activation energy.
B) slows down a reaction by raising the activation energy.
C) is always produced in the rate-determining step.
D) is always produced in an early step and consumed in a later step.
E) is always consumed in an early step and regenerated in a later step.

Question 55

The rate constant for the reaction 1 doubles on raising the temperature from 410 ^$\circ$ C to 425 ^$\circ$ C. The activation energy, in J/mol, is

A) 5.3 × 10⁵.
B) 1.8 × 10⁵.
C) 9.1 × 10⁴.
D) 6.7 × 10⁴.
E) -1.9 × 10⁵.

Question 56

The ____of a multistep reaction can be predicted unambiguously from the ____, but not vice versa.

A) mechanism; kinetic rate law
B) mechanism; Arrhenius Equation
C) activation energy; mechanism
D) kinetic rate law; mechanism
E) mechanism; activation energy

Question 57

If an elementary reaction involves the collision of two identical molecules, then the elementary reaction is ____, and the kinetics will be ____.

A) unimolecular; zeroth-order.
B) unimolecular; first-order.
C) unimolecular; second-order.
D) bimolecular; first-order.
E) bimolecular; second-order.

Question 58

Which of the following factors increases the rate of a chemical reaction by decreasing the activation energy barrier?

A) increasing the temperature
B) decreasing the temperature
C) adding a catalyst
D) increasing the surface area of a phase interface
E) increasing the concentration of reactants

Question 59

Which statement, comparing catalyzed and uncatalyzed versions of a reaction, is incorrect?

A) The initial and final energy states are the same.
B) The reaction mechanisms are different.
C) The reaction progress diagrams are different.
D) The balanced equation for the overall reaction is the same.
E) The activation energy is lower in the uncatalyzed reaction.

Question 60

Exhibit 12-2 For the following question(s), consider the following reaction mechanism:
Refer to Exhibit 12-2. The catalysts in this reaction are

A) Z and V.
B) X and Y.
C) Y only.
D) W and Y.
E) There are no catalysts in this reaction.

Question 61

The table below gives the rate constant for a certain reaction at
a. the activation energy for this reaction.
b. the frequency factor.
c. the value of k at 1100K.

Question 62

The table below gives the concentration of a substance A as a function of time at 473 K.

a. Establish whether the reaction is zeroth, first or second-order in A.
b. Calculate the rate constant at 473 K for this reaction.

Question 63

Phosgene, COCl₂, is formed from Cl₂ and CO at high temperatures. The mechanism is thought to be:

What is the rate law predicted for this mechanism?

Question 64

Consider this proposed mechanism for the reaction of NO₂ and CO:
a. Write the overall reaction represented by this mechanism.
b. What are the intermediates in this reaction, if any?
c. What are the catalysts in this reaction, if any?
d. Write the rate law predicted by the mechanism above.
e. Write the rate law you would predict if the reaction went in a single step.

Question 65

The conversion of substance A to substance B has DE = -35 kJ mol^-1 and an activation energy, E_a, of 25 kJ mol^-1. Sketch a reaction diagram, and on it clearly indicate A, B, the activated complex (C), DE and E_a.

Question 66

When studying kinetics, it is more accurate to measure instantaneous reaction rates at particular times rather than the average rates of reaction over time intervals. Explain why.