CHAPTER 14 SMARTBOOK

Which elementary step in the proposed mechanism is the rate-determining step?

(2) X (g) + C (g) → Y (g); [slow]

match each rate law to the correct units for the rate constant k - rate = k - rate = k[A] - rate = k[A][B] - rate = k[A][B]^2

- M/s - 1/s - 1/Ms - 1/M^2s

In a given reaction, the rate may be expressed as either rate = - Δ[Y]Δt, or as rate = Δ[X]Δt. What must be true about X or Y if the reaction is proceeding in the forward direction? Select all that apply.

Y is a reactant. X is a product.

Consider the mechanism shown. If the observed rate law is rate = k[NO]2[Br2] is the proposed mechanism valid? What is the rate law for the proposed mechanism? Select all that apply.

Yes, this mechanism is valid. rate = k2 k1k−1[NO]2[Br2] or rate = k[NO]2[Br2]

The equation below is associated with what order with respect to reactant A? [A]t = -kt + [A]0

Zero-order

If only reactants are initially added to a reaction flask, which species decrease in concentration during the course of the reaction N2 (g) + 3H2 (g) → 2NH3 (g)? Select all that apply.

[H2] [N2]

The characteristics of a first-order reaction (A → products) include ______. Select all that apply.

a rate law of the format: rate = k[A] a reaction rate that depends on the concentration of A raised to the first power

In the reaction A + B → AB → C + D, AB is a high-energy, temporary species formed by the reactant molecules. AB is called the _____. (Select both accepted names for this species).

activated complex transition state

The________ energy of a reaction is the energy threshold that colliding molecules must exceed in order to react.

activation

A reaction rate measured over a time period Δt is called the reaction rate.

average

Collision theory states that particles will react when they_____ with each other. For a reaction to be successful, the particles must have enough ______energy.

collide ; kinetic

Reaction rate is expressed as a change in the __________of reactants or products as a function of_________-

concentration ; time

Starting with only reactants, during the course of a reaction, reactant concentrations _____ and product concentrations _____.

decrease, increase

The individual steps that make up a reaction mechanism are called _____ reactions or steps.

elementary

For a collision to be effective, the colliding particles must have enough _____and the appropriate molecular__________

energy ; orientation.

If ln[A] = y, [A] = _____.

ey

If the rate doubles when [A] doubles, the reaction is If the rate quadruples when [A] doubles, the reaction is If the rate does not change when [A] doubles, the reaction is

first order with respect to A. second order with respect to A. zero order with respect to A.

In the Arrhenius equation (shown), the factor A is related to the ______of collisions between particles that result in a chemical reaction. k = Ae-Ea/RT

frequency

The _____ _______ of a reaction is the time required for the reactant concentration to reach half its initial value.

half life

One way to determine the validity of a proposed reaction mechanism is to devise experiments to detect one or more ________that are produced in an early step and consumed in a later step.

intermediate

Match each term in the Arrhenius equation k = Ae-Ea/RT with the correct definition

k = Rate constant A= Frequency factor Ea= Activation energy R= Universal gas constant T= Absolute temperature

chart in notes Slight variations in the calculated values of k are due to experimental deviations. Given the following data (shown), match the rate constant at each time to the calculated value of k for the first order reaction: A → products.

k at t = 0.0 - 1.00 × 10−1 s−1 k at t = 1.5 - 1.08 × 10−1 s−1 k at t = 2.5 s- 1.05 × 10−1 s−1 k at t = 5.0 s - 1.03 × 10−1 s−1

The half-life equation for a first-order reaction (t1/2 = 0.693k) is derived from what equation?

ln[A]t =−kt [A]0

Which of the following is the integrated form of a first-order rate law for the reaction A → products?

ln[A]t = -kt + ln[A]0

The _____ the activation energy, or the _____ the temperature, the larger the fraction of sufficiently energetic collisions, the greater the value of k, and the greater the reaction rate.

lower; higher

Enzymes are _____ that catalyze biological ____.

proteins; reactions

Chemical kinetics is studies the ___________ in which reactions take place

rate

The _________constant, given the symbol k, expresses the relationship between reactant concentration and reaction rate for a given reaction at a given temperature.

rate

Select all of the following rate laws where the rate constant k has units equal to M−2·s−1. All rates are represented with units of M·s−1.

rate = k[A][B][C] rate = k[A]2[B] rate = k[A]3

For the reaction A → B, which of the following equations correctly express the reaction rate at time t? Select all that apply.

rate = Δ[B]/Δt rate = - Δ[A]/Δt

In the Arrhenius equation k = Ae-Ea/RT if T increases the magnitude of the negative exponent becomes _____, which means k becomes _____, and the rate _____.

smaller, larger, increases

Which of the following is the correct expression for the half-life of a first-order reaction?

t1/2 = 0.693/k

Which of the following is the correct expression for the half-life of a second-order reaction?

t1/2 = 1 ---- k[A]0

The reaction 2NO (g) + 2H2 (g) → N2 (g) + 2H2O (g) is found to obey the following rate law rate = k[NO][H2]2, where k = 5.0 x 102 M-2s-1. Rank the following sets of concentrations in order of reaction rates, with the highest rate conditions appearing at the top of the list.

2x10= 1 1x10= 1 2x10= 5 4x10= 1

Calculate the average rate of reaction between t = 50.0 s and t = 100.0 s for the reaction between Br2 and HCOOH, given the data provided.

3.28 x 10-5 M/s

At 25°C, Hl breaks down very slowly to form H2 and I2. The reaction follows second-order kinetics and k at 25°C is 2.4 * 10−21 L/mol.s. If 0.0100 mol of Hl(g) is placed on a 1.0-L container, how long will it take for the concentration of Hl to reach 0.00900 mol/L?

4.6

Calculate the rate of the reaction A + B → C when [A] = 0.200 M and [B] = 0.400 M, given that the rate law for the reaction is rate = k[A][B]2. The rate constant for the reaction is 3.0 x 10-2 s-1M-2.

9.6 x 10-4 M/s

Match the changes in temperature and activation energy (Ea) for a given reaction with their effect on the rate and rate constant of a reaction.

A decrease in Ea leads to = an increase in rate. A decrease in temperature leads to = a decrease in rate. k increases = if temperature increases. k decreases = if Ea increases.

For the reaction 2A → B, which of the following statements best describes the relationship between the rates of change of reactants and products? Select all that apply.

A is used up twice as fast as B forms. The rate of decrease of A is equal to the rate of increase of B multiplied by two.

Select all the statements that correctly describe a reaction mechanism.

A reaction mechanism helps to depict how a reaction actually takes place through showing each bond-breaking and bond-making step. The sequence of steps that sum to give the overall reaction is called the reaction mechanism.

Select all of the statements that correctly describe a second-order reaction. Use the example: A → products.

A second-order reaction involving a single reactant has a rate that depends on the concentration of that reactant raised to the second power. If the initial concentration of A is doubled the reaction rate increases by a factor of 4. The rate law for a reaction that is second order in reactant A is: rate = k[A]

Select all the statements that correctly describe an elementary reaction.

An elementary reaction occurs in a single collision of the reacting particles. An elementary reaction is a single step in a reaction mechanism.

Which of the following conditions must be satisfied for a collision between reactants to be effective? Select all that apply.

Appropriate molecular orientation Enough energy to overcome Ea

Which of the following statements best describes the effect of reactant concentration on reaction rate?

As reactant concentration increases, the number of reactant collisions increases, which in turn increases the reaction rate.

The experimentally determined rate law for the overall reaction (A + B2 + C → AB + CB) is: rate = k[B2]. Select all of the plausible mechanisms from the options shown.

B2 → 2B slow A + B → AB fast C + B → CB fast B2 → 2B slow A + C → AC fast AC + 2B → AB + CB fast

What may be included in a reaction mechanism? Select all that apply.

Bond-breaking steps Formation of a new species that does not appear in the overall reaction A catalyst Bond-making steps

A reaction is first order with respect to reactant A. Doubling the initial concentration of A would have what effect on the initial reaction rate? A → products.

Double the reaction rate

Select all the statements that correctly describe enzymes.

Enzymes are biological catalysts. An enzyme is typically a large protein molecule that contains one or more active sites where interactions with substrates take place. Enzymes can increase the rate of biochemical reactions by factors ranging from 10⁸ to 10¹⁸

Select all of the following statements that correctly describe experimental support for reaction mechanisms.

Experiments are used to determine whether or not a proposed reaction mechanism is plausible. Isotope labeling experiments can be used to distinguish between different possible reaction schemes

For the general reaction A → products, a plot of ln[A] vs. time gives a straight line. What is the order of this reaction with respect to A?

First order

Which of the following statements correctly describe the average rate of a reaction? Select all that apply.

For the reaction A → B, the average reaction rate is expressed as [B]final − [B]initial tfinal − tinitial. The average reaction rate generally decreases with time. Average reaction rate expresses the decrease in reactant particles with time.

A catalyst that is in a different phase than the reactants is a(n) catalyst.

Heterogeneous

What are common advantages of homogeneous catalysts? Select all that apply.

Homogeneous catalysts can be designed to function selectively for particular types of reactions. Homogeneous catalysts commonly cost less than the alternative heterogeneous catalysts (precious metals). Reactions including homogeneous catalysts can often be carried out under atmospheric conditions.

Which of the following statements correctly relate experimental observations with reaction order? Select all that apply. (Assume A is a reactant in all cases.)

If [A] is doubled and the reaction rate increases by a factor of 4, the reaction is second order with respect to A. If [A] is decreased to one-half of its original value and the reaction rate is halved, the reaction is first order with respect to A

Which of the following statements correctly describe how a rate law is determined? Select all that apply.

If there is more than one reactant present the reaction orders will usually be determined individually. The rate law is determined by measuring the initial reaction rate for different reactant concentrations. The value of k can be calculated from a single experiment once the reaction orders are known.

What type of reaction rate is typically used to find other kinetic parameters?

Initial rate

Which of the following options correctly describe instantaneous rate? Select all that apply.

Instantaneous rate is equal to the slope of a tangent to the curve of a concentration vs. time plot at any particular time. Instantaneous rate is the rate for a specific instant in time.

Which units may be used to express reaction rate?

M/s

Which of the following statements correctly describe rate of reaction? Select all that apply.

Rate of reaction avoids reporting the rate of disappearance or appearance of a specific chemical species in a reaction. Rate of reaction = 1coefficientΔ[product]Δtime where the coefficient is the number in front of the product in the balanced chemical equation.

Which of the following statements correctly describe the relationship between reaction rate and concentrations of reactants and products? Select all that apply.

Reactants (or products) with the same coefficient have the same rates of disappearance (or appearance). Reactants (or products) with large coefficients have relatively high rates of disappearance (or appearance).

What are the most common types of heterogeneous catalysts?

Solids

Which of the following criteria must be met for a reaction mechanism to be considered valid? Select all that apply.

The elementary steps must add up to the overall balanced reaction. The mechanism must correlate with the experimentally determined rate law. The elementary steps must be physically reasonable.

Which of the following statements correctly describe a rate law for a reaction? Select all that apply. (Assume that the reaction is not reversible to any appreciable extent.)

The rate law includes the rate constant for the reaction. The rate law relates the rate of reaction to the concentrations of the reactants.

Which of the following statements correctly describe a zero-order reaction? Select all that apply.

The reaction rate is independent of the reactant concentration. When [A]t is plotted against t, a straight line results.

Which of the following statements correctly describe the straight-line plot obtained from concentration and time data for the first-order reaction A → products? Select all that apply.

The slope of the line is negative. A straight line is obtained by plotting ln[A] versus time. The rate constant for the reaction is related to the slope of the straight line.

Solve for the rate constant k given the following information for the first-order reaction represented by A → products. Time (s) [A] (M)Rate (M/s)0.00.02504.5 × 10−4250.01803.2 × 10−4500.00781.4 × 10−4

0.018

Calculate the value of [A]t using the equation given below, if k = 2.0 × 10-2 s-1, t = 30.0 s, and [A0] = 0.50 M. ln[At] = -kt + ln[A0]

0.28 M

The reaction AB (g) → A (g) + B (g) follows second-order kinetics. If k = 0.200 L/mol⋅s and [AB]0 = 1.50 M, what is the concentration of AB after 10.0 s?

0.375 M

The concentration of oxygen is being monitored in a reaction flask. The amount of O2 present is found to be 45.0 ppm after 15 s and 40.0 ppm after 25 s. What is the average rate of change of oxygen (in ppm/s) during this time interval?

0.50 ppm/s

A reaction is first order with respect to reactant A and the rate constant is equal to 1.5 x 10-3 s-1. What is the concentration of A after 5.0 minutes, if the initial concentration of A is 1.0 M?

0.64 M

How long does it take for the concentration of a reactant in a first-order reaction to decrease to 50% of the initial concentration, if the rate constant is equal to 1.00 s-1?

0.69 s

Which of the following plots will give a straight line for a reaction that follows second-order kinetics? The reaction is represented by A → products.

1 ---- vs. time [A]

Which of the following options gives the correct expression for the integrated rate law for a reaction that follows second-order kinetics? The reactant is represented by the equation A → products.

1 = kt + 1 [A] [A]0

Correctly order the steps necessary to assess the validity of the mechanism shown, if the observed rate law is rate = k[A][B][C]. 1. A (g) + B (g) -> X (g) [fast] 2. X (g) + C (g) -> Y (g) [slow] 3. Y (g) -> D (g)

1. fast step: rate = k[A][B] and rate = k[X]slow step: rate = k2[X][C]; the rate of a reaction is determined by its slowest step. analyze the slowest step and steps that precede it 2. [X] = k1/k-1 [A][B] 3. rate = k2k1/k-1 [A][B][C] or k[A][B][C]; the rate for the slowest step is rewritten in terms of [reactants]. the rate law matches the observed rate law

Which of the following statements correctly describe chemical kinetics? Select all that apply.

Chemical kinetics is the study of reaction rates. Chemical kinetics measures how fast reactants are converted into products.

The mechanism shown has been proposed for the destruction of ozone (O3) by highly reactive chlorine atoms. Match each species in this mechanism with its correct role.

Cl = catalyst ClO = intermediate O3 = reactant O2 = product

Which of the following options correctly describe the principles of collision theory? Select all that apply.

Collision theory states that in general particles must collide in order to react. When particles collide, some of their kinetic energy is converted to vibrational energy. Reaction rate is directly proportional to the number of collisions per second.

Instantaneous rate is equal to the slope of a tangent line at any particular point of which of the following plots?

Concentration versus time

Which of the following statements correctly describe the half-life of a reaction? Select all that apply.

The half-life of a first-order reaction is independent of the initial concentration. The half-life of a reaction is related to its rate constant. The half-life of a reaction is the time taken for [A]0 to drop to half its original value.

Which of the following options correctly describe the half-life of a second-order reaction? Select all that apply.

The half-life of a second-order reaction is inversely proportional to the initial reactant concentration. The half-life of a second-order reaction is related to the rate constant.

Which of the following statements correctly describe initial rate or the use of initial rate? Select all that apply.

The initial rate is an instantaneous rate. Initial rate is commonly measured at different reactant concentrations in order to determine a rate law for a reaction.

The rate law for the reaction NO2 (g) + CO (g) → NO (g) + CO2 (g) is given by rate = k[NO2]2. Consider the mechanism depicted in the illustration. Which options correctly describe the validity of this mechanism? Select all that apply.

The mechanism does agree with the experimental rate law. The elementary steps add up to the overall balanced equation.

The experimentally determined rate law for the overall reaction (2A + B2 → A2B2) is: rate = k[A][B2]. A student proposed the following reaction mechanism. What makes this mechanism NOT plausible? A + B2 → AB2 slow 2A + B2 → A2B2 fast

The mechanism shown is not consistent with the overall reaction.

Which of the following statements correctly describes the activation energy of a reaction?

The minimum energy that the colliding particles must have in order to react

What is molecularity? Select all that apply.

The number of reactant particles that react in an elementary step The number of reactant particles involved in a single collision

Which of the following statements correctly describe the rate constant k for a reaction? Select all that apply.

The rate constant has a constant value for a given reaction at a constant temperature. The rate constant relates reaction rate to reactant concentration.

Which of the following factors are affected by an increase in reactant concentration? Select all that apply.

The rate of reaction The number of reactant collisions The number of reactant particles in a given volume

Select all the statements that correctly describe the rate-determining step of a reaction mechanism.

The rate-determining step is the slowest step in a reaction mechanism. The rate of an overall reaction is determined by the rate of the rate-determining step in a reaction mechanism. The rate law for an overall reaction is based on the rate-determining step.

Which of the following statements describe an integrated rate law? Select all that apply.

The reactant concentration at any time in the reaction can be determined from the integrated rate law. An integrated rate law includes time as a variable. The value of the reaction constant, k, can be determined using an integrated rate law.

Which of the following statements correctly describe the transition state of a reaction? Select all that apply.

The transition state species is highly unstable. The transition state only exists at the instant of highest potential energy in the reaction

For the reaction A → product, in which of the following situations should the equation ln[A]t = -kt + ln[A]0 be used?

This equation should be used to calculate the concentrations of reactant A at a given time if the reaction is first order in A.

Match each type of elementary reaction to the correct description

Unimolecular elementary reaction - One reactant molecule decomposes in a single reaction step. Bimolecular elementary reaction - Two reactant molecules collide in a single reaction step. Termolecular elementary reaction - Three reactant molecules collide in a single reaction step.

Which question(s) can be answered from an integrated rate law that cannot be answered by other forms of the rate law? Select all that apply.

What is [A] after y minutes of the reaction? How long will it take to use up x moles per liter of A?