SmartBook Assignment Chapter 14: Chemical Kinetics
The rate law for the reaction 2NO2 (g) + F2 (g) → 2NO2F (g) is found to be rate = k[NO2][F2]. Which of the following proposed mechanisms is consistent with this rate law?
1: NO2 + F2 → NO2F + F (slow) 2: NO2 + F → NO2F (fast)
Write a balanced chemical equation for the reaction that has the following rate equation: see image
2N2O5 (g) → 4NO2 (g) + O2 (g)
A first-order reaction has a half-life of 4.1 minutes. The percentage of reactant remaining after 20 minutes is approximately equal to ______.
3.1% Reason: 20 minutes represents 20/4.1 or approximately 5 half-lives. Since the reactant concentration is reduced by half during each half-life, in 5 half-lives it is reduced to 1/2^5 = 0.031 or 3.1% of its original value.
At 25°C, HI 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 HI(g) is placed in a 1.0-L container, how long will it take for the concentration of HI to reach 0.00900 mol/L?
4.6 × 10^21 s
The table shown gives concentration vs. time data for the decomposition of nitrogen dioxide, shown below. What is the average rate of the reaction over the time interval from 20.0 s to 30.0 s? NO2 (g) → NO (g) + 1/2 O2 (g)
5.0 × 10^-4 M/s Average rate = - ΔM/Δt = - 0.013M-0.018M/30.0s-20.0s = 5.0 × 10^-4 M/s
Iodine-123 breaks down in a first-order process with a half-life of 13.1 h. What is the rate constant for this process?
5.29 × 10^-2 h^-1
Select all the options that correctly describe a reaction intermediate.
A reaction intermediate is formed in one step of a reaction and used up in a later step. The formation of a reaction intermediate is described in the reaction mechanism.
Identify the reaction intermediate(s) in the reaction mechanism shown.
(CH3)3C^+ (CH3)3C-OH2^+
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 × 10^-5 M/s average rate = - 0.00846−0.0101/100−50 = +3.28 × 10^-5 M/s
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
Which of the following statements correctly describe catalysts? Select all that apply.
Catalysts lower the activation energy of the rate-determining step in the reaction. Catalysts provide a different mechanism for the reaction.
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. Chemical kinetics describes chemical reactions using collision theory.
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. Reaction rate is directly proportional to the number of collisions per second. When particles collide, some of their kinetic energy is converted to vibrational energy.
Which of the following can provide experimental data that supports a proposed reaction mechanism?
Detection or characterization of a reaction intermediate Isotope labeling experiments
A the value of the rate constant k was studied at different temperatures for a particular second-order reaction. From the data given, determine the value of Ea for the reaction, as well as the value of the frequency factor A.
Ea = 163 kJ/mol A = 4.3 × 10^10
Select all the options that correctly identify the components of the Arrhenius equation (given).
Ea is the activation energy for the reaction. k is the rate constant for the reaction. T is the temperature in K.
What is an elementary reaction?
Each step that makes up a reaction mechanism
Consider the reaction 2H2 (g) + O2 (g) → 2H2O (g). At a particular moment in the reaction, H2 (g) is disappearing at a rate of 0.024 M/s. Select all the statements that correctly describe this reaction.
H2O (g) is being formed at a rate of 0.024 M/s. O2 (g) is being used up at a rate of 0.012 M/s.
Which of the following attributes are characteristic of enzymes? Select all that apply.
High specificity High rates of activity
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.
How long will it take to use up x moles per liter of A? What is [A] after y minutes of the reaction?
Which of the following the statements correctly relate experimental observations with reaction order? Select all that apply. (Assume A is a reactant in all cases.)
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. If [A] is doubled and the reaction rate increases by a factor of 4, the reaction is second order with respect to A.
Which options correctly describe how to assess the validity of a mechanism proposed for a multistep reaction with one or more fast initial steps, given the experimentally determined rate law? Select all that apply.
Intermediate concentrations are expressed in terms of reactant concentrations using rate laws for the fast steps. The overall rate law will reflect the concentrations of any reactants involved in all steps up to and including the rate-determining step.
Which of the following elementary reactions are unimolecular? Select all that apply.
N2O4 (g) → 2NO2 (g) PCl5 (g) → PCl3 (g) + Cl2 (g)
Which of the following conditions must be fulfilled in order to calculate reaction rate using change in pressure over time rather than change in concentration? Select all that apply.
One reactant or product must be a gas. The temperature at which the reaction is carried out must be known.
Which of the following are examples of heterogeneous catalysis? Select all that apply.
Oxides of nitrogen are converted to O2 and N2 in a catalytic converter. Liquid petroleum products react with hydrogen gas on the surface of platinum metal.
NO(g) reacts with H2(g) according to the following balanced equation: 2NO (g) + 2H2 (g) → N2 (g) + 2H2O (g). Which of the following overall rate laws is consistent with the proposed mechanism shown?
Rate = k[H2][NO]2
Determine the overall rate law and the value of the rate constant, k, for the reaction 2HI (g) → H2 (g) + I2 (g) from the data provided.
Rate = k[HI]^2 k = 4.00 × 10^−2 M^−1·s^−1
A plot of concentration versus time for a particular reaction is shown in the accompanying graphic. What can be deduced about the rate of reaction from this plot?
Rate is decreasing over the course of the reaction.
Which of the following species will be included in the overall rate law for a multistep reaction involving 2 or more initial fast steps?
Reactants involved in all steps up to and including the rate-determining step
Which of the following factors will influence the rate of a chemical reaction? Select all that apply.
Surface area of a solid reactant Reaction temperature Reactant concentration
What is the molecularity of the elementary step A (g) + B (g) + C (g) → D (g)?
Termolecular
Select all the options that correctly describe the straight-line plot obtained for rate constant and temperature data.
The activation energy for the reaction can be determined from the slope. The values plotted on the x axis are given by 1/T where T is the absolute temperature.
Which statement correctly describes the active site of an enzyme?
The active site is where the enzyme-catalyzed reaction occurs.
Which of the following statements correctly describe average rate, instantaneous rate, and initial rate? Select all that apply.
The average rate is an average of instantaneous rates for a given period of time. The initial rate is an instantaneous rate at t = 0.
The reaction of HBr with oxygen gas is represented by the following balanced equation: 4HBr (g) + O2 (g) → 2H2O (g) + 2Br2 (g) A proposed mechanism for this reaction is shown. The rate law is found to be rate = k[HBr]{O2]. Select all the options that correctly describe how this mechanism meets, or does not meet, the criteria for validity.
The elementary steps are logical and physically reasonable. The rate law is consistent with the proposed mechanism.
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 rate law for the rate-determining step must correlate with the experimentally determined rate law.
A chemical reaction occurs in three elementary steps and is exothermic overall. The first step is the slowest step. Select all the statements that correctly describe the potential energy profile for this reaction.
The energy of the products will be lower than the energy of the reactants. There will be three peaks in the energy profile.
Which of the following statements correctly describes the activation energy of a reaction?
The energy threshold that the colliding particles must exceed in order to react
Which of the following options correctly describe the different models of enzyme action? Select all that apply.
The enzyme active site can modify its shape to a certain extent to accommodate different substrates. The lock-and-key model proposed that the active site of an enzyme provides a precise fit for its substrate. The enzyme forms an enzyme-substrate complex, which is then converted to product.
Which of the following statements correctly describe the half-life of a reaction? Select all that apply.
The half-life of a reaction is the time it takes for [A]0 to drop to half its original value. The half-life of a reaction is related to its rate constant. The half-life of a first-order reaction is independent of the initial concentration.
Which of the following factors are affected by an increase in reactant concentration? Select all that apply.
The number of reactant collisions The number of reactant particles in a given volume The rate of reaction
What is molecularity?
The number of reactant particles that react in an elementary step
Select all the statements that correctly interpret the experimental data provided for the reaction 2Mg (s) + O2 (g) → MgO (s).
The overall reaction order is 3. The reaction is second order with respect to [O2]. The reaction is first order with respect to Mg.
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.
The order of a reaction and its rate constant can be determined graphically. Select all the statements that correctly describe this process.
The rate constant is related to the slope of the straight line obtained. The function of [A] versus time that gives a straight line plot indicates the reaction order.
The concentration of a reactant in a hypothetical reaction is plotted vs. time in the image shown. Which of the following statements correctly interpret the information shown? Select all that apply.
The rate decreases as the reaction proceeds. The rate of the reaction is changing with time.
Select the statements that correctly describe the kinetics of an elementary reaction.
The rate law for an elementary step can be deduced from its balanced chemical equation. The elementary reaction 2HI (g) → H2 (g) + I2 (g) follows second-order kinetics. For an elementary reaction, reaction order equals molecularity.
Select all the statements that correctly describe the rate-determining step in any reaction.
The rate law for the rate-determining step is determined using experimental data obtained for the overall reaction. The rate-determining step is an elementary step. The rate-determining step is always the slowest step in the reaction.
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 how a rate law is determined? Select all that apply.
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. If there is more than one reactant present the reaction orders will usually be determined individually.
Select all the statements that correctly describe a zeroth-order reaction. The reaction is represented by A → products.
The rate of a zeroth-order reaction is independent of reactant concentration. The rate constant of a zeroth-order reaction is equal to the reaction rate. For a zeroth-order reaction, a plot of [A]t versus time will give a straight line.
H2O2 (aq) decomposes rapidly in the presence of iodide ion (I^−). Select all the statements that correctly interpret the experimental data provided for this reaction.
The reaction is first order with respect to I^−. Rate = k[H2O2][I^−]
Which of the following options correctly interpret the rate law rate = k[H2][NO]^2? Select all that apply.
The reaction is second order with respect to NO. If [NO] is doubled while [H2] remains the same, the reaction will occur 4 times faster.
Select all the options that correctly interpret the potential energy profile shown.
The second step of this reaction is rate-determining. This reaction involves 2 steps. The overall reaction is endothermic.
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. The 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.
Why does a hot steel nail placed in oxygen burn slowly, while the same mass of a piece of steel wool burns much more quickly?
The steel wool has a greater surface area for reaction.
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.
Select the correct value and units for the rate constant, k, for the reaction 4A (g) + 3B (g) → 2C (g), given the data provided. The experimentally determined rate law is rate = k[A]^2[B].
The units for k are M^-2⋅s^-1. The value of k is 5.00 × 10^5.
Select all the statements that describe an integrated rate law.
The value of the reaction constant, k, can be determined using an integrated rate law. An integrated rate law includes time as a variable. The reactant concentration at any time in the reaction can be determined from the integrated rate law.
True or false: The rate law for an elementary step can be deduced from its reaction stoichiometry (i.e., from the balanced equation for the step).
True
What is the reaction intermediate in this two-step mechanism? (1) X + O3 → XO + O2 (2) XO + O → X + O2
XO
Match each point on the energy profile shown with the correct description.
a --- transition-state energy b --- activation energy c --- energy of reactants d --- energy of products
The _____ energy of a reaction is the energy threshold that colliding molecules must exceed in order to react.
activation
A _____ is a substance that increases the rate of a reaction without being consumed. This type of substance is often used up and regenerated during the reaction.
catalyst
The individual steps that make up a reaction mechanism are called _____ reactions or steps.
elementary
A(n) _____ is a protein that acts as a biological catalyst.
enzyme
The use of finely powdered palladium metal to catalyze the reaction shown below is an example of ______ catalysis. C6H12 (l) + H2 (g) → C6H14 (l)
heterogeneous
A ______ catalyst is in a difference phase from the reactants, whereas a ______ catalyst is dispersed in the same phase as the reactants.
heterogeneous, homogeneous
The addition of aqueous H2SO4 to catalyze the hydrolysis shown below would be an example of ______ catalysis. C4H8O2 (aq) + H2O (l) → C2H4O2 (aq) + C2H6O (aq)
homogeneous
The Arrhenius equation shows that the rate constant for a reaction (k) _____ when temperature increases and _____ when activation energy increases.
increases, decreases
The fact that a proposed reaction mechanism is consistent with the observed rate law is not sufficient to prove that the mechanism is correct. Additional experiments are typically necessary to validate a proposed mechanism. These may include attempts to detect a(n) ______; additionally, the paths of specific atoms through a reaction mechanism can be traced through the use of ______.
intermediate; isotopes
The decomposition of NO2 (g) proceeds according to the reaction below: 2NO2 (g) → 2NO (g) + O2 (g) The rate law for this reaction is rate = k[NO2]^2. Using the data shown, calculate the value of k, as well as the half-life for the initial concentration given.
k = 2.5 s^-1 ⋅ M^-1 t1/2 = 2.0 s
When two molecules in motion collide, ______ energy is partially converted to ______ energy; this may result in the breaking of bonds.
kinetic; vibrational
The units for reaction rate are given by _____.
mol/L⋅s
An elementary step is characterized by its _____, which is equal to the number of reactant particles in the step.
molecularity
The rate of a reaction is expressed as Δ[A]/Δt. If A is a reactant a _____ sign must be added in front of this expression in order for the rate to have a _____ value. This is not necessary if A is a product.
negative; positive
An energy profile for a reaction plots ______ energy versus reaction ______.
potential; progress
Chemical kinetics studies the _____ of chemical reactions, i.e., how fast reactants are converted into products.
rate, rates, or speed
The measured rate constants at different temperatures for a particular reaction are provided in the data table. Select the options that correctly represent the steps required to calculate the activation energy for this reaction from the data provided.
slope = 3.13−3.00/0.0077−0.0083 = -2.17 × 10^2 Ea = 1.80 kJ/mol
For a proposed reaction mechanism to be valid, the rate law determined from experimental data must match the rate law for the ______.
slowest reaction step
The dimerization of butadiene proceeds according to second-order kinetics. If the rate constant at a particular temperature is 0.100 M^-1 ⋅ min^-1, calculate the half-life for an initial reactant concentration of 0.35 M.
t1/2 = 29 min
The Arrhenius equation relates the rate constant of a reaction to ______. Select all the options that would complete this sentence correctly.
the frequency of collisions between particles the absolute temperature the activation energy
Which of the following can be calculated using the two-point form of the Arrhenius equation (given)?
the rate constant at different temperatures the activation energy of the reaction
In an example of homogeneous catalysis, the reactant(s) and the catalyst are in ______ phase(s).
the same
True or false: For a zeroth-order reaction, the rate constant k is equal to the rate of the reaction.
true
For the general rate law rate = k[A]^x[B]^y, the overall reaction order is given by _____.
x + y
Which of the following are described by a reaction mechanism? Select all that apply.
An explanation of the way in which the individual reactants are transformed to products The sequence of individual steps that make up the reaction The identities of any reaction intermediates
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.
Why does average reaction rate change over time? (Assume the reaction is not reversible to any appreciable extent.)
As the reaction proceeds, reactant concentrations decrease; hence rate decreases over time.
The rate law for the reaction 2NO (g) + H2 (g) → N2O (g) + H2O (g) is found to be rate = k[H2][NO]2. Which of the following mechanisms is consistent with this rate law?
B
Calculate the activation energy for the reaction 2NOCl (g) → 2NO (g) + Cl2 (g), if the rate constant k is equal to 0.286 L/mol⋅s at 500. K and 0.175 L/mol⋅s at 490. K.
1.00 × 10^2 kJ/mol
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/[A] versus time
Every chemical reaction proceeds through a high-energy unstable species as reactants are converted to products and bonds are in the process of being formed and/or broken. This unstable species is called a(n) _____ complex or _____ state.
Blank 1: activated Blank 2: transition
Collision theory states that particles will react when they _____ with each other. For a reaction to be successful, the particles must have enough _____ energy.
Blank 1: collide Blank 2: kinetic
Reaction rates are expressed as the change in _____ over a period of _____.
Blank 1: concentration or molarity Blank 2: time
Reaction rate is expressed as a change in the _____ of reactants or products as a function of _____.
Blank 1: concentration, concentrations, amount, amounts, molarity, or molarities Blank 2: time
As a chemical reaction proceeds, the concentrations of the reactants will _____. This results in a _____ in the rate of the reaction with time.
Blank 1: decrease, lessen, go down, drop, or change Blank 2: decrease, drop, lowering, lessening, or change
For a collision to be effective, the colliding particles must have enough _____ and the appropriate molecular _____.
Blank 1: energy, E, or kinetic energy Blank 2: orientation, alignment, positioning, arrangement, configuration, or conformation
The _____-_____ of a reaction is the time required for the reactant concentration to reach half its initial value.
Blank 1: half Blank 2: life
A species that is formed during one step of a reaction and used up in a subsequent step is called a reaction _____.
Blank 1: intermediate
An increase in temperature causes the reaction rate to increase. At a higher temperature the average _____ energy of the particles is greater, and the fraction of particles with enough energy to exceed the _____ energy is greater.
Blank 1: kinetic Blank 2: activation
Kinetic data are often analyzed graphically. Depending on the reaction order, a function of [A] is plotted versus time to obtain a straight _____ plot. The value of the rate constant is determined from the _____ of the plot. (Note: There is no actual graph given in this question.)
Blank 1: line Blank 2: slope
The reaction _____ provides information about the individual steps of a reaction, and identifies any reaction _____, which are formed during one step and consumed in a subsequent step.
Blank 1: mechanism Blank 2: intermediates
In the rate law rate = k[A]^x[B]^y, the exponents x and y are called the reaction _____ . If x = 1, the reaction is said to be _____ _____ with respect to reactant A.
Blank 1: orders Blank 2: first or 1st Blank 3: order
The _____ constant, given the symbol k, expresses the proportionality between reactant concentration and reaction rate for a given reaction. The value of k is constant at constant _____.
Blank 1: rate Blank 2: temperature or T
The _____ step in any reaction sequence determines the rate law for the overall reaction. This step is called the rate-_____ step.
Blank 1: slowest, slow, or slower Blank 2: determining or limiting
An enzyme binds to a reactant, which is called the _____, at the _____ site of the enzyme.
Blank 1: substrate Blank 2: active
If a lump of solid reactant is ground finely it will have a greater _____ area for reaction, leading to a larger number of _____ between particles and therefore an increase in reaction rate.
Blank 1: surface Blank 2: collisions or effective collisions
Using a plot of [reactant] versus time, the instantaneous reaction rate is calculated by finding the slope of a(n) _____ to the curve at any particular time. Instantaneous rate _____ as [reactant] decreases. (Note: this question contains no graph.)
Blank 1: tangent, tangent line, or line tangent Blank 2: decreases, lowers, drops, or diminishes
When two reacting particles collide, some of their kinetic energy is converted to _____ energy. If their kinetic energy is large enough, the particles will vibrate strongly, causing chemical _____ to break.
Blank 1: vibrational Blank 2: bonds
Which of the following species will appear in the overall rate law for the overall reaction 2NO (g) + Br2 (g) → 2NOBr (g), given the mechanism shown?
Br2 NO
Select all the statements that correctly describe how a chemical reaction is affected by an increase in temperature.
Collisions between reacting particles are more frequent. The average collision energy increases. The reaction rate increases.
True or false: Based on the balanced equation N2 (g) + 3H2 (g) → 2NH3 (g), the rate law is given by rate = k[N2][H2]^3.
False
Which of the following changes will cause an increase in the rate of a chemical reaction? Select all that apply. (Assume the reaction is not reversible to any appreciable extent.)
Increasing the concentration of a reactant Grinding a large piece of solid reactant into a fine powder
Select all the statements that correctly describe the instantaneous reaction rate.
Instantaneous rate is calculated by finding the slope of a tangent to the curve of concentration versus time at any point on the curve. The instantaneous reaction rate is proportional to reactant concentration.