Mod 5
Select common laboratory methods for determining the rate of a reaction
- Change in pressure over time - Spectrometric methods - Change in electrical conductivity
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 interpret the rate law rate = k[HNO2]4/[NO]2? Select all that apply.
- If [NO] doubles the rate decreases by a factor of 4. - The reaction order is -2 with respect to NO.
Which of the following changes would cause an increase in the reaction rate? Select all that apply.
- Increasing the concentration of the reactants - Increasing the surface area of the reactants
Which of the following factors are affected by an increase in reactant concentration? Select all that apply.
- The number of reactant particles in a given volume - The number of reactant collisions - The rate of 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 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.
Which of the following options correctly describe the effect of increasing temperature on a reacting system? Select all that apply.
-The reaction rate increases. -Particles collide more frequently. -Particles collide more energetically.
t1/2 is directly proportional to [reactant]initial
0
Select the correct form of the zero-order integrated rate law for one reactant.
[A]t - [A]0 = -kt
In order to determine reaction orders experimentally, a series of experiments are run in which the initial concentration of one reactant is _____ while the initial concentration of the other is kept ______. The effect of the initial concentration on the initial rate is then evaluated in each case.
changed constant
t1/2 = 0.693/k
first order reaction
The ____ - ______ of a reaction is the time required for the reactant concentration to reach half its initial value.
half life
Select the correct form of the first-order integrated rate law for one reactant.
ln[A]0/[A]t = kt
Chemical kinetics studies the ________ of chemical reactions, i.e., how fast reactants are converted into products.
rate
Select the correct form for the half-life expression for a second-order reaction.
t1/2 = 1/k[A]0
A plot of [reactant] vs. time is linear
zero order
t1/2 = [A]0/2k
zero order half life
If the rate does not change when [A] doubles, the reaction is
zero order with respect to A.
For the reaction A2 + B2 → 2AB, Ea(fwd) = 125 kJ/mol and Ea(rev) = 85 kJ/mol. Assuming the reaction occurs in one step, which of the following statements are correct? Select all that apply.
ΔH°rxn = 40 kJ/mol. In the reaction energy diagram, the products will appear at higher energy than the reactants.
Which of the following options correctly express rate for the reaction H2 (g) + O2 (g) → H2O2 (l)? Select all that apply.
Δ[H2O2]/Δt - (Δ[H2]/Δt)
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. - If there is more than one reactant present the reaction orders will usually be determined individually. - The value of k can be calculated from a single experiment once the reaction orders are known.
Which of the following statements correctly describe the transition state of a reaction? Select all that apply.
- The transition state only exists at the instant of highest potential energy in the reaction. - The transition state species is highly unstable.
Which of the following statements correctly describe an integrated rate law? Select all that apply.
- The value of the reaction constant, k, can be determined using an integrated rate law. - 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.
Express the rate of reaction in terms of the change in concentration of each of the reactants and products in the reaction A (g) + 2B (g) → C (g). Multiple choice question.
- Δ[A]/Δt = - 1/2 Δ[B]/Δt = Δ[C]/Δt
The reaction of acetone with bromine under acidic catalyzed conditions can be represented by the balanced equation CH3COCH3 + Br2 → CH3COCH2Br + H+ + Br-. The rate of disappearance of Br2 was measured for several different concentrations of the various reactants at a certain temperature. The data are summarized in the table below. Given that the rate law is given by rate = k[CH3COCH3][H+], the rate constant at this temperature is equal to ________.
0.0038 M-1⋅s-1
Cyclopropane undergoes isomerization in a first-order reaction with a rate constant of 0.1 s-1. If the initial concentration is 1.0 M, what is the concentration of the isomer propylene product after 10 seconds?
0.63 M
Place the steps necessary to calculate the rate constant for the reaction A (g) + B (g) + C (g) → D (g) from experimental data. Start with the first step at the top of the list.
1. compare reaction rates when the concentration of one reactant changes while the other remains constant 2. determine the reaction orders for each reactant individually 3. write a rate law, using the experimentally determined reaction orders 4. solve for the rate constant in the rate law by using the data from any of the experiments
Place the steps necessary to study the kinetics of a reaction in the correct order, starting with the first step at the top of the list.
1. gather experimental data consisting of a series of plots of concentration vs time 2. determine the initial rates 3. determine the reaction order 4. determine the rate constant and the actual rate law
Place the steps required to calculate the activation energy (Ea) from the Arrhenius equation in the correct order. Start with the first step at the top of the list. k = Ae-Ea/RT
1. plot ln k vs 1/T 2. slope of the lines equals - Ea/R 3. calculate Ea using the known value R
Place the steps necessary to determine reaction order from an integrated rate law in the correct order, starting with the first step at the top of the list.
1. rearrange each rate law into an equation for a straight line 2. plot y vs x for each integrated rate law 3. the linear plot indicates the order of reaction
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 × 102 kJ/mol
Select the correct form of the second-order integrated rate law for one reactant.
1/[A]t - 1/[A]0 = kt
At 25°C, HI breaks down very slowly to form H2 and I2. The rate law is given by rate = k[HI]2 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
Which of the following statements correctly describe a reaction mechanism? Select all that apply.
A reaction mechanism describes a series of single reaction steps that sum to the overall chemical equation. -A reaction mechanism is a hypothesis.
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 steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. B
Compare the initial rates when the concentration of one reactant changes and the other is held constant (and vice versa).
The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. B.
Compare the initial rates when the concentration of one reactant changes and the other is held constant (and vice versa).
The illustration depicts the fraction of collisions versus the collision energy for two of the same gas samples at different temperatures. Choose the statement(s) that accurately describe the illustration.
Curve b represents the sample at higher temperature. Curve b represents the sample that will exhibit a higher reaction rate.
The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. A
Determine the slope of tangent at t0 for each plot.
The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. A.
Determine the slope of tangent at t0 for each plot.
What is an elementary step?
Each step that makes up a reaction mechanism
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 Reason: The rate law can only be determined from experiment. It cannot be deduced from a balanced chemical equation unless the equation represents an elementary step in the mechanism.
A plot of ln [reactant] vs. time is linear.
First order
The molecular scenes show three experiments at a given temperature involving reactants X (black) and Y (green), with their initial rates (in mol/L⋅s). If the rate law is rate = k[X]2, which options correctly identify the initial rate for Expt 2 and the expected appearance of the scene for Expt 3?
Initial rate for Expt 2 = 0.25 x 10-5 Expt 3 would depict 6 black spheres.
Spectrometric methods
Monitor reactions where one component absorbs or emits a specific wavelength of light
change in pressure over time
Monitors reactions involving gaseous reactants or products
Change in electrical conductivity
Monitors reactions involving ionic reactants or products
Which of the following elementary reactions are unimolecular? Select all that apply.
N2O4 (g) → 2NO2 (g) PCl5 (g) → PCl3 (g) + Cl2 (g)
The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. C
Substitute the initial rates, orders, and concentrations into the rate equation and solve for k.
The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required. C.
Substitute the initial rates, orders, and concentrations into the rate equation and solve for k.
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
Transition state theory focuses on which of the following?
The high-energy species that exists at the moment of an effective collision
Which of the following options correctly describe the reaction CH3CHO (g) → CH4 (g) + CO (g) where the rate = k[CH3CHO]^-3/2? Select all that apply.
The overall order is 3/2. The reaction order is 3/2 with respect to CH3CHO.
What do the dashed lines represent in the illustration of the transition state of a reaction between between BrCH3 and OH-?
The partial bonds forming and breaking in the reactants
Correctly interpret the plot of rate vs. reactant concentration, [A], in the attached figure, where a, b, and c refer to different reaction orders.
The rate does not change no matter what the value of [A] is for a zero-order reaction. The rate is directly proportional to [A] for a first-order reaction. The rate increases exponentially with [A] for a second-order reaction.
Which of the following statements correctly describe the kinetics of an elementary step? Select all that apply.
The rate law for an elementary step can be deduced from its balanced chemical equation. The elementary step 2HI (g) → H2 (g) + I2 (g) follows second-order kinetics. For an elementary step, reaction order equals molecularity.
Why does a hot steel nail placed in oxygen burn slowly (illustration on left) but the same mass of a piece of steel wool burns quickly (illustration on right)?
The steel wool has a greater surface area for reaction.
True or False: In rate law expressions, reaction orders may be negative or fractions.
True - Reaction orders may be zero, positive, or negative integers, or fractions.
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?
Which of the following are possible forms of a reaction order? Select all that apply.
Zero Fractional numbers Negative integers Positive integers
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]
Transition theory focuses on the formation of the _____ complex, which exists at the moment of highest ______ energy in a reaction.
activated potential
The _____ energy of a reaction is the energy threshold that colliding molecules must exceed in order to react.
activation
When determining reaction orders by experiment, the most common procedure is to ______.
change the concentration of one reactant while keeping the other concentrations constant
Each reaction has its own characteristic reaction rate, which is determined by the nature of the reacting species. However, reaction rate can be influenced by increasing the frequency and energy of the _______ between molecules.
collisions
During the course of a reaction, reactant concentrations _____ and product concentrations _____.
decrease; increase
The rate of a reaction _____ over time because as the reactant is used up, there are _____ reactant particles to collide.
decreases; fewer
The individual steps that make up a reaction mechanism are called _____ reactions or steps.
elementary
A reaction energy diagram plots potential _____ versus ______ reaction.
energy, progress
For the reaction AB (g) → A (g) + B (g), rate = k[AB]2, k = 0.200 L/mol⋅s, and [AB]0 = 1.50 M. What is [AB] after 10.0 s?
f1[A]t = kt + 1[A]0 1[A]t = 0.200 Lmol⋅s x 10.0 s + 11.50M 1[A]t = 2.67, and [A]t = 0.375 M
If the rate doubles when [A] doubles, the reaction is
first order with respect to A.
The frequency factor, A, is the product of collision ______ (Z) and a(n) ______ probability factor (p).
frequency orientation
Raising the temperature of a reaction increases the reaction rate by increasing the ____ and ______of the collisions.
frequency energy
An increase in temperature causes a _____ collision frequency and energy and therefore _____ the rate dramatically.
greater; increases
Select the correct form of the first-order integrated rate law for one reactant. Select all that apply.
ln[A0]/[At] = kt ln[A]t - ln[A]0 = -kt
A first-order reaction of A → B has a rate constant of 3.5 s-1. How long will it take for the concentration of A to decrease to 25% of its initial amount?
ln[A]0[A]t = ln[A]00.25[A]0 = 1.39 = kt t = 1.393.5s−1 = 0.40 s
Which form of the Arrhenius equation can be conveniently used to calculate Ea for a reaction?
lnk1/k2 = -EaR(1/T2-1/T1)
A reaction _____ is a hypothesis about how a reaction occurs. This hypothesis is tested to see if it fits with the experimentally observed ______ law.
mechanism rate
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
Reactant concentration decreases _____ quickly in a reaction with a higher rate than in a reaction with a lower rate.
more
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
Collision frequency provides an upper limit on how fast a reaction can take place. The laws of probability tell us that the number of collisions depends on the _____ of the number of reactant particles, not their _____.
product; sum
The ___ - ___ step in a reaction is the one that limits how fast the overall reaction proceeds.
rate determining
What is the predicted overall rate law from the proposed mechanism shown?
rate = k[(CH3)3C-Br]
k
rate constant
[A],[B]
reactant concentrations
m, n
reaction orders
A plot of 1/[reactant] vs. time is linear
second order
t1/2 = 1/k[A]0
second order half life
If the rate quadruples when [A] doubles, the reaction is
second order with respect to A.
In the Arrhenius equation, k = Ae-Ea/RT. if T increases, the value of the negative exponent becomes _____, which means k becomes _____ and the rate _____.
smaller; larger; increases
For the reaction of A (g) + B (g) → AB (g), how many unique collisions between A and B are possible if there are four particles of A and three particles of B present in the vessel?
12
t1/2 does not depend on [reactant]initial.
1st
Write a balanced chemical equation for the reaction that has the following rate equation: Rate = - 1/2Δ[N2O5]/Δt = 1/4Δ[NO2]/Δt = Δ[O2]/Δt
2N2O5 (g) → 4NO2 (g) + O2 (g)
t1/2 is inversely proportional to [reactant]initial.
2nd
Determine the overall reaction order for the reaction 4A (g) + 3B (g) → 2C (g) from the experimental data provided.
3
What is the half-life (t1/2) for the reaction in the illustration? In the illustration compound X (blue) undergoes a slow, first-order transformation to Y (orange).
3.0 min
Determine the rate constant for the reaction 4A (g) + 3B (g) → 2C (g) given the accompanying experimental data. The rate law for this reaction is given by rate = k[A]2[B].
5.00 × 10^3 L^2/mol^2⋅min
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
All other factors being equal, which of the following mixtures will have the fastest reaction rate?
An aqueous reactant mixed with another aqueous reactant with a stirring device
Why does the rate of a reaction generally decrease over time?
As reactant is used up, there are fewer reactant particles to collide and therefore fewer effective collisions.
The first-order reaction has a rate constant of 1.74 x 10-5 s-1 at a temperature of 298 K. The value of k at 328 K is 7.59 x 10-4 s-1. Which of the following options correctly reflect the steps required to calculate the activation energy?
(1T2-1T1) = -3.07 x 10^-4 Ea = 102 kJ/mol
Which term represents the frequency factor in the Arrhenius equation? k = Ae-Ea/RT
A
The reaction _____ typically has units of mol⋅L-1⋅s-1.
rate
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 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.
What is molecularity?
The number of reactant particles that react in an elementary step
