Chem 27 Chapter 15
Which of the following elementary reactions are bimolecular reactions? (select all that apply) A. 2HI--> H2+I2 B. NO2+CO-->NO+CO2 C. N2O4--> 2NO2 D. C4H8-->2C2H4
A. 2HI--> H2+I2 B. B. NO2+CO-->NO+CO2 Explanation: Bimolecular reactions involve two reactant molecules or atoms. Bimolecular reactions are of the form A+B→products or 2A→products.
Regarding collision theory for bimolecular and higher order reactions,which of the following are FALSE? (select all that apply) A. An increase in concentration leads to fewer collisions between molecules B. Collisions with the correct orientation will always result in a reaction C. Reactions can occur without properly oriented collisions D. More collisions generally mean a faster reaction rate
A. An increase in concentration leads to few collisions between molecules B. Collisions with the correct orientation will always result in a reaction C. Reactions can occur without properly oriented collisions Explanation: Collision theory explains that reactions can only occur when reactants collide in the right way. Typically, reaction rates increase with higher concentrations because this causes the molecules to collide more frequently. However, adequate activation energy is also required for a reaction to take place.
Quiz 2 (Reading) Different from overall reactions, for elementary steps, it is possible to predict the rate law by looking at the reaction. Match the correct rate law to each of the following kinds of elementary steps.
1. A--> rate=k[A] 2. A+B-->rate=k[A][B] 3. A+B+C-->rate=k[A][B][C] 4. 2A-->rate=k[A]^2 5. 3A-->rate=k[A]^3 6. 2A+B-->rate=k[A]^2[B]
Quiz 2 (Reading) Match the rate equation to the associated rate law for the dummy reaction A (aq) ⟶ products (Sorry, no sub/superscripts or italicization in matching questions. "^" means superscript and "_" means subscript.)
1. Differential zeroth: -d[A]/dt=k 2. Differential first order: -d[A]/dt=k[A] 3. Differential second order: -d[A]/dt=k[A]^2 4. Integrated zeroth order: [A]=[A]_0-kt 5. Integrated first order: ln[A]=ln[A]_0-kt 5. Integrated second order: 1/[A]=1/[A]_0+kt
The rate constant for the reaction 2N2O5(g)→4NO(g)+3O2(g) is 0.425 L/mol s at 630. K and 5.02L/mol s at 680. K. Using the Arrhenius equation, what is the activation energy for this reaction?
176000J/mol Explanation: See IPad.
The rate constant for the reaction C12H22O11(aq)+H2O(l)→2C6H12O6(aq) is 4.52×10^−4 s^−1 at 355 K and 6.71×10^−3 s^−1 at 405 K. Using the Arrhenius equation, what is the activation energy for this reaction?
64500 J/mol Explanation: Same set up as #21
Choose the elementary reaction below that is a unimolecular reaction. A. C4H8--> 2C2H4 B. O+O3-->2O2 C. 2NO+O2-->2NO2 D. NO2+CO-->NO+CO2
A. C4H8--> 2C2H4 Explanation: Unimolecular reactions involve only one reactant molecule or atom. Unimolecular reactions take the form A→products.
Of the following, which are TRUE about collision theory? (select all that apply) A. Collision theory explains why reaction rates tend to increase with higher concentrations B. Collision theory states that reaction can occur without properly oriented collisions C. Collision theory states that, in addition to a collision in the proper orientation, adequate activation energy is required for a reaction to occur D. Collision theory explains that more frequent collisions lead to a faster reaction rate
A. Collision theory explains why reaction rates tend to increase with higher concentrations C. Collision theory states that, in addition to a collision in the proper orientation, adequate activation energy is required for a reaction to occur D. Collision theory explains that more frequent collisions lead to a faster reaction rate Explanation: Collision theory explains that reactions can only occur when reactants collide in the right way. Typically, reaction rates increase with higher concentrations because this causes the molecules to collide more frequently. However, adequate activation energy is also required for a reaction to take place.
The term molecularity best fits which of the following descriptions? A. Molecularity denotes the number of reactant species involved in an elementary reaction B. Molecularity is the degree to which the reactants are subdivided in a chemical process C. Molecularity is the value of an exponent in a rate law D. Molecularity denotes the number of reactant species involved in an overall reaction
A. Molecularity denotes the number of reactant species involved in an elementary reaction Explanation: Molecularity is the term used to describe the number of reactant species involved in an elementary reaction.
Which elementary reaction is bimolecular? A. NO(g)+O2(g)--> NO3(g) B. 2NO2(g)--> N2O4(g) C. O3(g)+O(g)--> 2O2(g) D. H2O(l)=H+(aq)+OH-(aq)
A. NO(g)+O2(g)--> NO3(g) B. 2NO2(g)--> N2O4(g) C. O3(g)+O(g)--> 2O2(g) Explanation: A bimolecular reaction involves collision of two particles (molecules or atoms) to produce an activated complex. The reactions above are all bimolecular.
For the reaction NO3− + MoCl62− ⟶ NO2− + OMoCl5− + Cl− which of the following mechanisms is consistent with the rate law: d[NO2^−]/dt = k [NO3^−] [MoCl6^2−] ? A. NO3^− + MoCl6^2− ⟶ NO2− + OMoCl6^2− (slow) OMoCl6^2− ⟶ OMoCl5^− + Cl^− (fast) B. MoCl6^2− ⟶ Cl^− + MoCl5^− (slow) MoCl5^− + NO3^− ⟶ OMoCl5^− + NO2^− (fast) C. NO3^− + MoCl6^2− ⇌ NO3MoCl6^3− (fast) NO3MoCl6^3− ⟶ NO2^− + OMoCl5^− + Cl^− (slow) *See IPad
A. NO3^− + MoCl6^2− ⟶ NO2− + OMoCl6^2− (slow) OMoCl6^2− ⟶ OMoCl5^− + Cl^− (fast) C. NO3^− + MoCl6^2− ⇌ NO3MoCl6^3− (fast) NO3MoCl6^3− ⟶ NO2^− + OMoCl5^− + Cl^− (slow) Explanation: *See IPad
Which substance is an intermediate in the reaction mechanism for the following decomposition of ozone into diatomic oxygen: Step 1=O3-->O2+O Step 2=O+O3-->2O2 A. O B. O2 C. O3 D. None of the above
A. O Explanation: Monoatomic oxygen is not present in the overall reaction, so it is considered a reaction intermediate because it forms in one elementary step and is consumed in another.
For integrated rate law calculations for zero order reactions, we are assuming a rate law that resembles: A. Rate=K B. Rate=K[A] C. Rate=k[A][B] D. Rate=K[A]^2
A. Rate=K Explanation: For zero order reactions, the differential rate law is as follows: Rate=k[A]0=k A zero order reaction thus has a rate that is constant—in fact, its rate is equal to the rate constant k itself. It does not depend on the concentration of its reactants at any point in time.
Which of the following is FALSE regarding reaction mechanisms? A. Reaction mechanisms with more than one step do not always contain intermediates B. Elementary reactions occur exactly as written C. Reactions do not need to involve intermediates D. Intermediates are produced in one step and consumed in a subsequent step
A. Reaction mechanisms with more than one step do not always contain intermediates Explanation: Reaction mechanisms describe the discrete steps by which reactions occur. Some overall reactions are truly one-step (elementary) on a molecular level; that is, they occur exactly as written and cannot be broken down into simpler collisional steps. However, most reactions are not elementary: any reaction that proceeds by two or more elementary steps must necessarily involve intermediates. Intermediates are species produced in one step of a mechanism and consumed in another.
Choose the options below that are true. Select all that apply: A. The rate law for a given reaction can be determined from a knowledge of the rate-determining step in that reaction's mechanism B. The rate laws of all chemical reactions can be determined directly from their net equations. C. The rate laws of bimolecular elementary reactions are second order overall D. The rate law for a given reaction can be determined from its reaction mechanism, without the accompanying rates of each elementary step in the mechanism
A. The rate law for a given reaction can be determined from a knowledge of the rate-determining step in that reaction's mechanism C. The rate laws of bimolecular elementary reactions are second order overall Explanation: The rate laws of overall chemical reactions cannot be determined from chemical equations, because these reactions often occur in multiple steps. Given the mechanism of the reaction with the accompanying rate of each step, we can identify the rate-determining step and use the coefficients of the species therein to write the rate law for the overall reaction. The elementary reactions in each step of a reaction mechanism are either first, second, or third order based on whether they are unimolecular, bimolecular, or termolecular, respectively.
What is the order of a reaction with a half-life that is directly proportional to initial concentration? A. Zero order B. First order C. Second Order D. None of the above
A. Zero order Explanation: A zero order reaction has a half-life that is equal to the initial concentration over twice the rate constant. The half-life equation for zero-order reactions is: t1/2=[A]0/2k
Quiz 3 (Reading) Question 4 Which of the following are not altered by the presence of a catalyst? A. reaction thermodynamics (ΔH, etc) B. reaction mechanism C. equilibrium constant D. stoichiometry E. reaction rate F. activation energy
A. reaction thermodynamics (ΔH, etc) C. equilibrium constant D. stoichiometry
Quiz 3 (Reading) Question 1 For the following reaction 2NH3 (aq) + Ag^+ (aq) ⟶ (Ag(NH3)2)^+ (aq) which of the following is a reasonable expression for the rate law (for some k, p, q, or r), at the time when no products are yet present? A. −d[Ag+]/dt = k [NH3]^p [Ag+]^q B. −d[Ag+]/dt = k [NH3]^p [Ag+]^q [(Ag(NH3)2)+]^r C. −d[NH3]/dt = k [NH3]^p [Ag+]^q D. −d[Ag+]/d[NH3] = k [NH3]^p [Ag+]^q E. d[Ag+]/dt = k [NH3]^p [Ag+]^q F. d[(Ag(NH3)2)+]/dt = k [NH3]^p [Ag+]^q
A. −d[Ag+]/dt = k [NH3]^p [Ag+]^q C. −d[NH3]/dt = k [NH3]^p [Ag+]^q F. d[(Ag(NH3)2)+]/dt = k [NH3]^p [Ag+]^q
Quiz 3 (Lecture) Question 2 The rate constant for the following reaction is 5.4×10−4 s−1 at 326 °C and 2.8×10−2 s−1 at 410 °C. H2 (g) + I2 (g) ⟶ 2HI (g) From these data, compute the activation energy for the rate-limiting step. [Hint: If you keep getting the same wrong answer, try keeping all your units!] A. 1.6 kJ/mol B. 1.6×10^2 kJ/mol C. 5.2×10^4 kJ/mol D. 1.6×10^3 kJ/mol E. 52. kJ/mol F. 1.6×10^5 kJ/mol
B. 1.6×10^2 kJ/mol Explanation: See IPad
An elementary step in a reaction mechanism: A. Can be broken down into more fundamental steps B. Can't be broken down any further C. Is equivalent to the overall reaction D. Depends on the mechanism
B. Can't be broken down any further Explanation: An elementary step in a reaction mechanism can't be broken down any further, it represents a discrete collision along the reaction pathway.
Quiz 3 (Reading) Question 2 It is possible to determine at least some of k, p, q, or r, in the previous question just from the chemical equation. A. True B. False
B. False
In the Arrhenius equation, A represents the: A. Atomic mass B. Frequency factor C. Activation energy D. None of the above
B. Frequency factor Explanation: In the Arrhenius equation, A represents a constant called the frequency factor, which is related to the frequency of collisions and the orientation of the reacting molecules.
Half-life expressions can be derived from: A. Rate Laws B. Integrated Rate Laws C. Relative Rate Laws D. None of the Above
B. Integrated Rate Laws Explanation: We rearrange integrated rate laws to derive half-life expressions
The rate law for a reaction can be derived from the: A. Stoichiometry of the overall reaction B. Molecularity of the rate-determining step C. Molecularity of the overall reaction D. None of the above
B. Molecularity of the rate-determining step Explanation: - The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step. - The rate law can never be determined by looking at the overall reaction, only the rate-determining step from the reaction mechanism can inform us about the rate law because an overall reaction cannot be faster than its slowest step.
Quiz 3 (Reading) Question 3 Consider the following elementary steps in a mechanism for A + B ⟶ D A + B ⇌ C r1 = k1 [A] [B] r−1 = k−1 [C] C ⟶ D r2 = k2 [C] Identify the steady state condition for intermediate C. A. 0 = k1 [A] [B] + k−1 [C] − k2 [C] B. 0 = k1 [A] [B] − k−1 [C] + k2 [C] C. 0 = k1 [A] [B] − k−1 [C] − k2 [C] D. −1 = k1 [A] [B] − k−1 [C] − k2 [C]
C. 0 = k1 [A] [B] − k−1 [C] − k2 [C] Explanation: *See IPad Steady state condition for an intermediate is always constant=0
For an endothermic forward reaction (ΔH forward>0 ), the activation energy of the reverse reaction will be equal to: A. Ea of the forward reaction B. Ea of the froward reaction plus ΔH forward C. Ea of the forward reaction minus ΔH forward D. None of the above
C. Ea of the forward reaction minus ΔH forward Explanation: Since the change in enthalpy gives us the distance between reactants and products, and the activation energy gives us the distance between reactants and the transition state, it is the difference of the two that gives us the distance between products and the transition state, which will represent the activation energy of the reverse reaction.
Identify the elementary reaction below that is a bimolecular reaction. A. H+O2+M-->HO2+M B. 2NO+Cl2-->2NOCl C. O3--> O2+O D. O+O3-->2O2
C. O3--> O2+O Explanation: Bimolecular reactions involve two reactant molecules or atoms. Bimolecular reactions are of the form A+B→products or 2A→products.
Given the elementary reaction below, what is the form of the rate law? 2NO+O2→2NO2 A. Rate=k[NO][O]^2 B. Rate=k[NO]^2[O]^2 C. Rate=k[NO]^2[O2] D. Rate=k[NO]^2[O2]^2
C. Rate=k[NO]^2[O2] Explanation: The rate law for an elementary reaction is determined directly from the chemical equation of the reaction. The rate law includes a factor for each of the reactants with an exponent determined by the coefficient of the reactant. Thus, the rate law for this reaction is rate=k[NO]^2[O2].
Reaction mechanisms tell us the: A. Rate of reaction B. Activation energy of a reaction C. Sequence of collisions that occurs D. None of the above
C. Sequence of collisions that occurs Explanation: A reaction mechanism tells us which molecules collide and in what sequence in order to generate the product of the overall reaction.
Chemical reactions can exhibit different rate constants at differing: A. Initial concentrations B. Volumes of container C. Temperatures D. None of the above
C. Temperatures Explanation: The value of the rate constant for a reaction is dependent on both the temperature at which the reaction is performed and the activation energy of the reaction.
The least common type of reaction is: A. Unimolecular B. Bimolecular C. Termolecular D. These are all equally common
C. Termolecular Explanation: It is much less statistically probable that three specific molecules all collide at the same time and with the correct orientation, so termolecular reactions are exceedingly rare.
The structure in a reaction that has the highest potential energy is the: A. Reactant B. Product C. Transition state D. Depends on the reaction
C. Transition state Explanation: The transition state will always have a higher potential energy than either the reactants or the products.
Quiz 1 (Lecture) Under certain UV illumination conditions, the following reaction data are collected. Determine the rate law from these data. *See IPad A. These data are not possible B. d[CCl4]/dt=k[Cl2]^2[HCCl3] k cannot be determined C. d[CCl4]/dt=(2.5 x 10^3)[Cl2]^1/2[HCCl3] D. d[CCl4]/dt=k[Cl2]^1/2[HCCl3] k cannot be determined E. C. d[CCl4]/dt=(8.0 x 10^4)[Cl2]^2[HCCl3]
C. d[CCl4]/dt=(2.5 x 10^3)[Cl2]^1/2[HCCl3] *See IPad
For a third order reaction where concentration is measured in molarity and time is measured in seconds, the units on the rate constant must be: A. M/s B. 1/(M s) C. 1/s D. 1/M^2 s)
D. 1/M^2 s) Explanation: 1/1/M^2 s) when combined with M^3, will give you M/s, an appropriate unit of rate
Quiz 2 (Reading) Choose the correct statement. A. A differential rate law gives the progress of a reaction directly and is determined by transforming the concentration-time plot. An integrated rate law gives the rate of a reaction directly and is determined by the method of initial rates. B. A differential rate law gives the progress of a reaction directly and is determined by the method of initial rates. An integrated rate law gives the rate of a reaction directly and is determined by transforming the concentration-time plot. C. A differential rate law gives the rate of a reaction directly and is determined by transforming the concentration-time plot. An integrated rate law gives the progress of a reaction directly and is determined by the method of initial rates. D. A differential rate law gives the rate of a reaction directly and is determined by the method of initial rates. An integrated rate law gives the progress of a reaction directly and is determined by transforming the concentration-time plot.
D. A differential rate law gives the rate of a reaction directly and is determined by the method of initial rates. An integrated rate law gives the progress of a reaction directly and is determined by transforming the concentration-time plot.
Which elementary reaction is unimolecular? A. NO(g)+O2(g)--> NO3(g) B. 2NO2(g)--> N2O4(g) C. O3(g)+O(g)--> 2O2(g) D. H2O(l)=H+(aq)+OH-(aq)
D. H2O(l)=H+(aq)+OH-(aq) Explanation: A unimolecular reaction involves the rearrangement of a single reactant species to produce one or more molecules of product. Only the reaction H2O(l)=H+(aq)+OH-(aq) involves one reactant species forming products. The other reactions involve the reaction of more than one reactant particle.
Which of the following elementary reactions is a bimolecular reaction? A. H+O2+M-->HO2+M B. O3-->O2+O C. 2NO+Cl2--> 2NOCl D. NO2+CO-->NO+CO2
D. NO2+CO-->NO+CO2 Explanation: Bimolecular reactions involve two reactant molecules or atoms. Bimolecular reactions are of the form A+B→products or 2A→products.
A reaction mechanism can be derived from the: A. Stoichometry of the overall reaction B. Change in enthalpy of the overall reaction C. Change in entropy of the overall reaction D. None of the above
D. None of the above Explanation: The mechanism can't be discerned from anything about the overall reaction, it must be determined in some other way.
If a plot of 1/[A] vs. time for a set of concentration data does not yield a straight line: A. The reaction is first order B. The reaction is second order C. The reaction is zero order D. The reaction is not second order
D. The reaction is not second order Explanation: We can't be sure of the specific reaction order, other than it must not be second order. A plot of 1/[A]t versus t for a second order reaction will always be a straight line with a slope of k and a y-intercept of 1/[A]0. If a set of rate data are plotted in this fashion but do NOT result in a straight line, the reaction is not second order in A.
If a plot of [A] vs. time for a set of concentration data does not yield a straight line: A. The reaction is first order B. The reaction is second order C. The reaction is zero order D. The reaction is not zero order
D. The reaction is not zero order Explanation: We can't be sure of the specific reaction order, other than it must not be zero order. The integrated rate law for a zero order reaction also has the form of the equation of a straight line: [A]t=−kt+[A]0 y=mx+b A plot of [A]t versus t for a zero order reaction will always be a straight line with a slope of −k and a y-intercept of [A]0. If a set of rate data are plotted in this fashion, but do NOT result in a straight line, the reaction is not zero order in A.
The transition state of a reaction can easily be isolated: A. At high temperatures B. At low temperatures C. At low pressures D. Under no circumstance
D. Under no circumstance Explanation: It is typically impossible to isolate a compound in the transition state of a reaction, as it will too rapidly proceed towards product.
Quiz 3 (Lecture) Question 1 Determine the steady-state rate expression, given the following reaction steps *See IPad As a supplementary exercise, take the limits of your answer for very large k1,k−1 or k2, and verify that it is the same as what you get from assuming a fast first or second step, respectively. A. rate = ( k1 k2 [NO] [Cl2] ) / ( k−1 + k2[NO] ) B. rate = ( k1 k2 [NO]2 [Cl2] ) / ( k−1 + k2 ) C. rate = ( k1 k2 [NO] [Cl2]2 ) / ( k−1 + k2[NO] ) D. rate = ( k1 k2 [NO]2 [Cl2] ) / ( k−1 + k2[NO] )
D. rate = ( k1 k2 [NO]2 [Cl2] ) / ( k−1 + k2[NO] ) Explanation: See IPad
If a first-order reaction has a rate constant of 0.041 s^−1 at a particular temperature, how long in seconds will it take for the reactant to be reduced to 42% of its initial concentration? The answer has two significant figures (round your answer to a whole number).
Explanation: 1st order rxn: ln[A]=ln[Ai]-kt Rate constant (k)=0.041s^-1 Assume that initial concentration of A is 1.00 ln[1.00]=0 Then 42% of 1.00 is 0.42 so when rewriting the expression, you should get ln(0.42)=(0.041s^-1)(t) t=21.158--> 21s
Quiz 2 (lecture) Question 1 From the following plots, identify the orders of reactions I, II and III. *See IPad
I. 1st order II. 0th order III. 2nd order
Quiz 2 (lecture) Question 2 From the following time series of concentrations, identify the orders of reactions I, II and III. You can do this without plotting. First, look for things that are constant (rates, half-lives) and relationships among these (between and within series). *See IPad
I. 2nd order II. 0th order III. 1st order *See IPad Explanation: POE III is 1st order because there is a constant half life (1st order usually) II is 0th order because it decreases in concentration at a constant rate I. 2nd order (won't be able to identify anything uniquely...POE) Not constant/not half life p
The rate constant k of the second-order reaction 2O3→3O2 is 0.0190 L/mol s. The concentration of O3 at t=40.0 seconds is 0.465 mol/L. What was the initial concentration (in unit of molarity) of O3? Remember to use correct significant figures in your answer (round your answer to the nearest thousandth).
Initial concentration=0.719M See IPad
The second-order reaction 2NOCl→2NO+Cl2 has the rate constant k of 8.00×10^−2 1/Ms. If the concentration of NOCl at t=5.00 seconds is 0.231M, what was the initial concentration of NOCl? Your response should have three significant figures.
Second Order Integrated Rate Law: 1/[A]=kt+1/[Ai] 1/[0/231M]=(8.00x10^-2 1/Ms)(5.00s)/[Ai] [Ai]=0.255M
Define reaction rate. Distinguish between the initial rate, average rate, and instantaneous rate of a chemical reaction. Which of these rates is usually fastest? The initial rate is the rate used by convention. Give a possible explanation as to why.
The reaction rate is defined as the change in concentration of a reactant or product per unit time. Consider the general reaction: aA → products where rate = -d[A]/dt
Consider the zero order reaction 3A→4B+3C. The rate constant k is 0.378 mol/L s and [A]t=0.864 mol/L when t=0 seconds. What is the concentration of A at t=2.00 seconds? Your answer should have three significant figures (three decimal places).
[A]=0.108s See IPad
If a zero order reaction has a rate constant k of 0.0416M/min and an initial concentration of 2.29 M, what will be its concentration after 20.0 minutes? Your answer should have three significant figures.
[A]=1.46 M See IPad
C4H8→2C2H4 is a first order reaction. If the rate constant k for the reaction is 8.90×10^−3 s^−1, how long will it take until 20.0 percent of the initial amount of C4H8 remains? The answer has 3 significant figures (round to a whole number).
t=181s See IPad
For a first order linear plot of ln[A] vs. time, where ln[A] at 2.00 minutes was −1.25, and ln[A] at 4.00 minutes was −2.50. What is the rate constant for this reaction? The answer contains 3 significant figures (round your answer to 3 decimal places).
k=0.625 min^-1 Explanation: Calculate for slope change in y/change in x=(-2.50)-(-1.25)/(4 min-2.00 min)=-1.25/2.00 min=-0.625 min^-1 The rate constant is the opposite of the slope for a first order plot, so k=0.625 min^-1
The rate constant units can be determined by the overall reaction order. We are given that the rate law is rate=k[NO2]2. Since we can see that the reaction is second order, and since reactant amounts were measured in units of mol/L and time in units of s, k must have the units L mol−1s−1.
mol L−1s−1
The first order reaction 3A→2B+C has rate constant 0.538 s^−1. If the initial concentration of A is 0.867 mol L^−1, what is the half-life of the reaction, in seconds? Remember to use correct significant figures in your answer.
t(1/2)=1.29s See IPad
A first order reaction has a rate constant of 0.00489 s^−1. If the initial concentration of the reaction is 3.50 M, what is the half-life of the reactant, in seconds? Remember to use correct significant figures in your answer (round your answer to the nearest whole number).
t1/2=142 s See IPad
The rate constant k for the first-order reaction A→3B is 0.527 s^−1. After how many seconds will 45.0% of the initial amount of A remain? Your answer will have 3 significant figures.
t=1.51s See IPad
