Summary171 A Model for Reaction Ratesbull The rate of a chemical reaction is expressed as the
bull Reaction rates are generally calculated and expressedin moles per liter per second (mol(Lmiddots))
bull In order to react the particles in a chemical reactionmust collide in a correct orientation and with suffi-cient energy to form the activated complex
bull The rate of a chemical reaction is unrelated to thespontaneity of the reaction
172 Factors Affecting Reaction Ratesbull Key factors that influence the rate of chemical reac-
bull Catalysts increase the rates of chemical reactions bylowering activation energies
bull Raising the temperature of a reaction increases therate of the reaction by increasing the collision fre-quency and the number of collisions forming theactivated complex
chemical reaction at a given temperature and theconcentrations of reactants is called the rate law
bull The rate law for a chemical reaction is determinedexperimentally using the method of initial rates
bull The instantaneous rate for a chemical reaction iscalculated from the rate law the specific rate con-stant and the concentrations of all reactants
bull Most chemical reactions are complex reactions con-sisting of two or more elementary steps
bull A reaction mechanism is the complete sequence ofelementary steps that make up a complex reaction
bull For a complex reaction the rate-determining steplimits the instantaneous rate of the overall reaction
bull activated complex (p 532)bull activation energy (p 533)bull catalyst (p 539)bull collision theory (p 532)bull complex reaction (p 548)bull heterogeneous catalyst (p 541)
bull homogeneous catalyst (p 541)bull inhibitor (p 540)bull instantaneous rate (p 546)bull intermediate (p 548)bull method of initial rates (p 544)bull rate-determining step (p 549)
bull rate law (p 542)bull reaction mechanism (p 548)bull reaction order (p 543)bull reaction rate (p 530)bull specific rate constant (p 542)bull transition state (p 532)
554 Chapter 17 Reaction Rates
Go to the Chemistry Web site atchemistrymccom for additionalChapter 17 Assessment
Concept Mapping32 Complete the following concept map using the follow-
ing terms surface area collision theory temperaturereaction rates concentration reactivity catalyst
Mastering Concepts33 For a specific chemical reaction assume that the change
in free energy (G) is negative What does this infor-mation tell you about the rate of the reaction (171)
34 How would you express the rate of the chemical reac-tion A 0 B based on the concentration of reactant AHow would that rate compare with the reaction ratebased on the product B (171)
35 What does the activation energy for a chemical reac-tion represent (171)
36 What is the role of the activated complex in a chemi-cal reaction (171)
37 Suppose two molecules that can react collide Underwhat circumstances do the colliding molecules notreact (171)
38 How is the activation energy for a chemical reactionrelated to whether or not a collision between mole-cules initiates a reaction (171)
39 In the activated complex for a chemical reaction whatbonds are broken and what bonds are formed (171)
40 If A 0 B is exothermic how does the activationenergy for the forward reaction compare with the acti-vation energy for the reverse reaction (A 9 B) (171)
41 What role does the reactivity of the reactants play indetermining the rate of a chemical reaction (172)
42 Explain why a crushed solid reacts with a gas morequickly than a large chunk of the same solid (172)
43 What do you call a substance that increases the rate ofa chemical reaction without being consumed in thereaction (172)
44 In general what is the relationship between reactionrate and reactant concentration (172)
45 In general what is the relationship between reactionrate and temperature (172)
46 Distinguish between a homogeneous catalyst and aheterogeneous catalyst (172)
47 Explain how a catalyst affects the activation energyfor a chemical reaction (172)
48 Use the collision theory to explain why increasing theconcentration of a reactant usually increases the reac-tion rate (172)
49 Use the collision theory to explain why increasing thetemperature usually increases the reaction rate (172)
50 In a chemical reaction what relationship does the ratelaw describe (173)
51 What is the name of the proportionality constant in themathematical expression that relates reaction rate andreactant concentration (173)
52 What does the order of a reactant tell you about theway the concentration of that reactant appears in therate law (173)
53 Why does the specific rate constant for a chemicalreaction often double for each increase of 10 K (173)
54 Explain why the rates of most chemical reactionsdecrease over time (173)
55 In the method of initial rates used to determine the ratelaw for a chemical reaction what is the significance ofthe word initial (173)
56 If a reaction has three reactants and is first order inone second order in another and third order in thethird what is the overall order of the reaction (173)
57 What do you call the slowest of the elementary stepsthat make up a complex reaction (174)
58 What is an intermediate in a complex reaction (174)
59 Distinguish between an elementary step a complexreaction and a reaction mechanism (174)
60 Under what circumstances is the rate law for the reac-tion 2A 3B 0 products correctly written as Rate k[A]2[B]3 (173)
CHAPTER ASSESSMENTCHAPTER ASSESSMENT17
3
4
1
influenced byexplained by
6
7
5
2
chemistrymccomchapter_test
Assessment 555
CHAPTER 17 ASSESSMENT
61 How does the activation energy of the rate-determin-ing step in a complex reaction compare with the acti-vation energies of the other elementary steps (174)
Mastering ProblemsA Model for Reaction Rates (171)62 In the gas-phase reaction I2 Cl2 0 2ICl the [I2]
changes from 0400M at time 0 to 0300M at time 400 min Calculate the average reaction rate inmoles I2 consumed per liter per minute
63 If a chemical reaction occurs at a rate of 225 102
moles per liter per second at 322 K what is the rateexpressed in moles per liter per minute
64 On the accompanying energy level diagram match theappropriate number with the quantity it represents
a reactants c productsb activated complex d activation energy
65 Given the following data for the decomposition ofhydrogen peroxide calculate the average reaction ratein moles H2O2 consumed per liter per minute for eachtime interval
66 At a given temperature and for a specific time intervalthe average rate of the following reaction is 188 104 moles N2 consumed per liter per second
N2 3H2 0 2NH3
Express the reaction rate in moles H2 consumed perliter per second and in moles NH3 produced per literper second
Factors Affecting Reaction Rates (172)67 Estimate the rate of the reaction described in problem 63
at 332 K Express the rate in moles per liter per second
68 Estimate the rate of the reaction described in problem 63at 352 K and with [I2] doubled (assume the reaction isfirst order in I2)
Reaction Rate Laws (173)69 Nitrogen monoxide gas and chlorine gas react accord-
ing to the equation 2NO Cl2 0 2NOCl Use thefollowing data to determine the rate law for the reac-tion by the method of initial rates Also calculate thevalue of the specific rate constant
70 Use the following data to determine the rate law andspecific rate constant for the reaction 2ClO2(aq) 2OH(aq) 0 ClO3
(aq) ClO2(aq) H2O(l)
Instantaneous Reaction Rates andReaction Mechanisms (174)71 The gas-phase reaction 2HBr NO2 0 H2O NO
Br2 is thought to occur by the following mechanism
HBr NO2 0 HOBr NO H 42 kJ (slow)
HBr HOBr 0 H2O Br2 H 862 kJ (fast)
Draw the energy diagram that depicts this reactionmechanism On the diagram show the energy of thereactants energy of the products and relative activa-tion energies of the two elementary steps
Decomposition of H2O2
Time (min) [H2O2] (M)
0 250
2 212
5 182
10 148
20 100
2NO Cl2 Reaction Data
Initial [NO] Initial [Cl2] Initial rate (M) (M) (mol(Lmiddotmin))
050 050 190 102
100 050 760 102
100 100 1520 102
2ClO2(aq) 2OH(aq) Reaction Data
Initial [ClO2] Initial [OH] Initial rate(M) (M) (mol(Lmiddotmin))
00500 0200 690
0100 0200 276
0100 0100 138
2
3
4
Reaction progress
Ener
gy 1
556 Chapter 17 Reaction Rates
72 Are there any intermediates in the complex reactiondescribed in problem 71 Explain why or why not Ifany intermediates exist what are their formulas
73 Given the rate law Rate k[A][B]2 for the genericreaction A B 0 products the value for the specificrate constant (475 107 L2(mol2s)) the concentra-tion of A (0355M) and the concentration of B(00122M) calculate the instantaneous reaction rate
Mixed ReviewSharpen your problem-solving skills by answering thefollowing
74 Use the method of initial rates and the following datato determine and express the rate law for the reactionA B 0 2C
75 The concentration of reactant A decreases from 0400molL at time 0 to 0384 molL at time 400 minCalculate the average reaction rate during this timeperiod Express the rate in mol(Lmiddotmin)
76 It is believed that the following two elementary stepsmake up the mechanism for the reaction betweennitrogen monoxide and chlorine
NO(g) Cl2(g) 0 NOCl2(g)
NOCl2(g) NO(g) 0 2NOCl(g)
Write the equation for the overall reaction and identifyany intermediates in the reaction mechanism
77 One reaction that takes place in an automobilersquosengine and exhaust system is described by the equa-tion NO2(g) CO(g) 0 NO(g) CO2(g) This reac-tionrsquos rate law at a particular temperature is given bythe relationship rate 050 L(molmiddots)[NO2]
2 What isthe reactionrsquos initial instantaneous rate at [NO2] 00048 molL
78 At 232 K the rate of a certain chemical reaction is320 102 mol(Lmiddotmin) Predict the reactionrsquosapproximate rate at 252 K
Thinking Critically79 Using Numbers Draw a diagram that shows all of
the possible collision combinations between two mole-cules of reactant A and two molecules of reactant BNow increase the number of molecules of A from twoto four and sketch each possible AndashB collision combi-nation By what factor did the number of collisioncombinations increase What does this imply aboutthe reaction rate
80 Applying Concepts Use the collision theory toexplain two reasons why increasing the temperature ofa reaction by 10 K often doubles the reaction rate
81 Formulating Models Create a table of concentra-tions starting with 0100M concentrations of all reac-tants that you would propose in order to establish therate law for the reaction aA bB cD 0 productsusing the method of initial rates
Writing in Chemistry82 Research the way manufacturers in the United States
produce nitric acid from ammonia Write the reactionmechanism for the complex reaction If catalysts areused in the process explain how they are used andhow they affect any of the elementary steps
83 Write an advertisement that explains why CompanyArsquos lawn care product (fertilizer or weed killer) worksbetter than the competitionrsquos because of the smallersized granules Include applicable diagrams
Cumulative ReviewRefresh your understanding of previous chapters byanswering the following
84 Classify each of the following elements as a metalnonmetal or metalloid (Chapter 6)a molybdenumb brominec arsenicd neone cerium
85 Balance the following equations (Chapter 10)a Sn(s) NaOH(aq) Na2SnO2 H2b C8H18(l) O2(g) CO2(g) H2O(l)c Al(s) H2SO4(aq) Al2(SO4)3(aq) H2(g)
86 What mass of iron(III) chloride is needed to prepare100 L of a 0255M solution (Chapter 15)
87 H for a reaction is negative Compare the energy ofthe products and the reactants Is the reactionendothermic or exothermic (Chapter 16)
CHAPTER ASSESSMENT17
A B 0 2C Reaction Data
Initial [A] Initial [B] Initial rate (M) (M) (mol C(Lmiddots))
0010 0010 00060
0020 0010 00240
0020 0020 00960
Use these questions and the test-taking tip to preparefor your standardized test
1 The rate of a chemical reaction is all of the followingEXCEPT
a the speed at which a reaction takes placeb the change in concentration of a reactant per unit
timec the change in concentration of a product per unit
timed the amount of product formed in a certain period of
time
2 The complete dissociation of acid H3A takes place inthree steps
H3A(aq) rarr H2A(aq) H(aq)
Rate k1[H3A] k1 32 102 s1
H2A(aq) rarr HA2(aq) H(aq)
Rate k2[H2A] k2 15 102 s1
HA2(aq) rarr A3(aq) H(aq)Rate k3[HA2] k3 08 102 s1
overall reaction H3A(aq) rarr A3(aq) 3H(aq)
When the reactant concentrations are [H3A] 0100M [H2A
] 0500M and [HA2] 0200Mwhich reaction is the rate-determining step
a H3A(aq) rarr H2A(aq) H(aq)
b H2A(aq) rarr HA2(aq) H(aq)
c HA2(aq) rarr A3(aq) H(aq)d H3A(aq) rarr A3(aq) 3H(aq)
3 Which of the following is NOT an acceptable unit forexpressing a reaction rate
a Mmin c mol(mLmiddoth)b Ls d mol(Lmiddotmin)
Interpreting Tables Use the table to answer questions46
Reaction SO2Cl2(g) rarr SO2(g) Cl2(g)
4 What is the average reaction rate for this reactionexpressed in moles SO2Cl2 consumed per liter perminute
a 130 103 mol(Lmiddotmin)b 260 101 mol(Lmiddotmin)c 740 103 mol(Lmiddotmin)d 870 103 mol(Lmiddotmin)
5 On the basis of the average reaction rate what will theconcentrations of SO2 and Cl2 be at 2000 min
a 0130M c 039Mb 0260M d 052M
6 How long will it take for half of the original amountof SO2Cl2 to decompose at the average reaction rate
a 285 min c 385 minb 335 min d 500 min
7 Which of the following does NOT affect reaction rate
a catalystsb surface area of reactantsc concentration of reactantsd reactivity of products
8 The reaction between persulfate (S2O82) and iodide
(I) ions is often studied in student laboratoriesbecause it occurs slowly enough for its rate to bemeasured
S2O82(aq) 2I(aq) rarr 2SO4
2(aq) I2(aq)
This reaction has been experimentally determinedto be first order in S2O8
2 and first order in ITherefore what is the overall rate law for thisreaction
a Rate k[S2O82]2[I]
b Rate k[S2O82][I]
c Rate k[S2O82][I]2
d Rate k[S2O82]2[I]2
9 The rate law for the reaction A B C rarr productsis Rate k[A]2[C]
If k 692 105 L2(mol2middots) [A] 0175M[B] 0230M and [C] 0315M what is theinstantaneous reaction rate
a 668 107 mol(Lmiddots)b 877 107 mol(Lmiddots)c 120 106 mol(Lmiddots)d 381 106 mol(Lmiddots)
Standardized Test Practice 557
STANDARDIZED TEST PRACTICECHAPTER 17
Watch the Little Words Underline words likeleast not and except when you see them in testquestions They change the meaning of the question
Experimental Data Collected for Reaction
Time (min) [SO2Cl2] (M) [SO2] (M) [Cl2] (M)
00 100 000 000
1000 087 013 013
2000 074
chemistrymccomstandardized_test