Chemicalkinetics–Chapter12-Part1
-theareaofchemistryconcernedwiththestudyoftheratesofchemicalreactions.Thepurposesofkineticstudiesaretofindthe
factorsthataffectreactionratesanddeterminethereactionmechanism.Knowledgeofthefactorsthataffectreactionratesenables
chemiststocontrolrates.Findingthereactionmechanismmeanswecanidentifytheintermediatestepsbywhichreactantsare
convertedintoproducts.
Whatisrateorspeed?
-rateischangeinsomequantitywithtime.Thesymbolfor"thechange"isΔ.
Whatisrateofreactionorspeedofreaction?
-changeinconcentrationofareactantoraproductwithtime.
Changeinconcentration= (concentration)=concentrationfinal–concentrationinitial
changeintime= t=tfinal–tinitial
Forinstance,asthereactionA+B
Cprogresses,theconcentrationofCincreases.Therateisexpressedasthechangeinthe
molarconcentrationofC,
[C],duringthetimeinterval
t.ConcentrationofC=[C]
Foraspecificreactionweneedtotakeintoaccountthestoichiometry;thatis,weneedthebalancedequation.Forexample,let's
expresstherateofthefollowingreactionintermsoftheconcentrationsoftheindividualreactantsandproducts.
2NO(g)+O2(g)
2NO2(g)
Theseconcentrationscanbemonitoredexperimentallyasafunctionoftime.Noticefromthebalancedequation,that2molNO
reactswith1molO2;therefore,theconcentrationofNOwilldecreasetwiceasfastasthatofO2.
Sincetheratesofchangeofindividualreactantsandproductsmaydiffertheconventionistomakethereactionratecomeouttobe
thesamenomatterwhichreactantorproductisusedtocalculateit.Firstwedivideeachconcentrationchangebythecoefficient
fromthebalancedequation
Second,anegativesignisinsertedbeforetermsinvolvingreactants.ThechangeinNOconcentration, [NO],isnegativebecause
theconcentrationofNOdecreaseswithtime.Insertinganegativesignintheexpressionmakestherateofreactionapositive
quantity.
Forageneralequation:
aA+bB
cC
theratecanbeexpressedintermsofanyindividualreactantorproduct.
Nomatterwhichreactantorproductweuse,thereactionratewillbepositiveandhavethesamevalue.
1.Writeexpressionsfortherateofthefollowingreactionintermsofeachofthereactantsandproducts.
2N2O5(g)
4NO2(g)+O2(g)
Rateofreaction=
2.Inthereaction2NO(g)
O2(g)+N2(g)
IftherateofformationofO2is0.054M/s,whatistherateofchangeofNOconcentration?
AverageRate.Theaveragerateofreactionoveranytimeintervalisequaltothechangeintheconcentrationofareactant
ofaproduct [C]dividedbythetimeinterval, t,duringwhichthechangeoccurred.
[A],or
InstantaneousRate:Theslopeofthetangenttothecurveatanygiventimeorinstantwhenyouplotorgraph[reactant]vs.time.
InitialRate:TheinstantaneousrateattimeZero
ThreetypesofreactionratesforthereactionofO3andC2H4.
3.CalculationoftheAverageRate
Experimentaldataforthehypotheticalreactionarelistedinthefollowingtable:
A
2B
Time(s)
[A](mol/L)
0.00
1.000
10.0
0.891
20.0
0.794
30.0
0.707
40.0
0.630
a.Calculatetheaverageratesofchangeof[A],andtheaveragereactionratesforthetwotimeintervalsfrom0to10sandfrom30
to40s.
0to10s
30to40s
averagerateofchange
mol/L·s
mol/L·s
averagereactionrate
mol/L·s
mol/L·s
b.Whydoestheratedecreasefromonetimeintervaltothenext?
4. Writeexpressionsfortherateofreactionintermsofeachofthereactantsandproducts.
N2(g)+3H2(g)
2NH3(g)
5.
6.
Thiosulfateionisoxidizedbyiodineinaqueoussolutionaccordingtotheequation
2–
2–
–
2S2O3 (aq)+I2(aq)
S4O6 (aq)+2I (aq)If0.025molofisconsumedin0.50Lsolutionperminute:
2–
i. CalculatetherateofremovalofS2O3 inM/s=
ii. WhatistherateofremovalofI2inM/s=
N2O5isanunstablecompoundthatdecomposesaccordingtothefollowingequation.
2N2O5
4NO2+O2
Thefollowingdatawasobtainedat50°C.
[N2O5](M)
Time(s)
1.00
0
0.88
200
0.78
400
0.69
600
0.61
800
0.54
1000
0.48
1200
0.43
1400
a.
WhatistheaveragerateofN2O5disappearanceinthetimeinterval200-400s=___________
b.
WhatistheaveragerateofN2O5disappearanceinthetimeinterval800-1000s=___________
c.
WhatistherateofO2productioninthetimeinterval800-1000s?=_____________
Answers for 4-6
4.
5.a.
b.
-4
-4
-4
6.a.5.0x10 M/sb.3.5x10 M/sc.1.8x10 M/s
RATELAWS:Part2
FactorsthatAffectReactionRate
1.ConcentrationsofreactantsinsolutionorPressureofgaseousreactants
2.Temperature
3.Catalysts
4.Surfaceareaofasolidreactant
5.Extentofmixing
6.Natureofreactants
EffectofConcentration.Therateofareactionisproportionaltothereactantconcentrations.ForthereactionNO+
therateisproportionaltotheconcentrationsofNOandO2.
x
O 2
NO2,
y
Theratelaw(orrateequation)forthereactionis:rate=k[NO] [O2] Theproportionalityconstantkiscalledtherateconstant.Thevalueofkdependsonthereactionandthetemperature.xandyare
exponentswhichcouldbeintegers,fractionornegative.
Theexponentsxandydeterminehowstronglytheconcentrationaffectstherate.Theexponentxiscalledtheorderwithrespectto
NO,andyistheorderwithrespecttoO2.Thesumx+yistheoverallorder.Thevaluesofxandymustbedeterminedfrom
experiment,andcannotbederivedbyanyothermeans.Wewilldiscusshowtodeterminetheorderofreactioninthenextsection.
Fornowwewilljustusetheresults.FortheNOreactionwithO2experimentsshowthatx=2andy=1.Therefore,theratelawfor
thisreactionis:
2
rate=k[NO] [O2]
Thisreactionissecondorderinnitricoxide,andfirstorderinoxygen.Itisthirdorderoverall.
ThefactthatthereactionisfirstorderinO2meansthattherateisdirectlyproportionaltotheO2concentration.If[O2]doublesor
triples,theratewilldoubleortriplealso.Wecanshowthismathematically.Considertwoexperiments.Inexpt1theconcentration
ofO2isc.Inexpt2theconcentrationofO2isdoubledfromcto2c.IftheconcentrationofNOisthesameinbothexperiments,itwill
havenoeffectontherate.Useoftheratelawallowsustowritetheratioofthetworates:
Asdiscussed,
weseethatdoublingtheconcentrationofareactantthatisfirstorderwillcausetheratetodouble.IftheconcentrationofO2is
heldconstantintwoexperimentsandtheconcentrationofNOdoubles(fromcto2c),theratelawpredictsthattheratewill
quadruple.
ThefactthatthereactionissecondorderinNOmeansthattherateisproportionaltothesquareoftheconcentrationofNO.
Doublingortriplingof[NO]causestheratetoincreasefour-ornine-fold,respectively.
Ingeneral,iftheconcentrationofonereactantisdoubledwhiletheotherreactantconcentrationisunchanged,andtherateis:
1.
2.
3.
unchanged,theorderofthereactioniszeroorderwithrespecttothechangingreactant.
doubled,theorderofthereactionisfirstorderwithrespecttothechangingreactant.
quadrupled,theorderofthereactionissecondorderwithrespecttothechangingreactant.
1.ConcentrationEffectontheRate
3
ThereactionA+2B
productswasfoundtohavetheratelaw:rate=k[A][B] .Bywhatfactorwilltherateofreactionincreaseif
theconcentrationofBisincreasedfromxto3x,whiletheconcentrationofAisheldconstant?___________
TheIsolationMethod.Oneprocedureusedtodeterminetheratelawforareactioninvolvestheisolationmethod.Inthismethod
theconcentrationofallbutonereactantisfixed,andtherateofreactionismeasuredasafunctionoftheconcentrationoftheone
reactantwhoseconcentrationisvaried.Anyvariationintherateisduetothevariationofthisreactant'sconcentration.Inpractice
theexperimenterobservesthedependenceoftheinitialrateontheconcentrationofthereactant.
TodeterminetheorderwithrespecttoAinthefollowingchemicalreaction
2A+BàC
theinitialratewouldbemeasuredinseveralexperimentsinwhichtheconcentrationofAisvariedandtheconcentrationofBisheld
constant.TodeterminetheorderwithrespecttoB,theconcentrationofAmustbeheldconstantandtheconcentrationofBis
variedinseveralexperiments.
2.FindingtheRateLaw
Thefollowingratedatawerecollectedforthereaction:2NO+2H2àN2+2H2O
Experiment
[NO]0
(M)
[H2]0
(M)
Δ[N2]/Δt
(M/h)
1
0.60
0.15
0.076
2
0.60
0.30
0.15
3
4
1.20
0.60
0.60
0.60
1.21
???
a.Determinetheratelaw.
rate=
b.Calculatetherateconstantanditsunits.
k=
c.rateofappearanceofN2&rateofdisappearanceofH2inexp.4are=________&_______respectively.
2
3.Theratelawforthereaction2A+B
Cwasfoundtoberate=k[A][B] .IftheconcentrationofBistripledandthe
concentrationofAisunchanged,byhowmanytimeswillthereactionrateincrease?
4.
Usethefollowingdatatodetermine(a)theratelawand(b)therateconstantforthereaction2A+B
Experiment
[A]0
[B]0
Rate(M/s)
1
0.25
0.10
0.012
2
0.25
0.20
0.048
3
0.50
0.30
0.216
ratelaw:
rateconstant:_______;itsunits=__________
C