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PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comChemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.;and Bruce E. BurstenChapter 14Chemical KineticsChemicalKinetics1

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comKinetics Chemical Kinetics is the study of therate at which a chemical processoccurs. Besides information about the speed atwhich reactions occur, kinetics alsosheds light on the reaction mechanism(exactly how the reaction occurs).ChemicalKinetics2

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comFactors That Affect Reaction Rates Physical State of the Reactants In order to react, molecules must come incontact with each other. If the reaction ishappening between a solid and a liquid itwill react only on the surface. The more homogeneous the mixture ofreactants, the faster the molecules canreact.ChemicalKinetics3

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comFactors That Affect Reaction Rates Concentration of Reactants As the concentration of reactants increases,so does the likelihood that reactantmolecules will collide.ChemicalKinetics4

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comFactors That Affect Reaction Rates Temperature At higher temperatures, reactantmolecules have more kinetic energy,move faster, and collide more often andwith greater energy.ChemicalKinetics5

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comFactors That Affect Reaction Rates Presence of a Catalyst Catalysts speed up reactions bychanging the mechanism of thereaction. Catalysts are not consumed duringthe course of the reaction.ChemicalKinetics6

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesRates of reactions can be determined bymonitoring the change in concentration ofeither reactants or products as a function oftime.ChemicalKinetics7

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq)In this reaction, theconcentration ofbutyl chloride,C4H9Cl, wasmeasured at varioustimes.ChemicalKinetics8

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comThe average rate of thereaction over eachinterval is the change inconcentration dividedby the change in time: Average rate [ X ]t2 - [ X ]t1t 2 -t1 [C4H9Cl]Average rate t [X] tChemicalKinetics9

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq)Average rate [ 0.0905] - [0.1000]50-0 -0.0095 1.9 x 10-450ChemicalKinetics10

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq) Note that the averagerate decreases as thereaction proceeds. This is because as thereaction goes forward,there are fewercollisions betweenreactant molecules.ChemicalKinetics11

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com calculate the average rate of appearance of Bover the time interval from 0 to 40 s.ChemicalKinetics12

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq) A plot of concentrationvs. time for this reactionyields a curve like this. The slope of a linetangent to the curve atany point is theinstantaneous rate atthat time.ChemicalKinetics13

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction RatesC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq) All reactions slow downover time. Therefore, the bestindicator of the rate of areaction is theinstantaneous rate nearthe beginning.ChemicalKinetics14

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comAverage vs. instantaneous rxnrate Average rxn rate change of chemical concentration between beginning of rxn and end [A]rate of reaction (rate) total time elapsed t Instantaneous rxn rate (tangent to curve) change of chemical concentration overashortperiodoftime [A]rate of reaction (rate) the short time elapsed tUsually in chemistry, we are interested in the instantaneous rxn rate at the veryChemicalbeginning (at t 0)KineticsINITIAL REACTION RATE15

NaHCO3 citric acid -- sodium citrate H2O CO2initial [A] 1 tablet2Instantaneousrate at t 0(initial rate)initial [A] 2 tablets1.8concentration (tablet/100 mL)PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comAverage vs. instantaneous rxnrate1.6Average rate1.41.210.80.60.40.2000.51time (minutes)1.52ChemicalKinetics16

A -- 2 CA -- B[A]22[B]1.8co n cen tration (m ol/L )1.8concentration (m ol/L)PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comComparing concentrationsof chemicals in a 51time (hours)1.52[C]1.60.20[A]00.51time (hours)1.52ChemicalKinetics17

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comWhat we’re after in kinetics We want to be able to understand and predict how much reactantsremain (or products get formed) in a certain amount of timeChange with time is rateRate means calculus must be used, so we need to be careful whichrate we are talking aboutWe will use instantaneous rate at the start of a reaction (initial rate)Several factors affect the rate of a reaction Initial concentration Temperature Amount of reactant exposed (surface area) Presence of a catalystA theory (equations) that predicts kinetics must explain theseChemicalKinetics18

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comCalculate the instantaneous rate of disappearance of C4H9Cl at t 0 (theinitial rate).ChemicalKinetics19

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comDetermine the instantaneous rate of disappearance of C4H9Cl at t 300 s.Answer: 1.1 10–4M/sChemicalKinetics20

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction Rates and StoichiometryC4H9Cl(aq) H2O(l) C4H9OH(aq) HCl(aq) In this reaction, the ratioof C4H9Cl to C4H9OH is1:1. Thus, the rate ofdisappearance ofC4H9Cl is the same asthe rate of appearanceof C4H9OH.Rate - [C4H9Cl] t [C4H9OH] tChemicalKinetics21

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction Rates and Stoichiometry What if the ratio is not 1:1?2 HI(g) H2(g) I2(g)in such a case , [H2 ] [ I2 ]1 [ HI ] Rate 2 t t tChemicalKinetics22

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comReaction Rates and Stoichiometry To generalize, then, for the reactionaA bBcC dD1 [A]1 [B]1 [C]1 [D]Rate a tb tc td tChemicalKinetics23

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comKey points about kinetics so far Study the vocabularyReaction rate has weird unitsBe clear whether you are talking about average reaction rate orinstantaneous reaction rateThe equations which we will study all talk about instantaneousreaction rate, most easily measured at the beginning of areaction (initial rate)Relating rates at which products appear ( ) and reactantsdisappear (-) has to do with inverse stoichiometric coefficientsReaction rate depends on several factors: initial concentration,temperature, surface area of reactants exposed, presence of acatalystChemicalKinetics24

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com(a) How is the rate at which ozone disappears related tothe rate at which oxygen appears in the reaction(b) If the rate at which O2 appears, Δ[O2] /Δ t, is 6.0 10–5 M/sat a particular instant, at what rate is O3 disappearing at thissame time, –Δ[O3] /Δ t?ChemicalKinetics25

SAMPLE EXERCISE 14.3 continuedPDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comThe decomposition of N2O5 proceeds according to the following equation:If the rate of decomposition of N2O5 at a particular instant in a reaction vessel is 4.2 10–7 M/s, what is the rateof appearance of (a) NO2, (b) O2?Answers: (a) 8.4 10 –7 M/s, (b) 2.1 10 –7 M/sChemicalKinetics26

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConcentration and RateOne can gain information about the rateof a reaction by seeing how the ratechanges with changes in concentration.ChemicalKinetics27

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConcentration and RateNH4 (aq) NO2 (aq)N2(g) 2 H2O(l)Comparing Experiments 1 and 2, when [NH4 ]doubles, the initial rate doubles.ChemicalKinetics28

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConcentration and RateNH4 (aq) NO2 (aq)N2(g) 2 H2O(l)Likewise, comparing Experiments 5 and 6,when [NO2 ] doubles, the initial rate doubles.ChemicalKinetics29

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConcentration and Rate This meansRate [NH4 ]Rate [NO2 ] Rate [NH ] [NO2 ]or Rate k [NH4 ] [NO2 ] This equation is called the rate law, andk is the rate constant.ChemicalKinetics30

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com If you want your concepts to be clearerdo the visualizing concepts part of theproblems at the back of your chapter.ChemicalKinetics31

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comRate Law An equation that shows how the ratedepends on the concentration of thereactants is called the rate law The exponents tell the order of thereaction with respect to each reactant. This reaction isFirst-order in [NH4 ]First-order in [NO2 ]ChemicalKinetics32

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comRate Laws The overall reaction order can be foundby adding the exponents on thereactants in the rate law. This reaction is second-order overall.ChemicalKinetics33

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com For a general reaction:aA bB cC dDRate law has the formRate k [A]m[B]nwhere k is the rate constant and isdependant on temperature.ChemicalKinetics34

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comIf we know the rate law and the rate for aset of reaction conditions we cancalculate the value of k.ChemicalKinetics35

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comTo calculate the rate constant:Rate lawRate k [A]m [B]nRate k[NH4] [NO2]For experiment 1:5.4X10-7 M/s k (0.0100M)(0.200M)k 5.4X10-7 M/sk (0.0100M)(0.200M)2.7X10 -4 /MsChemicalKinetics36

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comExponents in the rate lawThe exponent in the rate law indicateshow the rate is affected by theconcentration of each reactant.ChemicalKinetics37

SAMPLE EXERCISE 14.4 Relating a Rate Law to the Effect of Concentration on RatePDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConsider a reactionfor which rate k[A][B]2. Each of the following boxes represents areaction mixture in which A is shown as red spheres and B as purple ones. Rank these mixtures in order ofincreasing rate of reaction.ChemicalKinetics38

SAMPLE EXERCISE 14.4 Relating a Rate Law to the Effect of Concentration on RatePDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comConsider a reactionfor which rate k[A][B]2. Each of the following boxes represents areaction mixture in which A is shown as red spheres and B as purple ones. Rank these mixtures in order ofincreasing rate of reaction.Each box contains 10 spheres. The rate law indicates that in this case [B] hasa greater influence on rate than [A] because B has a higher reaction order.Hence, the mixture with the highest concentration of B (most purple spheres)should react fastest. This analysis confirms the order 2 1 3.ChemicalKinetics39

Assuming that rate k[A][B], rank the mixtures represented in thisSample Exercise in order of increasing rate.PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comAnswer: 2 3 1ChemicalKinetics40

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.comUnit for k depends on the overallorder of the reaction.For example for a second order reactionUnit of Ratek (Units of concentration)2M/sM 2 /Ms2MM sChemicalKinetics41

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com(a)What are the overall reaction orders for the rate lawsdescribed in Equations 14.9 and 14.10?(b) What are the units of the rate constant for the rate law for Equation14.9?ChemicalKinetics42

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com(a)What are the overall reaction orders for the rate lawsdescribed in Equations 14.9 and 14.10?2N 2 O5 (g) 4NO 2 (g) O 2 (g)Rate k [ N 2O 4 ]CHCl3 (g) Cl2 (g) CCl4 (g) HCl(g)Rate k[CHCl3 ] [Cl2 ]1/2Answer:The rate of the reaction in Equation 14.9 is first order in N2O5 and first orderoverall. The reaction in Equation 14.10 is first order in CHCl3 and one-halforder in Cl2. The overall reaction order is three halves.(b) What are the units of the rate constant for the rate law for Equation14.9?Answer:Rate k [ N 2 O 4 ]SoNotice that the units of the rate constant change as the overall order of the reaction changes.ChemicalKinetics43

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com1. The reaction is first order in a reactantIf the concentration of a particular component isdoubled the rate doubles and triples when theconcentration triples.ChemicalKinetics44

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com2. If the reaction is second order with respect toa component:If the rate is second order with respect to aparticular component, doubling itsconcentration increases the rate by a fa