1、Reaction Kinetics (1) Xuan ChengXiamen UniversityPhysical Chemistry1Physical ChemistryReaction KineticsFor example-16.63-237.19Thermodynamics: spontaneous reactions (possible to take place)Thermodynamics: cant answer (1) how to make them happen (2) how fast they will take place (3) the reaction mech

2、anismReaction Kinetics2Physical ChemistryReaction KineticsT, P, catalystT, catalystChemical Kinetics:Chemical KineticsThe study of the rates and mechanisms of chemical reactionsA reacting system is not in equilibriumReaction Kinetics3Physical ChemistryRate of ReactionConsider the homogeneous reactio

3、n (occurs entirely in one phase)(17.1)The rate of conversion J(17.2)At equilibrium, J = 0Reaction Kinetics4Physical ChemistryRate of Reaction(17.3)(17.4)*At constant VThe rate of reaction rmol dm-3 s-1mol cm-3 s-1Reaction Kinetics5Physical ChemistryRate LawsFor many reactions (17.5)*k: rate constant

4、 (rate coefficient), k = f (T, P), , : order or partial order+ + + n : overall orderThe expression for r as a function of concentrations at a fixed temperature is called the rate law.Reaction Kinetics6Physical ChemistryReaction OrdersZero-orderFirst-orderSecond-orderThird-orderNegative First-order1.

5、5-orderNo simple orderReaction Kinetics7Physical ChemistryRate LawsReaction Rate LawReaction Kinetics8Physical ChemistryReaction Mechanisms(17.7)intermediate(17.8)Step (a)Step (b)Step (c)Elementary reactionReaction actually occursReaction Kinetics9Physical ChemistryMeasurement of Reaction RatesChemi

6、cal MethodThe concentration of a reactant or product as a function of timeReaction vesselsAt constant TAt intervalsSlows down or stop the reactionRapidly analyze chemical compositions of the mixtureCooling the sampleremoving a catalystDiluting the mixtureAdding a speciesReaction Kinetics10Physical C

7、hemistry反應(yīng)速率的測(cè)定方法化學(xué)方法 不同時(shí)刻取出一定量反應(yīng)物，設(shè)法用驟冷、沖稀、加阻化劑、除去催化劑等方法使反應(yīng)立即停止，然后進(jìn)行化學(xué)分析。Reaction Kinetics11Physical ChemistryMeasurement of Reaction RatesPhysical MethodMeasures a physical property of the reaction system as a function of time. This allows the reaction to be followed continuously as it proceeds.Mo

8、re accurate and less tedious物理方法 用各種物理性質(zhì)測(cè)定方法（旋光、折射率、電導(dǎo)率、電動(dòng)勢(shì)、粘度等）或現(xiàn)代譜儀（IR、UV-VIS、ESR、NMR、ESCA等）監(jiān)測(cè)與濃度有定量關(guān)系的物理量的變化，從而求得濃度變化。物理方法有可能做原位反應(yīng)。Reaction Kinetics12Physical ChemistryIntegration of Rate LawsAssumptions(a) The reaction is carried out at constant temperature (k is constant)(b) The volume is const

9、ant (r is given by (17.4)(c) The reaction is “irreversible” (ko is very large)(17.4)*Integrate it to find A as a function of time: A = g(t)Reaction Kinetics13Physical ChemistryFirst-Order ReactionsSuppose the reactionis first-order with(17.4)*(17.5)*(17.10)define(17.11)Reaction Kinetics14Physical Ch

10、emistryFirst-Order Reactions(17.12)(17.13)ifatatand(17.14)*The concentration of A (and r) decreases exponentially with time for a first-order reaction.(17.10)Integrated rate lawReaction Kinetics15First-order reaction: lnA vs tFirst-Order ReactionsPhysical Chemistry(17.14)*A/AotFirst-order reaction:

11、A/Ao vs tLnAtReaction Kinetics16First-Order ReactionsPhysical ChemistryHalf-life:(17.13)First-order reaction(17.15)*The time needed for A to drop to half its valueIndependent of AoA useful indication for the chemical reaction rateReaction Kinetics17Second-Order ReactionsPhysical Chemistry(17.10)defi

12、ne(17.11)Reaction Kinetics18Second-Order ReactionsPhysical Chemistry(17.16)A plot of 1/A versus t gives a strait line of slope kA if (17.17)Reaction Kinetics19A-1tSecond-order reaction: 1/A vs tSecond-Order ReactionsPhysical Chemistry(17.16)Half-life:Second-order reaction with r = kA2Reaction Kineti

13、cs20Second-Order ReactionsPhysical ChemistrySecond-order reaction with r = kA2First-order reaction with r = kA(17.18)Three variablesReaction Kinetics21Second-Order ReactionsPhysical Chemistryx=0 when t=0Reaction Kinetics22Second-Order ReactionsPhysical Chemistry(17.22)Reaction Kinetics23Second-Order ReactionsPhysical ChemistryIf Ao=Bo(17.23)Half-life:Reaction Kinetics24Third-Order ReactionsPhysical Chemistry(Problems 17.17 and 17.24)(17.24)Reaction Kinetics25nth-Order ReactionPhysical ChemistryConsider (17.25)Integration gives(17.26)For n 1(17.27)For n 1(17.28)React