Physical chemistr Reaction Kinetics(4) Xuan Cheng Xiamen University
1 Reaction Kinetics (4) Xuan Cheng Xiamen University Physical Chemistry
choical Chemistry Reaction Kinetic Temperature Dependence of Rate Constants Arrhenius equation E/RT k=Ae a (17.66) A& ea: constants characteristic of the reaction(Arrhenius parameters E: Arrhenius activation energy A: pre-exponential factor(Arrhenius A factor)(frequency factor) E In k=hn A-u log10 k=log10 A 2.303RT (17.67) rT Some reactions are not Arrhenius-like. IfEa independent of temperature Re dInk (17.68) dnk a R d(1/7 A=kEa/ rt (17.69)
2 Physical Chemistry Temperature Dependence of Rate Constants Arrhenius Equation Ea RT k Ae− / = (17.66)* A & Ea : constants characteristic of the reaction (Arrhenius parameters) Ea : Arrhenius activation energy A: pre-exponential factor (Arrhenius A factor) (frequency factor) (17.67) RT E k A a ln = ln − RT E k A a 2.303 log10 = log10 − Some reactions are not Arrhenius-like, (1/ ) ln d T d k Ea = −R If Ea independent of temperature dT d k Ea RT 2 ln (17.68) Ea RT A ke / (17.69) Reaction Kinetics
choical Chemistry Reaction Kinetic Temperature Dependence of Rate Constants E In k=hn A- log 10 k= log 10 A 2.303RT (17.67) rT Plot Ink(logo)VS. 1/T 200 100}-1 Slope =-EOR 20F-- E2(3)>E2(2)>E()0- 20001000 463376 T/K
3 Physical Chemistry Temperature Dependence of Rate Constants (17.67) RT E k A a ln = ln − RT E k A a 2.303 log10 = log10 − a a a E E E (3) (2) (1) Slope = -Ea /R Plot lnk (log10k) vs. 1/T Reaction Kinetics
Ch ical chemistr Reaction Kinetic Temperature Dependence of Rate Constants Arrhenius Equation h=Ae-Ea/Rt (17.66)2 Tolman用统计平均的概念对基元反应的活化能下了一个 定义:活化分子的平均能量与反应物分子平均能量之差 值,称为活化能 设基元反应为 A P 正、逆反应的活化能可以用图表示。 活化状态 活化状态 Ea Ea Ea 作用物 P △Ea △Ea生成物 生成物作用物 放热反应 吸熱反应
4 Physical Chemistry Temperature Dependence of Rate Constants Arrhenius Equation Ea RT k Ae− / = (17.66)* Tolman 用统计平均的概念对基元反应的活化能下了一个 定义：活化分子的平均能量与反应物分子平均能量之差 值，称为活化能。 设基元反应为 A P 正、逆反应的活化能可以用图表示。 Reaction Kinetics
Ch ical chemistr Reaction Kinetic 复杂反应的活化能 复杂反应的活化能无法用简单的图形表示,它 是组成复杂反应的各基元反应活化能的数学组合。 组合的方式决定于基元反应的速率系数与表观速 率系数之间的关系,这个关系从反应机理推导而得。 例如: k(表观)=kk2/k1 则E(表观)=E1+E-E a 这表观活化能也称为总包反应活化能或实验活化能
5 复杂反应的活化能 复杂反应的活化能无法用简单的图形表示，它只 是组成复杂反应的各基元反应活化能的数学组合。 则 （ ） E E E E a a,1 a,2 a, 1 表观 = + − − 这表观活化能也称为总包反应活化能或实验活化能。 组合的方式决定于基元反应的速率系数与表观速 率系数之间的关系，这个关系从反应机理推导而得。 例如： 1 2 1 k k k k ( / 表观）= − Physical Chemistry Reaction Kinetics
Chical chemistr Reaction Kinetico 温度对反应速率影响的类型 通常有五种类型 7 5 (1)反应速率随温度的升高而逐渐加快,它们之 间呈指数关系,这类反应最为常见。 (2)开始时温度影响不大,到达一定极限时,反 应以爆炸的形式极快的进行
6 温度对反应速率影响的类型 r T r T r T r T r T (1) (2) (3) (4) (5) 通常有五种类型： （1）反应速率随温度的升高而逐渐加快，它们之 间呈指数关系，这类反应最为常见。 （2）开始时温度影响不大，到达一定极限时，反 应以爆炸的形式极快的进行。 Physical Chemistry Reaction Kinetics
Chical chemistr 温度对反应速率影响的类型 7 (3) (5) (3)在温度不太高时,速率随温度的升高而加 快,到达一定的温度,速率反而下降。如多相催 化反应和酶催化反应。 (4)速率在随温度升到某一高度时下降,再升 高温度,速率又迅速增加,可能发生了副反应。 (5)温度升高,速率反而下降。这种类型很少, 如一氧化氮氧化成二氧化氮
7 温度对反应速率影响的类型 （3）在温度不太高时，速率随温度的升高而加 快，到达一定的温度，速率反而下降。如多相催 化反应和酶催化反应。 （4）速率在随温度升到某一高度时下降，再升 高温度，速率又迅速增加，可能发生了副反应。 （5） 温度升高，速率反而下降。这种类型很少， 如一氧化氮氧化成二氧化氮。 r T r T r T r T r T (1) (2) (3) (4) (5) Physical Chemistry
Ch ical chemistr Reaction Kinetic Chain reactions Free- Radical polymerizations Chain reaction A series of steps: a reactive intermediate is consumed Reactants are converted to products Cycle is repeated The intermediate is regenerated The structure of chain reactions Chain carriers: the intermediates responsible for the propagation of a chain reaction Radical chain carriers the chain carriers are radicals Radicals: Species with unpaired electrons Ions, neutrons(in nuclear fission
8 Chain Reactions & Free-Radical Polymerizations Physical Chemistry A series of steps: a reactive intermediate is consumed Chain reaction Reactants are converted to products The intermediate is regenerated Cycle is repeated The structure of chain reactions Chain carriers: the intermediates responsible for the propagation of a chain reaction Radical chain carriers: the chain carriers are radicals Radicals: Species with unpaired electrons Ions, neutrons (in nuclear fission) Reaction Kinetics
Ch ical chemistr Reaction Kinetic Chain reactions Free- Radical polymerizations a chain reaction leads to a complicated rate law H+b→2HB d[Hbr] kh2[Br2]3/ dt [Br2]+k'LHBr] d[HBr]k[H2I[Br21/2 dt 1+j[HBr]/[ Br Derive the rate law for the formation of hBr according to the mechanism given below
9 Physical Chemistry Chain Reactions & Free-Radical Polymerizations A chain reaction leads to a complicated rate law. H Br 2HBr 2 + 2 → [ ] '[ ] [ ] [ ][ ] 2 3/ 2 2 2 Br k HBr k H Br dt d HBr + = 1 [ ]/[ ] [ ] [ ][ ] 2 1/ 2 2 2 j HBr Br k H Br dt d HBr + = Derive the rate law for the formation of HBr according to the mechanism given below. Reaction Kinetics
Ch ical chemistr Reaction Kinetic Chain reactions Free- Radical polymerizations The following radical chain mechanism has been proposed a initiation B→2b· r=kalBr2l (b) Propagation:Br·+H2→>BB+H =kb[B°[H2] H+B→HB+b· r=kblH-LBr2l (C) Retardation:H·+B→H2+B· kchollHBrI (d) Termination:B·+·B+M→B2+Mr=kJ|Br2 The third body m removes the energy of recombination; the constant concentration of m has been absorbed into the rate constant
10 Physical Chemistry Chain Reactions & Free-Radical Polymerizations The following radical chain mechanism has been proposed: (a) Initiation: Br →2Br • 2 [ ] Br2 r k = a (b) Propagation: Br •+H → HBr + H • 2 [ ][ ] Br H2 r kb = • H •+Br → HBr + Br • 2 [ ][ ] 2 ' r k H Br b = • (c) Retardation: H •+HBr → H + Br • 2 r k [H ][HBr] c = • (d) Termination: Br •+•Br + M → Br2 + M 2 r = k [Br•] d The third body M removes the energy of recombination; the constant concentration of M has been absorbed into the rate constant. Reaction Kinetics
