Physical chemistr Reaction Kinetics(6) Xuan Cheng Xiamen University
1 Reaction Kinetics (6) Xuan Cheng Xiamen University Physical Chemistry
Ch ical chemistr Reaction Kinetic Theories of reaction rates Hard-Sphere collision Theory of gas-Phase reactions assumptions )The molecules are hard spheres (2) For a reaction to occur between B and C, the two molecules must collide (3)Not all collisions produce reaction. Reaction occurs if and only if the reactive translational kinetic energy along the line of centers of the colliding molecules exceeds a threshold energy ethr (4)The Maxwell-Boltzmann equilibrium distribution of molecular velocities is maintained during the reaction
2 Physical Chemistry Theories of Reaction Rates Hard-Sphere Collision Theory of Gas-Phase Reactions Assumptions (1) The molecules are hard spheres (2) For a reaction to occur between B and C, the two molecules must collide (3) Not all collisions produce reaction. Reaction occurs if and only if the reactive translational kinetic energy along the line of centers of the colliding molecules exceeds a threshold energy thr (4) The Maxwell-Boltzmann equilibrium distribution of molecular velocities is maintained during the reaction Reaction Kinetics
1h ical Chemistr Reaction Kinetic 简单碰撞理论的基本假设 该理论的基本假设(即理论模型) (i)反应物分子可看作简单的硬球,无内部结构和相互作用; (i)反应分子必须通过碰撞才可能发生反应; (ⅲ〕并非所有碰撞都能发生反应,相互碰撞的两个分子—碰撞 分子对的能量达到或超过某一定值c-称为阈能时,反应才能 发生,这样的碰撞叫活化碰撞; (iV)在反应过程中,反应分子的速率分布始终遵守麦克斯韦一 玻耳兹曼( Maxwell-Boltzmann)分布
3 (ii)反应分子必须通过碰撞才可能发生反应； (iii)并非所有碰撞都能发生反应，相互碰撞的两个分子—碰撞 分子对的能量达到或超过某一定值thr—称为阈能时，反应才能 发生，这样的碰撞叫活化碰撞； (iV)在反应过程中，反应分子的速率分布始终遵守麦克斯韦— 玻耳兹曼(Maxwell-Boltzmann)分布。 简单碰撞理论的基本假设 该理论的基本假设(即理论模型）： (i)反应物分子可看作简单的硬球，无内部结构和相互作用； Physical Chemistry Reaction Kinetics
Ch ical chemistr Reaction Kinetic Theories of reaction rates Hard-Sphere Collision Theory of Gas-Phase Reactions The number of B reacting in a bimolecular reaction B+C→ Products EL/RT B Ethr= Naeth (17.3) thr /RT 丿Vadt e The predict rate constant k=BCe - Eier/RT r=kBc NA[B[C」 EL/RT k=<Bce NALBJLC (232) The use of(15.62 )for ZBC k=M八/8R7(111)71/2 e thr/RT forB≠C(23.3) B
4 Physical Chemistry Theories of Reaction Rates Hard-Sphere Collision Theory of Gas-Phase Reactions The number of B reacting in a bimolecular reaction B + C → Products E RT BC thr Z e − / Ethr NA thr = − dt dn V V a J r 1 1 A (17.3) A thr N E RT r ZBCe / − = r = k[B][C] [ ][ ] / N B C Z e k A E RT BC − thr The predict rate constant = (23.2) [ ][ ] / N B C Z e k A E RT BC − thr = The use of (15.62) for ZBC (23.3) E RT B C A B C thr e M M RT k N r r / 1/ 2 2 8 1 1 ( ) − = + + for B C Reaction Kinetics
choical Chemistry Reaction Kinetic Theories of reaction rates Hard-Sphere Collision Theory of Gas-Phase Reactions For the bimolecular reaction 2b-> Products 1 dB 2=B2 E./RT The rate of disappearance of B d[B]-2zBce k 1 d[B/dt E/RT BB N4[BI2 r=kB The use of (15.63 )for ZBB 1/2 aRT E/RT for B=C(23.4) TB
5 Physical Chemistry Theories of Reaction Rates Hard-Sphere Collision Theory of Gas-Phase Reactions For the bimolecular reaction 2B → Products 2 [ ] [ ] 2 1 k B dt d B r = − = A thr N E RT ZBCe dt d B / 2 [ ] − − = 2 2 r = k[B] / 2 [ ] [ ] [ ]/ 2 1 N B Z e B d B dt k A E RT BB − thr = − = The rate of disappearance of B The use of (15.63) for ZBB (23.4) E RT B A B thr e M RT k N d / 1/ 2 2 1/ 2 8 2 1 − = for B = C Reaction Kinetics