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Atkins & de Paula: Atkins’ Physical Chemistry 9e

Atkins & de Paula: Atkins’ Physical Chemistry 9e. Chapter 22: Reaction Dynamics. Chapter 22: Reaction Dynamics. REACTIVE ENCOUNTERS 22.1 Collision theory  rate constant, k r  encounter rate  minimum energy requirement  steric requirement. 22.1(a) Collision rates in gases

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Atkins & de Paula: Atkins’ Physical Chemistry 9e

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  1. Atkins & de Paula: Atkins’ Physical Chemistry 9e Chapter 22: Reaction Dynamics

  2. Chapter 22: Reaction Dynamics REACTIVE ENCOUNTERS 22.1 Collision theory rate constant,kr encounter rate  minimum energy requirement  steric requirement. 22.1(a) Collision rates in gases collision density, the number of (A,B) collisions in a region of the sample in an interval of time divided by the volume of the region and the duration of the interval:

  3. Chapter 22: Reaction Dynamics collision cross-section 22.1(b) The energy requirement

  4. Chapter 22: Reaction Dynamics

  5. Chapter 22: Reaction Dynamics

  6. Chapter 22: Reaction Dynamics • 22.1(c) The steric requirement • steric factor,P = σ*/σ. • reactive cross-section, σ*, the area within which a molecule must approach another molecule for reaction to occur. • rate constant from collision theory, • harpoon mechanism, a process in which electron transfer precedes atom extraction. (Exercise Example 22.2!)

  7. Chapter 22: Reaction Dynamics 22.1(d) The RRK model The Rice–Ramsperger–Kassel model (RRK model), a model that takes into account the distribution of energy over all the bonds in a molecule. Lindemann-Hinshelwood mechanism RRK model s Exp. data for unimolecular isomerization of trans-CHD=CHD

  8. Chapter 22: Reaction Dynamics • 22.2 Diffusion-controlled reactions • cage effect, the lingering of one molecule near another on account of the hindering presence of solvent molecules.

  9. Chapter 22: Reaction Dynamics 22.2(a) Classes of reaction diffusion-controlled limit, a reaction in which the rate is controlled by the rate at which reactant molecules encounter each other in solution. activation-controlled limit, a reaction in solution in which the rate is controlled by the rate of accumulating sufficient energy to react.

  10. Chapter 22: Reaction Dynamics 22.2(b) Diffusion and reaction

  11. Chapter 22: Reaction Dynamics 22.3 The material balance equation

  12. Chapter 22: Reaction Dynamics • TRANSITION STATE THEORY • transition state theory (or activated complex theory, ACT), a theory of rate constants for elementary bimolecular reactions. • transition state, the arrangement of atoms in an activated complex that must be achieved in order for the products to form. • 22.4 The Eyring equation Our task!!

  13. Chapter 22: Reaction Dynamics • 22.4(a) The rate of decay of the activated complex • transmission coefficient, κ, the constant of proportionality between the rate of passage of the complex (k‡) through the transition state and the vibrational frequency along the reaction coordinate (‡); k‡ = κ‡. • 22.4(b) The concentration of the activated complex

  14. Chapter 22: Reaction Dynamics 22.4(c) The rate constant 22.4(d) The collision of structureless particles

  15. Chapter 22: Reaction Dynamics • 22.4(e) Observation and manipulation of the activated complex • Na+I- decay • Photoreaction of IH∙∙∙OCO van der Waals complex • IH∙∙∙OCO  HOCO resembles the activated complex of H + CO2[HOCO] ‡ HO+CO

  16. Chapter 22: Reaction Dynamics 22.5 Thermodynamic aspects 22.5(a) Activation parameters correlation analysis, a procedure in which ln K (=-ΔrGθ/RT) is plotted against ln k (proportional to -Δ‡G /RT). liner free energy relation (LFER), a linear relation obtained in correlation analysis; reaction becomes thermodynamically more favorable.

  17. Chapter 22: Reaction Dynamics 22.5(b) Reactions between ions kinetic salt effect, the effect of a change in ionic strength on the rate constant of a reaction. Exercise Example 22.3!

  18. Chapter 22: Reaction Dynamics THE DYNAMICS OF MOLECULAR COLLISIONS 22.6 Reactive collisions 22.6(a) Experimental probes of reactive collisions infrared chemiluminescence, a process in which vibrationally excited molecules emit infrared radiation as they return to their ground states. IR chemiluminescence O+CSCO+S

  19. Chapter 22: Reaction Dynamics laser-induced fluorescence (LIF), a technique in which a laser is used to excite a product molecule from a specific vibration–rotation level and then the intensity of fluorescence is monitored.

  20. Chapter 22: Reaction Dynamics • multiphoton ionization (MPI), a process in which the absorption of several photons by a molecule results in ionization. • resonant multiphoton ionization (REMPI), a technique in which one or more photons promote a molecule to an electronically excited state and then additional photons are used to generate ions from the excited state. A laser pulse excites electrons in a semiconductor surface (10 layers C 60 on a Cu(111) substrate) which in turn pass their energy to adsorbed molecules (NO). REMPI measures the motion of the desorbed molecules.

  21. Chapter 22: Reaction Dynamics • reaction product imaging, a technique for the determination of the angular distribution of products. Reaction products detected in the Streamer Chamber when a 1.1-GeV-per-nucleon beam of holmium-165 collided with a holmium-165 target at the Bevalac.

  22. Chapter 22: Reaction Dynamics 22.7 Potential energy surfaces potential energy surface, the potential energy as a function of the relative positions of all the atoms taking part in the reaction. HA + HB-HC HA-HB + HC

  23. Chapter 22: Reaction Dynamics saddle point, the highest point on a potential energy surface encountered along the reaction coordinate. HA + HB-HC HA-HB + HC

  24. Chapter 22: Reaction Dynamics saddle point, the highest point on a potential energy surface encountered along the reaction coordinate. HA + HB-HC HA-HB + HC

  25. Chapter 22: Reaction Dynamics Example of potential energy surfaces. Ultrafast reaction dynamics of the complete photo cycle of an indolylfulgimide studied by absorption, fluorescence and vibrational spectroscopy

  26. Chapter 22: Reaction Dynamics 22.8 Some results from experiments and calculations HA + HB-HC HA-HB + HC

  27. Chapter 22: Reaction Dynamics HA + HB-HC HA-HB + HC

  28. Chapter 22: Reaction Dynamics 22.8(a) The direction of attack and separation 300

  29. Chapter 22: Reaction Dynamics 22.8(b) Attractive and repulsive surfaces attractive surface, a potential energy surface in which the saddle point occurs early on the reaction coordinate. repulsive surface, a potential energy surface in which the saddle point occurs late on the reaction coordinate. H + Cl2 HCl +Cl attractive surface repulsive surface

  30. Chapter 22: Reaction Dynamics 22.8(c) Classical trajectories  direct mode process, a bimolecular process in which the switch of partners takes place very rapidly. complex mode process, a bimolecular process in which the activated complex survives for an extended period. direct mode process complex mode process

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