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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

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5 Chapter 22: Reaction Dynamics

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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

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22.2 Diffusion-controlled reactions cage effect, the lingering of one molecule near another on account of the hindering presence of solvent molecules.

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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.

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22.2(b) Diffusion and reaction

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22.3 The material balance equation

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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!!

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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

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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

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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.

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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

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 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.

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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.

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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

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 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

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

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22.8 Some results from experiments and calculations HA + HB-HC  HA-HB + HC

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22.8(a) The direction of attack and separation 300

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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

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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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