山大物理化学英文下6.ppt

The great discrepancies between experimental and calculated A were recognized around 1925. The equation was then modified by introduction of an empirical factor P called the steric factor / probability factor. Steric factor (P), ranging between 1~10-9, represents the fraction of energetically suitable collisions for which the orientation is also favorable, can be only determined experimentally. SCT can not give any clue to calculate P. §9.6 Rate Theories of elementary reaction Extensive reading: Levine, pp. 879-881 Two important empirical rules: Rate equation (law of mass action) Arrhenius equation Type of reaction Unimolecular reaction Bimolecular reaction Termolecular reaction A 1013 s 1011 mol-1?dm3?s-1 109 mol-2?dm6?s-1 A seems related to collision frequency. Boltzmann distribution term It is obvious that a molecule of A cannot react with a molecule of B unless the two reactant molecules can somehow interact. This interaction can only take place if they come within a certain distance of each other, i.e., collides with each other. A reaction can take place only if the molecules of the reactants collide. Therefore, the rate constant of the reaction may be predicted by calculation of the collision frequency of the reactants. Collision theory is proposed independently by Max Trautz in 1916 and William Lewis in 1918. Thereafter, C. Hinshelwood made modification on its. Basic consideration and brief history /wiki/Collision_theory 6.1 Fundamental assumptions of SCT for gaseous bimolecular reaction 1) The reaction rate of reaction is proportional to the collision frequency (Z), which can be solved by kinetic theory of molecule; where ZAB is the collision frequency of A with B per unit cubic meter per second, q is the portion of effective collision. reaction rate can be expressed as: 2) The collision can be either non-reactive (elastic) collision or reactive collision. Only the molecules posses energy excess to a critical valu