Energy Distribution Among Reagents and Products of Atomic Reactions

Abstract

A simple valence bond resonance description of the activated complex in exothermic reactions A+BC→AB+C, coupled with experimental and theoretical evidence concerning the efficiency of transfer of vibrational energy at a collision, leads to the prediction that almost the entire heat of reaction will be contained in vibration of the bond being formed. The predicted activated complex configuration, which involves extended internuclear separation in the bond being formed, may be connected with increased collision diameters. The high efficiency of association reactions A+A+M→A₂+M requires that in these reactions also the product contains almost the entire heat of reaction in vibration of the bond being formed. An attempt is made to account for the hitherto unexplained negative activation energy of these reactions in terms of the collisional redissociation of the highly vibrating product; a value for E_act of the required order of magnitude is obtained. The rate of the endothermic processes which constitute the reverse of the above reactions will be greatest if the molecule under attack is vibrationally excited. This is discussed in relation to the rate equation and is illustrated from experiment. For ``linear'' reaction the argument of the paper can be formulated in terms of potential‐energy surfaces; this is done in order to illustrate points of similarity and divergence from earlier theories. Further evidence is adduced for the predicted activated complex configuration by an alternative method of calculation applied to the reaction H+Cl₂→HCl+Cl. Twenty‐three particular cases of exothermic exchange reactions and association reactions are discussed, in each of which some experimental evidence exists for the presence of more than equilibrium vibrational energy in the bond formed.