Hot Radical Reactions The Transfer of Energy

The heat exchange in a chemical reaction must appear in the newly formed products if it occurs in one step or in the intermediates as well if it is a complex reaction. Even if the reaction is close to being thermoneutral, it will generally have a slow step which requires at least a moderate activation energy, and for an energy balance, cither the products of this step must appear with considerable energy or the products of some subsequent step will. Thus, although the simple bimolccular production of HI from H2 + I2 has a AIP = —2.4 Kcal  

In the same reaction the recombination of Br atoms in a tennolccular reaction, with for example Br2 as the third body, will produce two Br2 molecules sharing an excess of 46 Kcal  

It is of some importance to examine the properties of these excited species and inquire into their fate in the reaction system. Although it is possible to find such excited species in thermal reactions, they can be most conveniently produced with varying amounts of energy in photochemical reactions. In a typical photolysis, the excess energy of the photolysis fragments produced by the primary process will be equal to the energy of the light quantum minus the heat of the reaction. In the photolysis of HI by 

20 f ()2 — 2O2 1 118 Tvccil 2CH3 + CII3I cyie + CTT3I + 85 Kcal 

For linear molecules this becomes conservation of the component of electronic angular momentum along the molecular axis. This rule is useful principally in cases in which the original collision complex can be considered a linear molecule, in which event it can be used to predict which electronic states of the two product species are compatible with the initial collision complex. 

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