18 HAUIATION UIOLOGV 



the total number of part ides involved, are truly bimolecuhir and can occur 

 in dilute f>;as as readily as in a condensed system. When such reactions 

 are exothermic, their heats of acti\ation are usually low, ratifying from 

 to 10 kcal. It does not follow that any such reaction which can be pos- 

 tulateil will occur in practice. For example, the reaction 



D + C'M4-^CH3l) + H 



apparently does not take place under ordinary conditions, and theoretical 

 calculations indicate that its heat of activation may be as high as 40 kcal. 

 The reaction between atomic sodium and cyaiingen, 



Na + C2N2 -^ NaC^X + CN, 



is slow o\>ing to the snialiness of its probability factor, although its energy 

 of activ'ation is approximately zero. 



When sufficient energy is available an atom may react with a molecule 

 to form two radicals. A step of this kind (Lewis and von Elbe, 1938), 



H + U^^ OH + H. 



undoubtedly plays an important role in the explosive combination of 

 hydrogen and oxygen. 



Combination of two atoms results in the formation of a molecule pcjs- 

 sessing more oscillational energy than is necessary to dissociate it. As a 

 result, the life of the quasi molecule is equal to the time of a single oscilla- 

 tion (about 10~^'^ sec), and at ordinary pressures it dissociates before it 

 has a chance to make a stabilizing collision with a normal molecule. In 

 other words, combination of atoms in the gas phase as a bimolecular reac- 

 tion cannot occur. However, two radicals, or an atom and a moderately 

 large radical, can combine to form a molecule whose energy can be dis- 

 tributed among several degrees of freedom and whose mean life will there- 

 fore be comparable to the time between collisions in an ordinary gas. 

 Examples of this type are 



H + C2H4 -> C2H5 

 and 



2C2H.^ — > ( '4H11). 



There is, of course, no restriction on the combination of atoms in a con- 

 densed system, since the colliding atoms will be continuously in collision 

 with the surrounding solvent molecules. 



Disproportionation and probably metathelical reactions occur and 

 influence the mechanisms of reactions. The reaction 



2C2H:,-. C2H4 + r>\u 



is detectable under suitable conditions but is apparently less probable 

 than the simph? combination of the ra(li<'als to form l)utane. 



