66 REACTIONS INDUCED BY IONIZING RADIATION 



ordinary reactions. The following three equations (8, 25) are possible 

 examples of such reactions. The third of these may require some energy 

 of activation. 



Br2+ + H -^ HBr+ + HBr 



H2+ + Bra -^ H + Br+ + HBr 



C0+ + CO -^ C+ + CO2 



Similar processes undoubtedly occur in solution, but here their energies 

 are greatly affected by the solvation of ions. 



Solvent Effects in Aqueous Solution 



Radiation chemistry of aqueous solutions differs in many important 

 respects from the corresponding chemistry of gaseous systems. The 

 energy of solvation greatly changes the energy of ionic reactions in 

 aqueous solutions. For example, the heat of recombination of hydrogen 

 and hydroxyl ions in a gas is about 350 kcal per mole. For the same 

 reaction of the solvated ions in aqueous solution, the heat is about 14 

 kcal per mole. Ions which are formed by impact in a solution will 

 ordinarily exist long enough to come into equilibrium with the sur- 

 rounding solvent molecules, before they can diffuse together and neutral- 

 ize one another. For this reason they have some of the properties of 

 ordinary electrolytic ions. 



The following two equations (27) represent the reactions which largely 

 determine the chemical characteristics of irradiated aqueous solutions. 



H2O -}- H2O+ -^ H3O+ + OH 



H2O + e- -^ OH" + H 



The resulting H and OH radicals react readily wuth oxidizing and 

 reducing agents, respectively. However, in the absence of such reagents, 

 recombination of the radicals greatly reduces the ion-pair yield. Hydro- 

 gen peroxide can be formed by combination of two hydroxyl radicals or, 

 more efficiently, if dissolved oxygen is present, by a reaction between a 

 hydrogen atom and an oxygen molecule to form the perhydroxyl radical. 

 Since hydrogen peroxide is a relatively stable but reactive oxidizing 

 agent, its presence undoubtedly influences the properties of irradiated 

 water. However, the primary radicals, H, OH, and HO2 are very 

 probably of greater importance. 



