Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

70 EXPERIMENTAL METHODS FOR SLOW REACTIONS(f) Addition of foreign gasesThe addition of foreign gases in radiation chemistry studies achieves several purposes.Radical scavengers such as NO, 0,, C2H4, C3H6, Cl,, Br,, I,, HBr and HIeffectively remove t hermalised free radicals. Similarly, electrons may be removedby scavengers such as N,O, SF, and CCl,. These gases modify the charge neutralisationof an ion. In the radiolysis of propane, the decrease in G(H,) togetherwith the value of G(N,) on addition of N,O, and the effect of propene, leads tothe conclusion that positive ionlpositive ion interaction does not contribute tothe formation of H,. Also, positive ion + electron recombination can give H atomsbut not molecular H,199. These added gases will also act in the capacity of energytransfer agents, as well as do the inert compounds CF4 and C2F6.Apart from conventional trapping reactions, propene and NO, especially thelatter, may also take part in other types of process. The addition of 0.25 % ofdeuteropropene to cyclohexane causes all parent ions produced in the radiolysisto undergo H, transfer"'The charge transfer process of C6H:Zwith NO (ionisation potential 9.25 eV)competes very effectively with this reaction but the same process with 0, (ionisationpotential 12.1 eV) does not. Similar charge transfer processes occur with theinert gases. Xe and Kr inhibit the formation of HD in the radiation-induced exchangebetween H2 and D, due to the reactions2"H;+D, -+ H,+HD: (23)H: +Kr -+ HZ+KrH+ (24)Inert gases also cause sensitised radiolysis in gaseous system. For methane, thexenon sensitisation is via energy transfer, whereas with Kr and Ar an ionic mechanismis operative202. The considerable number of ions produced in the a-particleradiolysis of ethylene is greatly reduced by irradiating Xe-C2H4 (50: 1) mixtures.In the xenon-sensitised experiments, C2H,f is the major ion formed by chargetransferzo3. Hence a much simpler system is produced.It is obvious from the above experimental results, that the addition of foreigngases to radiolytic systems may lead to detailed information about mechanismsfor these processes. However, as with thermal systems, the results can be misleading.In the radiolysis of propane to low conversions (0.002 %), the yields of unsaturatedproducts are very much higher in the absence than in the presence of added