Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

16 EXPERIMENTAL METHODS FOR SLOW REACTIONSand-d[A]/dt = k,[A]’ = k0[AI2 at low pressures (0)= k,k,,[A]/k, = k,[A] at high pressures (P)= k, k,,[A]2/(k,,+ k,[A]) at intermediate pressures (Q)Equation (R) predicts that a graph of l/kexp versus 1/[A] should be linear andthat the slopeis Ilk, and the intercept l/km. Useful data may be obtained from thistype of plot provided that the pressures at which the particular system is studied isnot close to that at which the rate coefficient is k,.In an opposite sense to these reactions, pressure effects occur in systems wherehot molecules or hot radicals are produced6,. Here quenching of the hot speciesmay be followed in competition with spontaneous decomposition or isomerisationof the hot species (A*).A* + productsA*+M + A+MRAIRproducts = kl2tMlIkl1Provided k,, can be estimated, a graph of the LHS of the equation versus [MIyields a value for k, which may be compared with the rate coefficient for spontaneousdecomposition or isomerisation derived from the classical HRRKM theory63or the quantum mechanical theory64.All three of these types of process are intimately concerned with the use of energytransfer agentst. Experimental work on energy transfer in gases has been reviewedby Cottrell and M~Coubrey~~, and with particular reference to unimolecularreactions by Trotman-Dickenson66 (see also Volume 3, last chap.). The mostcommon energy transfer agents that are used are N,, CO, COZY SF, and the inertgases. However, should they react chemically in the particular system studied,their effect as an energy transfer agent will be obscured and any deductions may becompletely erroneous. Thus in the di-tert.-butyl peroxide/SF, system, it is found thatabove 140°, significant reaction of the hexafluoride occurs with methyl radicals6’.The relative efficiencies of various energy transfer agents for several reactions areshown in Table 1’. A general conclusion about the efficiency of energy transfer isthat it increases with increasing complexity of the molecule. (For translationaltranslationalenergy transfer, maximum efficiency occurs when collisions occurbetween species of equal mass.) The chances are high that the most efficient is thet For a recent theoretical treatment see ref. 47b, p. 346.See also ref. 4, p. 171.