Termolecular reactions temperature dependence

Numerous other reactions of this type, including a sequence involving HO2, exist and the relative importance of each reaction depends on the mixture composition. The rates of these termolecular processes increase with increasing pressure but have Httle or no temperature dependence. 

However, the formation of the dimer in the ter-molecular reaction is sufficiently fast under stratospheric conditions that the bimolecular reactions are not important. For example, using the recommended termolecular values (DeMore et al., 1997) for the low-pressure-limiting rate constant of /c,3()0 = 2.2 X 10-32 cm6 molecule-2 s-1 and the high-pressure-limiting rate constant of k3()0 = 3.5 X 10-12 cm3 molecule-1 s-1 with temperature-dependent coefficients n = 3.1 and m = 1.0 (see Chapter 5), the effective rate constant at 25 Torr pressure and 300 K is 1.6 X 10-14 cm3 molecule-1 s-1, equal to the sum of the bimolecular channels (Nickolaisen et al., 1994). At a more typical stratospheric temperature of 220 K and only 1 Torr pressure, the effective second-order rate constant for the termolecular reaction already exceeds that for the sum of the bimolecular channels, 2.4 X 10-15 versus 1.9 X 10-15 cm3 molecule-1 s-1. 

Nickolaisen, S. L., R. R. Friedl, and S. P. Sander, Kinetics and Mechanism of the CIO + CIO Reaction Pressure and Temperature Dependences of the Bimolecular and Termolecular Channels and Thermal Decomposition of Chlorine Peroxide, J. Phys. Chem., 98, 155-169(1994). 

It is true that they are very rare. The reaction 2C0 + 02 = 2C02 proceeds, not homogeneously, but as a wall reaction under ordinary conditions. The explosion wave, which is set up at higher temperatures, must indeed depend upon a homogeneous change, but this, as Dixon has shown, requires the presence of water, whereby the termolecular reaction is probably replaced by a series of bimolecular changes... 

The velocity constant of a reaction can be expressed in the form xe E RT- With bimolecular reactions the variations in x> which depends on the collision number, are small from reaction to reaction compared with the variations of many powers of ten in the exponential term. Similarly, even in termolecular reactions the exponential term appears to play the principal part in determining the region of temperature in which the velocity of reaction shall attain an assigned value. The gaining of the energy of activation appears to be the principal determining factor in simple reactions, and it is often roughly true to... 

From the slope of plots of lc2i versus pressure Kircher and Sander determined the termolecular rate coefficients and their temperature dependence for M = Ar and N2. The values of these are k(Ar) = 8.4 X 10- exp (1100 K) and k(N2) = 1.9 X lO " exp(980/T) in cm molecule" s . Thus the termolecular rate coefficient in N2 is about 5.1 X 10 at 298 K. The effect of one atmosphere of N2 is to increase the effective bimolecular rate coefficient from about 1.6X10 to about 2.9 X10 cm molecule s . If water vapor is present, a further enhancement about 70% is found with 10 torr H2O at 298 K. The recommended expression for modelling the HO2 + HO2 reaction in air at high pressures and in the presence of water vapor is given by the expression ... 

The temperature dependence of ki>0/[SF6] derived from experiments at 298 K and 341 K is relatively weak and close to that observed experimentally for Cl + NO —> C1NO (ocT14)4 5 and T + NO —> INO ( T 10).17 The values of ki,o/[M] for other bath-gases, obtained by refit to available experimental data, fulfil the inequality Ne < He < Ar < N2 < C2F6 < C02 < SF6 < CF4, which describes the relative efficiencies of different bath-gases observed for termolecular reactions.252... 

Because both forward and reverse reactions involve the participation of air molecules (M), the reaction rale coefficients for the forward and reverse reactions are represented by the general termolecular form (3.25). The reverse (decomposition) reactions in these types of reactions are typically highly temperature-dependent. Reaction rate coefficients for these two reactions are given in both the IUPAC (Atkinson et al. 2004) and JPL (Sander et al. 2003) kinetic data evaluations. In the IUPAC evaluation both forward and reverse rate coefficients are given. The JPL evaluation presents the forward rate coefficients and the equilibrium constants for the reactions. At equilibrium the rates of the forward (/) and reverse (r) reactions are equal, so the equilibrium constant, K r, is directly related to the forward, kf, and reverse, kf, rate coefficients. For example, for N2Os formation,... 

The primary emphasis in shock tube interferometric studies of the hydrogen-oxygen reaction has been on induction period phenomena. Recently, however, the entire postshock density profiles of a selection of rich, lean and near stoichiometric Ha-Oa-Ar mixtures have been studied by numerical integration of an assumed reaction mechanism. In this manner it was shown that the characteristic features of the profile prior to the end of the density plateau are essentially independent of the recombination kinetics. Thereafter, however, the shape of the profile is largely accounted for by termolecular reactions (e)-(g). Systematic variation of the termolecular rate coefficient values in experimental regimes where recombination is most sensitive to reactions if) or ig)> respectively, has yielded temperature-dependent expressions of the form kf< = AT for kf and kf believed valid over the range 1400-3000 K. The expression of Jacobs et al. was found satisfactory for kf. In all three cases, variation with temperature is small (1-0 m 0-5). Values at 1700 K, kf = 5-9 x 10 (cited above), kf z= 1-9 X 10 , and kf = 3-6 x 10 cm mole sec, are in excellent accord with those listed in Table 2.2. 

ON 00 NO would be a true transition state in the sense ofEyring theory and represent the only and rate-determining step. It has been shown that this pathway is possible by the expected rate of termolecular encounters, and even the unusual temperature dependence of the gas-phase kinetics can be accounted for (6). However, the idea of a reaction with a negative enthalpy of activation is not convincing, because the alternatives are steady-state formulations with normal chemical physics. The kinetics of many multistep chemical reactions has been successfully explained by applying this model. 

In 1967 Sullivan [14] reported experimental measurements of the rate of the overall reaction H2 +1 4-1 HI -I- HI. Using a low-temperature photochemical source to produce I atoms Sullivan measured their reaction rate with H2 and determined the (apparent) rate constant for the termolecular reaction and its temperature dependence. Extrapolation of the rate constant to the higher temperature range of the thermal reaction data showed that the former could accoimt for the entire thermal rate. It was thus shown that the dominant mechanism for the thermal reaction of H2 with I2 at temperatures below about 700 K is either the termolecular reaction H2 + I + I HI + HI or another mechanism which is kinetically indistinguishable from it. 

Our recent electronic structure calculations 3deld a potential energy surface adequate to explain, at least qualitatively and within the uncertainties due to an incomplete knowledge of relaxation rates, the available experimental observations for the hydrogen-iodine reaction. The rate expressions, the rate constants, their temperature dependence, the vibrational excitation of HI products, the excitation and/or dissociation of reactant I2, the photochemical rates - all are compatible with the recent ab initio potential energy surface and with the classical trajectory calculations carried out with a similar surface. And all are compatible with either the bimolecular or termolecular mechanisms. It appears most likely that both mechanisms contribute, but the matter is not resolved as yet. 

In the first step (4) an excited complex is formed which either dissociates back to the reactants, or is stabilized by radiation (5) or by collisbn with a third body (6). Then the radiative association can be linked to the termolecular process if the radiative lifetime of the complex and the third body stabilization efficiency are known (Bates, 1984,1985 Herbst, 1982,1987). The termolecular rate coefficient has been extensively studied between 80 and 300K, generally displaying a T temperature dependence. The question arises whether such a dependence is valid at the lowest temperatures. Bdhringer et al (1983) have studied the association reaction ... 

Nickolaisen, S.L., Friedl, R.R., Sander, S.P. Kinetics and mechanism of the chlorine oxide CIO -I-CIO reaction pressure and temperature dependences of the bimolecular and termolecular channels and thermal decomposition of chlorine peroxide. J. Phys. Chem. 98, 155-169 (1994) Nickolaisen, S.L., Roehl, C.M., Blakeley, L.K., Friedl, R.R., Francisco, J.S., Liu, R.F., Sander, S. P. Temperature dependence of the HO2-1-CIO reaction. 1. Reaction kinetics by pulsed photolysis-ultraviolet absorption and ab initio studies of the potential surface. J. Phys. Chem. A 104, 308-319 (2000)... 

Rate coefficients of bimolecular and termolecular gas reactions prove to depend on one variable only, namely temperature. Unimolecular reaction rate coefficients also depend on the total molar concentration [M] (Chapter 4). One could be explicit and write k T) or k T, [M]) rather than k alone. Indeed, in combustion processes rate coefficients do change by orders of magnitude from the cool to the hot parts of a flame. It is not customary to write these dependences explicitly, however, and so one must just keep in mind that rate coefficients are not constants even though they often bear that name. 

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