Pre-exponential constant

The final rate expressions, which were used in the present work, were given by Hou and Hughes (2001). In these rate expressions all reaction rate and equilibrium constants were defined to be temperature-dependent through the Arrhenius and van t Hoff equations. The particular values for the activation energies, heats of adsorption, and pre-exponential constants are available in the original reference and were used in our work without alteration. 

Equation 10.76 has the same form as the modified Arrhenius form of the rate constant, Eq. 9.83, with the pre-exponential constant approximated as... 

Note that v = 0 for a unimolecular reaction. As mentioned before, this expression is only expected to be accurate in the high-pressure limit. Comparing Eq. 10.97 with the usual Arrhenius expression, we can write the CTST expression for the high-pressure Arrhenius pre-exponential constant as... 

The constant Adoo is the high-pressure Arrhenius pre-exponential constant for the uni-molecular decomposition of the (stable) C molecule to form A and B. 

Can this simple reaction mechanism reproduce the general trends seen experimentally What value for the pre-exponential constant provides the best match to experiment Is this pre-exponential within the range of physically reasonable values ... 

Aoo High-pressure limit of pre-exponential constant in Arrhenius expression varies... 

Seed coefficients are in the order kOl, eal,k02,ea2,arl where KOI and k02 are arrhenius pre-exponential constants, eal and ea2 are activation energies, and arl is the area under peak 1. For insertion of a different model, p(l,7) will have to be added and the assignment of iX, the total number of coefficients, sill have to be revised. iX 5... 

This value of the activation energy, together with the corrected rate constants for each temperature, was employed to evaluate the pre-exponential constant, k0, in the Arrhenius rate constant model. The value thus derived was... 

Equation (5) is often applied to estimate the pre-exponential constant [77,78]. Another useful approximation for Kq was proposed by Adamson et al. [101], who expressed the pre-exponential constant in terms of the evaporation heat y ... 

The reaction rate is calculated (in statement 40) according to Equation (11.14), and we increase the duration to 106 years. The activation energy Ea (PARM (1)) and the pre-exponential constant A (PARM (2)) are obtained from the linear fit coefficients of Figure 11.3(b) see Equation (11.26). The results of both the phreeqc output and Equation (11.16) (Table 11.5) are shown in Figure 11.4. They are identical. 

Desorption parameters desorption order, pre-exponential constant, desorption activation energy, Ed, saturation coverage, 7Vmax, and adsorption state (a, j3 etc)... 

---