Rate of Reaction in the Nebula

The second question cannot be answered unequivocally because the kinetics of the Fischer-Tropsch reaction is not well enough understood to permit reliable extrapolations to very low pressures. Still, a tentative analysis based on Langmuir s adsorption isotherm suggests that the rate may be adequate even at 10 atm (Lancet, 1972). 

Since is much more strongly adsorbed than is CO and covers nearly the entire surface at pressures greater than 10 atm (Hayward and Trapnell, 1964), the Langmuir expression for the rate R simplifies to  

A conservative estimate of t, may be obtained from the experiments of Lancet (1972), who studied the rate of the Fischer-Tropsch reaction on a cobalt catalyst between 500 and 375 K (Fig. 12). The data give an activation energy of 27 1 kcal/ mole, similar to earlier determinations (Anderson, 1956). The latoratory half-time t extrapolated to 360 K is 8600 hr, or 1 yr. With = Pcl = atm, Phn = PcN obtain t = 3.4 x 1(P yr. 

However, this value almost certainly is a gross overestimate. Lancet s experiments were done in a static system at 1 atm, where the mean distance between gas molecules and catalyst was 10cm or 10 mean free paths. In the nebula, this distance would be only 0.03 cm or 0.01 mean free paths. A better estimate 

We must also check whether enough NHj was present in the nebula for synthesis of nitrogen compounds. At equilibrium, only about 1% of the total N will be present as NHj at 360 K and 10 atm, and even less at lower pressures (Fig. 1). Kinetic considerations give similar values, around 1% (Lewis and Prinn, 1980 Norris, 1980). However, Cl chondrites contain only 1.5% of their cosmic complement of N, and so even 1 % in the form of NHj may have sufficed. 

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