Composition of the Early Solar Nebula

One of the first applications of major geochemical interest was the calculated equilibrium composition of the early solar system. This work is summarized in classic papers by Grossman (1972) and Grossman and Larimer (1974). The method they used is interesting because they csdculated equilibrium constants from available free energies this makes it transitional between the two approaches described above. 

This calculation begins by finding the free energies for the 70 most important gaseous species consisting of the 15 elements (H, O, C, N, Mg, Si. Cl) thought 

Here and nn totai are the total number of moles of H2 molecules and of H atoms per liter, respectively. If A(X)/A(H) is the abundance of any element X relative to that of hydrogen atoms in the solar nebula, then the concentration in moles per liter of element X is simply 

We will assume that all species, whether molecules or single atoms, are at complete chemical equilibrium. This means we can write a chemical reaction describing formation of each gas molecule from its constituent gaseous atoms. For water, as an example, this is 

The free energies of all species are known at this T and P, so the equilibrium constants can be calculated for all such reactions (as in equations 19.44 and 19.45 above). We are assuming ideal gas behavior, so activities can be expressed by partial pressures of each gaseous species. For reaction (19.51) the equilibrium constant would be  

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