Concise discussion on reaction equilibria involving gases together with immiscible liquids and solids

Concise discussion on reaction equilibria involving gases together with immiscible liquids and solids 

The discussion of the preceding section will now be presented in more general terms. For a reaction 

In this expression we make the substitution in terms of partial pressures (or fugacities if the vapour is not perfect) only for the substances 1 to n. This gives 

The first term on the right-hand side refers to the standard chemical potentials, at the temperature Ty of those substances which are present only in the gaseous phase and whose partial pressures are contained in iTp. The second term refers to the chemical potentials of all the remaining substances, and these chemical potentials are the values as they actually occur in the reaction system, that is, at a particular pressure, composition and temperature. 

So far no assumption has been made with regard to the various solids and liquids, substances n+1 to N, being present in a pure state. If this assumption is now made, each of the Pi which occur in the second summation of (4 51) is a function of temperature and pressure only and is independent of the composition of the system. Thus for each of the substances n+1 to we have 

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