Chemical Equilibria in Terms of Fugacity

The concept of fugacity works so well because the criterion for chemical equilibria is just as simple as that using chemical potential. To derive this relationship for fugacity, we begin by equating the chemical potentials of phases a and )8  

Applying a mathematical relationship to the quotient in the logarithms and rearranging gives  

The first three terms are just a restatement of Equation (7.3) hence the remaining term must be equal to zero, that is, 

Equation (7.6) forms the criterion for chemical equilibrium in terms of fugacity. It is just as simple as that for chemical potential. Fugacity is also mathematically much better behaved. 

in practice, we can replace Equation (7.1) with Equation (7.6) in defining our 

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