ABSTRACT
This chapter demonstrates that the actual molar reactional Gibbs function must be null at equilibrium. However, there appears a problem, once the matter is to apply this rule practically. The actual molar reactional Gibbs function, indeed, is a well-definite linear combination of the chemical potentials of the remaining reactants and products at equilibrium. Actually, the notion of chemical potential is essentially abstract. Fortunately, G. N. Lewis has introduced two new functions: the activity and the fugacity. This is done by implying a standard state related to the corresponding activity. The standard state must be defined in terms of experimental variables. It is the fugacity. It has been introduced to study the behavior of imperfect gases. It is a kind of partial pressure. It is endowed with the units of pressure. It is based on the fact that it is admitted that, as a gas, a liquid or a solid may also be endowed with a value of fugacity.
