Physical Interpretations of Selected Conceptuals

First we consider the reversible addition of a small amount of pure component i to a mixture at fixed temperature and pressure. This process has already been discussed in 3.7.3 there we showed that, for each mole of substance added, the reversible shaft work is given by 

when we choose the pure-substance reference state to be at the same temperature and pressure as the mixture, then the activity of component i is simply related to the reversible isothermal-isobaric work involved in adding a small amount of pure i to the mixture. This provides a physical interpretation for the activity. 

For this process, the heat effect is given by the entropy balance (3.6.10), now written for one inlet and no outlets. Since the process is isothermal, the system (T), inlet (T ), and external boundary (Tg f) all have the same temperature, T = T( = Tg t, so (3.6.10) becomes 

For an isothermal-isobaric addition of a small amount of component i to a mixture, the reversible heat effect is given by the change in partial molar entropy on mixing. 

Similarly, for Lewis-rule ideal solutions, (6.3.2) again reduces to 

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