The Basic Thermodynamics of Chemical Reactions and Its Convenient Formulations

5 THE BASIC THERMODYNAMICS OF CHEMICAL REACTIONS AND ITS CONVENIENT FORMULATIONS 

From Eq. 6.4 we know that for any single-phase system 

This chapter concerns reactions in only one phase we will not need the phase superscript in this chapter. The next chapter considers reactions in more than one phase, where we will need it. For the net change in the Gibbs energy of the system to be zero, as we convert to ammonia, we must have 

Substituting the definition of fugacity (Eq. 7.1) for gj, three times we find 

Previously we have said little about g° other than that it was a function of temperature only and that each species had its own value of g°. We discussed standard states in Chapters 7, 8, and 9. A standard state is some state of matter that we will all agree upon as a suitable basis for constructing tables of properties. For most chemical reaction purposes we choose the standard state of some substance as the pure substance in its normal state (solid, liquid, or gas) at F = 1 atm or 1 bar, and an arbitrarily chosen T, normally = 25°C = 298.15 K for the tables of interest in this chapter. Alas, there are other standard states that are much more convenient for some problems, as discussed previously for vapor-liquid equilibrium calculations. However, if we put off for the moment saying what our standard state is, we can use the symbol ° to indicate a property in the standard state, and then say that, for any pure chemical element or compound (pure species) the partial molar Gibbs energy is the same as the pure species Gibbs energy, and in its standard state 

---