Criteria for Reaction Equilibrium

In this section we use the combined laws from 7.5 to obtain the criteria for reaction equilibria in both closed and open systems. The development here parallels that presented in 7.3 for phase equilibrium. 

For H independent reactions taking place in a closed system at fixed T and P, the combined laws are 

Just as in the nonreacting situations discussed in 7.3, the inequality in (7.4.49) applies to irreversible processes and the equality applies to reversible changes. Now we repeat the argument in 7.3.1 that establishes a correspondence between reversible changes and equilibrium states the consequence is that the equality in (7.4.49) applies both to reversible changes and to equilibrium states. Therefore, for reaction equilibrium at fixed T and P, we must have 

However, this is only necessary but not sufficient for identifying equilibrium. For example, when reactions are coupled it may happen that some terms in (7.6.1) are positive while others are negative, so the sum is zero nevertheless, reactions are in progress (vy 0) and the system is not at equilibrium. 

Equilibrium means that all driving forces are zero. For chemical reaction , the driving force is the affinity Aj reaction equilibrium (at fixed T and P) occurs when each affinity is zero. When this condition is met, the equality in (7.6.1) is satisfied term-by-term. 

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Thermod3mamic equilibrium encompasses thermal, mechanical, and chemical equilibrium. Chemical equilibrium, in turn, includes both diffusional and reaction equilibrium. In this section we have considered only nonreacting systems, and so, at this point, we have developed only the criteria for thermal, mechanical, and diffusional equilibrium criteria for reaction equilibrium are given in 7.6. 

CRITERIA FOR REACTION EQUILIBRIUM 303 7.6 CRITERIA FOR REACTION EQUILIBRIUM... 

For H independent chemical reactions taking place in a single phase at fixed temperature and pressure, the criteria for reaction equilibrium is (11.2.1) the affinities of all reactions become zero. Using the implicit stoichiometric coefficients from (11.2.21), we can write (11.2.1) as... 

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