6.3 Criteria of Thermodynamic Equilibrium
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The nature of thermodynamic equilibrium has been introduced in section 1.4. As we know, it involves simultaneous thermal and mechanical equilibrium within a system. Apart from these constraints, the system must also be in a state of chemical equilibrium. In the most general sense, chemical equilibrium subsumes the following restrictions: (i) the various phases that may exist within the system are in equilibrium with each other in that there is no mass transfer of any chemical species between the phases; (ii) all reactions occurring in the system are also equilibrated, i.e., there is no further progress of the reaction in terms of conversion of the reactants to products. The last two criteria are better understood in terms of the property called chemical potential, which we shall introduce in the following section. Here we focus on the overall, general criterion that must be in obeyed for any chemical system in equilibrium regardless of whatever species, phases or reactions that defines it.
Consider a closed system, which can be either homogeneous or heterogeneous, and which exists in a state of thermal and mechanical equilibrium with its surroundings. However, we assume that it is not under equilibrium with respect to possible inter-phase transfer of the chemical species or reactions between them. If the latter conditions prevail, all inter-phase transfer processes or reactive transformation of species must continue to occur till the point when the system is also at chemical equilibrium. In all real systems such changes are induced by finite gradients and are therefore are irreversible in nature. Applying the first law equation for all such changes for the system: | |||||||||||||||||||||||||||||||||||||||
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| (6.28) |
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