Gibbs Rule

The definition of reference thermometric fixed point is an equilibrium state of a definite substance the realization of a fixed point must depend only on the composition and on the substance . Hence boiling points, for example, cannot be considered fixed points, since they depend on pressure. Only triple points fulfil this definition as can be deduced from the Gibbs rule for pure substances ... 

The reader interested in a more complete and rigorous demonstration of the Gibbs rule may consult for instance the books by Gokcen and Reddy (1996) or Hillert (1998) where a classification of the different systems (reacting, non-reacting, etc.) to which the rule can be applied is also presented. 

Gibbs phase rule phys chem A relationship used to determine the number of state variables F, usually chosen from among temperature, pressure, and species composition in each phase, which must be specified to fix the thermodynamic state of a system in equilibrium F = C - P - M+2, where C is the number of chemical species presented at equilibrium, P is the number of phases, and M is the number of independent chemical reactions. Also known as Gibbs rule phase rule. gibz faz, rijl I... 

In a two-component system, the difference between surface and bulk composition of component 1 is given by Gibbs rule (18) ... 

Defay and Prigogine (55) pointed out that Gibbs rule, the exact thermodynamic law that determines surface enrichment, can be satisfied only if changes in the two outer layers are taken into account. If one takes into account two layers instead of one, the surface layer composition is given by Eq. (9a) instead of Eq. (8) ... 

Useful insights into the thermodynamics of a multiphase reacting system can be obtained by analyzing the situation when chemical and phase equilibrium (C PE) are achieved simultaneously. The Gibbs rule can be used to find the degrees of freedom F ... 

How many degrees of freedom do you have in a system composed of P phases with C components (Recall Gibbs rule.)... 

We will start this section with a derivation of Gibbs rule, which provides us with a relation between surface energy 7 and surface-layer composition. The presentation given here closely follows that of Guggenheimt l. 

Another problem to be discussed is the nonequilibrium state of the powdered polysulfides prepared by solid-gas interactions. In these cases the rate-controlling process is most often the sulfur diffusion in solids. A thick product layer formed on the unreacted starting solid inhibits the sulfur diffusion, resulting in a nonequilibrium state. To describe this state, one can use some indicators which are shared by many sulfide materials (Vaughan and Craig 1978). First, there is a greater number of phases on the surface of the solids than would follow from Gibbs rule the duration of the experiment has a dramatic effect on the number of phases and the compositional identity of the final products. 

Low solubility of a LVl could invoke the formation of new solid phases at the electrodes. The number of possible phases at the conditions not far from equilibrium ones is determined by Gibbs rule. At constant temperature and pressure, it states ... 

Both stoichiometric and correlative bonds decrease the number of parameters of a chemical system needed and sufficient for the description of dynamics of a system s composition during its transition from the initial indignant state to the final equilibrium state. However, in order to understand their physical sense, let us consider briefly the characteristic numbers of the equilibrium chemical system. A minimal number of parameters, the assigning of which completely and synonymously determines the state of thermodynamical system, represents its fundamental characteristic, that is the number of degrees of freedom. In accordance with the phase Gibbs rule c = k + 2-f (here k is the number of independent substances or system components, / is the number of phases, n is the number ol external parameters influencing the equilibrium state of system). Usually, these are pressure and the temperature and that is why most often the phase Gibbs rule is denoted in the form c = k + 2-/[9, 101. 

Equations of a relation between the parameters of a system based on a phase Gibbs rule follows from the principle of detailed equilibrium in its different displays. Thus, for a multi-phase system the principle of a detailed equilibrium requires the equilibrium of any two phases with each other. This permits to separate them and to consider them separately from others. General conditions for an equilibrium in an isolated system are reduced to partial conditions of thermal (temperature of all phases is equal), mechanical (at plain... 

Fig. 3.3 (a) Schematic representation of the Gibbs rule for a two-phase system, (b) the cell voltage profile shows a plateau in the composition range ai < x < for the bulk material... 

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