Gibbs free energy aggregation

When an element enters into a reaction, its standard Gibbs free energy and standard enthalpy of formation are taken as zero if its state of aggregation is that selected as the basis for... 

Polymorphism occurs whenever a given component exists under different aggregation states as a function of P and T. The stable state requires that the chemical potential of the component (hence, the Gibbs free energy of the phase for a phase composed of a single component) be at minimum at equilibrium. Figure 2.4 shows examples of G-T plots for Al2Si05 in various P conditions. 

The Gibbs free energy of phase y is represented by a straight line connecting the standard state potentials of the two end-members in the mixture. Because we use the term mixture, it is evident that the standard state of both end-members is the same and is that of pure component. The two components are totally immiscible in any proportion and the aggregate is a mechanical mixture of the two components crystallized in form y ... 

AG° is the standard Gibbs free energy change of reaction obtained from the standard energy of formation of pure components AG at 25 °C, P = 1 atm and a suitable aggregation state. Once having determined IQq in standard conditions, the van t Hoff equation may be used to calculate fQq at other temperatures, as follows ... 

When an element enters into a reaction, its standard Gibbs free energy and standard enthalpy of formation are taken as zero if its state of aggregation is identical to that selected as the basis for the determination of the standard Gibbs free energy and enthalpy of formation of its compounds. If AH° is negative, the reaction is said to be exothermic if AH is positive, the reaction is said to be endothermic. 

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