Unitary affinity and equilibrium

A chemical reaction proceeds in the direction of decreasing its affinity and reaches equilibrium at which the affinity vanishes. The equilibrium is thus the state at which the unitary affinity of the reaction equals minus the affinity of mixing of the reaction system. The equilibrium constant of a reaction is accordingly an exponential function of the unitary affinity of the reaction. This chapter discusses the role of the unitary affinity in reaction equilibrium 

Let us consider a chemical reaction in which reactants R. change into products P( as shown in Eq. 6.1  

In the summations, the stoichiometrical coefficient v, is negative for the reactants and positive for the products. In Eq. 6.2 the first term on the right hand side is the unitary affinity A, which comprises of the stoichiometrical sum of the unitary chemical potentials of the reactants and products, and the second term is the affinity of mixing AM, which comprises of the stoichiometrical sum of the chemical potentials of mixing for the reactants and products. By substituting the unitary affinity A for the first term on the right hand side of Eq. 6.2 and defining this to be equal to RT In K(T, p we obtain Eq. 6.3  

Since A = 0 at equilibrium, Eq. 6.4 yields Eq. 6.5 when the reaction is at equilibrium  

We call K(T, p) the equilibrium constant of the reaction. As realized from Eq. 6.2, the reaction equilibrium is established at which the unitary affinity A becomes equal to minus the affinity of mixing AM of the reaction. 

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