Chemical equilibrium in a mixture of ideal gases

AG° is the standard reaction Gibbs energy. Then, combining the logarithmic terms. 

The argument of the logarithm is called the proper quotient of pressures the numerator is the product of partial pressures of the chemical products each raised to the power of its stoichiometric coefficient, and the denominator is the product of the partial pressures of the reactants, each raised to the power of its stoichiometric coefficient. Ordinarily the quotient is abbreviated by the symbol Qp  

The sign of AG is determined by the sign and magnitude of In Qp, since at a given temperature AG° is a constant characteristic of the reaction. If, for example, we compose the mixture so that the partial pressures of the reactants are very large, while those of the products are very small, then Qp will have a small fractional value, and In Qp will be a 

Using the more general notation, we put the value of fii from Eq. (11.35) in Eq. (11.29) to obtain 

Note that since the for the reactants are negative, we have for the reaction in question 

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