Affinity, reaction quotient and activities

If we define the value Qr, known as the reaction quotient of the transformation r, by the relation  

Using relation [1.25], the Gibbs energy associated with the reaction takes the form  

The last two relations will be useful for the expression of the equihbrium constants (see section 3.1). 

As the affinity is a function of state, its differential, expressed on the basis of the chosen variables, will be of the form  

111 the case where that set of variables comprises only intensive variables Yi, the differential of the affinity is written  

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