Affinity and direction of reaction

At chemical equilibrium the total amount of mass is not changed, therefore the change of internal energy d due to chemical reaction is null  

The condition w = dl/dt = 0 implies a state of null reaction rate, which is not relevant. The non-obvious solution is obtained as = 0, which implies equilibrium of the affinity. Thus, in general, the equilibrium condition is given by 

The non-equilibrium reactions are classified according to the following four cases  

Because of (E.50), the products increase and the originals decrease, therefore the reaction moves to the right (forward reaction) if 0. If products decrease and the originals increase, the reaction moves to the left (backward reaction) if d /dt 0. On the other hand, if dUc = —Ad /dt 0, the reaction is exothermic, and ii dUc 0, the reaction is endothermic. Thus Case 1 is a forward exothermic reaction. Case 2 a forward endothermic reaction. Case 3 a backward exothermic reaction, and Case 4 a backward endothermic reaction. 

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