General Properties of Affinity

Affinity of a reaction is a state function, completely defined by the chemical potentials. It can be expressed as a function of V, T and Nk- For a closed system, since all the changes in Nk can only be due to chemical reactions, it can be expressed in terms of V, T, and the initial values of the mole numbers Nj. There are some general properties of affinities that follow firom the fact that chemical reactions can be interdependent when a reactant is involved in more than one reaction. 

The sign of affinity can be used to predict the direction of reaction. Consider the reaction X + F 2Z. The affinity is given by A = (px + M y 2pz)- The velocity of reaction (d /dt) indicates the direction of reaction, i.e. whether the net conversion is from X,Y to Z or from Z to X,Y. From the definition of it follows that if di /dt) 0 then the reaction proceeds to the right . X-fY Z if di,/dt) 0 then the reaction proceeds to the left  

2Z X -I- Y. The Second Law requires that A d /dt) 0. Thus we arrive at the following relation between the sign of A and the direction of the reaction  

A chemical reaction can be the net result of two or more successive chemical reactions. For instance 

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