Standard affinity and equilibrium constants

On comparing the definition (7.27) of the equilibrium constant with that of the standard affinity in table 7.2 we see immediately that 

When dealing with a reaction which is a linear combination of known reactions, it is advantageous to work with affinities rather than equilibrium constants since the standard affinity is obtained simply by adding together the corresponding standard affinities of the separate reactions in the same linear combination. 

Equation (7.80) holds whether a system is ideal or not, for the standard affinity of a reaction in a non-ideal system is the same as that in the corresponding reference system. 

The usefulness of these ideas will become apparent in chapter VIII. 

For an ideal system as shown in table 7.1, the standard heat of reaction is identical with the total heat of reaction. At a given pressure we have therefore  

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