Summation of Reaction-Thermodynamic Properties

Not infrequently thermodynamic data are lacking for a reaction of particular interest. Sometimes such data does exist for related reactions which can then be combined to solve our problem. Combination may be either by addition or subtraction of their log (or In ATe,) values as appropriate, 

We have produced our desired reaction and its value by writing the related reactions in such a way as to cancel out unwanted terms in the addition. Note that three of the four reactions have been written in their usual form, so their log K values are positive. The fourth and last is written in reverse so that log K2 must be given a negative sign (log I/K2 = - log K2). The same final answer for could 

Note that because AG° = -RT In we could also have added AG values for the four reactions to obtain the corresponding AG° value. The same approach can also be employed to obtain AH° by summing the enthalpies of related reactions. This is called Hess s law. As a caution, it must be remembered that the thermodynamic properties of each reaction have uncertainties. Consequently, as we increase the number of reactions combined to obtain the properties of a desired reaction, we increase the uncertainty in our result. 

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