Values From the Elements

As discussed in Section 12.6, any such compilation can only show the values of changes from one state to another. All energy values are relative to some arbitrary datum (see Section 2.7). For chemical reactions the simplest, most satisfactory, and almost universally used convention is to assign values of 

We have no way of determining changes in Gibbs energy directly. Instead, we compute them from 

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While most tables like A.8 are at 25 °C = 298.15 K, the widely used JANAF tables for combustion calculations [6] give Gibbs energy of formation values from the elements at a wide range of temperatures, in effect defining a new datum state for each temperature. This works perfectly well if we understand what the datum state is (see Problem 12.6). 

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