Calorimetric Measurement of Chemical Drives

If we were to abide by the usual rules of terminology, we would only call the first element heat and use the symbol Q for it (without the index e), even though the other contributions cause the same effects. We will overlook this limitation, though, because it means nothing to us and only creates an uimecessary obstacle. 

Gibbs-Helmholtz Equations One of these equations (of which there are different versions) will serve here as an example for applying the balance of entropy discussed above. The most common version of this equation describes the relation between drive and the heating effect that is observed when a reaction runs freely at constant p and T, meaning that the released energy Ji is burnt without any use (r] = 0), so that JL- = T ASg = Q. Again, we assume to be small. The fame of these equations can be traced to an error, the idea common at that time, that the emitted heat —Q or the heat tone as it was called was a measure of the drive of a reaction (Berthelot s principle, 1869). +Qe would have been 

Therefore, there was good reason to measure these heats and to collect the data. The error was only gradually dealt with by the work of Josiah Willard Gibbs, Hermann von Helmholtz, and Jacobus Henricus van t Hoff who all showed that the heat tone —Qe itself did not represent the correct measure for the drive, but that a positive or negative contribution corresponding to the latent heats Qf (such as was known from phase transitions) had to be added  

The concept is simple In a reaction, the released energy J A is burnt during a certain conversion A at given values of p and T and the generated entropy ASg is 

Unfortunately, the latent entropy becomes a problem because depending upon the type of substances, the entropy demands Sm and therefore the entropy content AS of the sample change in the reactor. A positive AS becomes noticeable as a negative contribution —AS in the calorimeter (Index ) so that not ASg but AS = ASg - AS = —ASe is measured there. 

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