Enthalpy changes unique values

Some other published log BCF-log Kow QSARs are listed in Table 15.1. For compounds with log Kow values below about 7, rectilinear log BCF-log Kow correlations are satisfactory (Connell, 1995 Devillers et al., 1996 Meylan et al., 1999), even for diverse compounds, despite the assertion of Streit (1992) that thermodynamic considerations indicate that a unique log BCF/log Kow relationship cannot exist, since enthalpy changes vary for different classes of chemical substances. ... 

In the unique case of diatomic molecules, however, bond energies and dissociation energies are, in principle, identical and exact (although, there may be disagreement in the literature as to what the exact values are). The enthalpy change for the reaction... 

Therefore, under relaxation conditions the composition of components of the reaction j changes until it reaches some minimum free enthalpy, which is equal AZ only under standard conditions. For convenience of use, this value is expressed in terms of the product of activities, which the reaction participants have at equilibrium under standard conditions. Indeed, in case of equilibrium under standard conditions all reaction components must be tied by the unique value of their activities product, which is called thermodynamic standard equilibrium constant of the reaction. For this reason... 

The measured enthalpy change for a reaction has a unique value only if the initial state (reactants) and final state (products) are precisely described. If we define a particular state as standard for the reactants and products, we can then say that the standard enthalpy change is the enthalpy change in a reaction in which the reactants and products are in their standard states. This so-called standard enthalpy of reaction is denoted with a degree symbol, AjH . 

In the enthalpy diagrams we have drawn, we have not written any numerical values on the enthalpy axis. This is because we caimot determine absolute values of enthalpy, H. However, enthalpy is a function of state, so changes in enthalpy, AH, have unique values. We can deal just with these changes. Nevertheless, as with many other properties, it is still useful to have a starting point, a zero value. 

Relationships between the enthalpy of reaction and the change in the O—H stretching frequency have been disputed. New values for enthalpies of reaction for phenol with several Lewis bases have been determined by calorimetry. A good linear relationship between AH and roH was reported. No unique relation exists between vibrational frequencies and lengths for O—H O hydrogen bonds but decreasing... 

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