Entropic factor estimation

II and II, are estimated degrees only there is no experimental value involved except for AIT and calculation of the entropic factor from equation (1) by Hass. The process is indicated in the chart below ... 

Interestingly, the standard entropies (and in turn heat capacities) of both phases were found to be rather similar [69,70]. Considering the difference in standard entropy between F2(gas) and the mixture 02(gas) + H2(gas) taken in their standard states (which can be extracted from general thermodynamic tables), the difference between the entropy terms of the Gibbs function relative to HA and FA, around room temperature, is about 6.5 times lower than the difference between enthalpy terms (close to 125 kJ/mol as estimated from Tacker and Stormer [69]). This indicates that FA higher stability is mostly due to the lower enthalpy of formation of FA (more exothermic than for HA), and that it is not greatly affected by entropic factors. Jemal et al. [71] have studied some of the thermodynamic properties of FA and HA with varying cationic substitutions, and these authors linked the lower enthalpy of formation of FA compared to HA to the decrease in lattice volume in FA. 

It is possible to estimate Kc when it deviates only slightly from the entropic factor of 4 by an amount related to the Coloumb repulsion energy of the metal charges (23). Thus,... 

Obviously, this estimation is only grossly approximate, and much experimental and theoretical work is needed in order to more precisely determine the entropic contribution for chymotrypsin and other enzymes. The striking feature of this analysis, however, is that entropic factors alone can make such a major contribution to the turnover rates of enzymes, even without inclusion of the advantages which the enzyme derives from differential stabilization of the TS versus the S. The detailed quantitative description of all of the factors which contribute to enzymic catalysis for individual enzymes remains a major goal for future work, but it now appears likely that enzymic catalysis can be accounted for on the basis of known chemical principles. 

The entropic hypothesis seems at first sight to gain strong support from experiments with model compounds of the type listed in Table 9.1. These compounds show a huge rate acceleration when the number of degrees of freedom (i.e., rotation around different bonds) is restricted. Such model compounds have been used repeatedly in attempts to estimate entropic effects in enzyme catalysis. Unfortunately, the information from the available model compounds is not directly transferable to the relevant enzymatic reaction since the observed changes in rate constant reflect interrelated factors (e.g., strain and entropy), which cannot be separated in a unique way by simple experiments. Apparently, model compounds do provide very useful means for verification and calibration of reaction-potential surfaces... 

Enthalpic and Entropic Contributions to the Excess Free Energy Molecular Picture of the Dissolution Process Model for Description of the Aqueous Activity Coefficient Box 5.1 Estimating Molar Volumes from Structure Illustrative Example 5.2 Evaluating the Factors that Govern the Aqueous Activity Coefficient of a Given Compound... 

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