Entropy predicting relative

Predicting Relative S° Values of a System Based on an understanding of systems at the molecular level and the effects of heat absorbed, we can often predict how the entropy of a substance is affected by temperature, physical state, dissolution, and atomic or molecular complexity. (All S° values in the following discussion have units of J/mohK and, unless stated otherwise, refer to the system at 298 K.)... 

SAMPLE PROBLEM 20.1 Predicting Relative Entropy Values... 

The following sections of this chapter focus on predicting relative acidities, which is an analysis of thermodynamics. The focus will be on enthalpy because it measures the intrinsic stabilities of the acids and bases on both sides of the equilibria. We do not consider entropy because an acid and a base exist in both the reactants and the products therefore, the number of molecules does not change during the reaction. Ffence, enthalpy is a good predictor for acid-base reaction equilibria. 

The First Law Does Not Predict Spontaneity The Sign of AH Does Not Predict Spontaneity Freedom of Motion and Dispersal of Energy Entropy and the Number of Microstates Entropy and the Second Law Standard Molar Entropies and the Third Law Predicting Relative S° of a System... 

From such crude data as are to be found in the literature we can calculate approximate values of the equilibrium constants, and hence of the free energies of dissociation for the various hexaarylethanes. From our quantum-mechanical treatment, on the other hand, we obtain only the heats of dissociation, for which, except in the single case of hexaphenylethane, we have no experimental data. Thus, in order that we may compare our results with those of experiment, we must make the plausible assumption that the entropies of dissociation vary only slightly from ethane to ethane. Then at a given temperature the heats of dissociation run parallel to the free energies and can be used instead of the latter in predicting the relative degrees of dissociation of the different molecules. 

Based on these contributions (a-d), we may arrive at the predictive scheme presented in Table 1. Because of the relatively large contribution from dehydration, essentially all proteins adsorb from an aqueous environment on apolar surfaces, even under electrostatically adverse conditions. With respect to polar surfaces, distinction may be made between proteins having a strong internal coherence ( hard proteins) and those having a weak internal coherence ( soft proteins). The hard proteins adsorb at polar surfaces only if they are electrically attracted, whereas the structural rearrangements (i.e., reductions in ordered structure) in the soft proteins lead to a sufficiently large increase in conformational entropy to make them adsorb at a polar, electrostatically repelling surface. 

The equation-of-state method, on the other hand, uses typically three parameters p, T andft/for each pure component and one binary interactioncparameter k,, which can often be taken as constant over a relatively wide temperature range. It represents the pure-component vapour pressure curve over a wider temperature range, includes the critical data p and T, and besides predicting the phase equilibrium also describes volume, enthalpy and entropy, thus enabling the heat of mixing, Joule-Thompson effect, adiabatic compressibility in the two-phase region etc. to be calculated. 

Robinson G. R. Jr. and Haas I L. Jr. (1983). Heat capacity, relative enthalpy and calorimetric entropy of silicate minerals An empirical method of prediction. Amer. Mineral, 68 541-553. 

From a computational point of view, the heat of formation, which is derived from the electronic energy of the molecule molecule> is the most difficult thermochemical quantity to predict accurately. Entropies and heat capacities are derived from vibration and rotational constants, all of which can be predicted with considerable accuracy using relatively low levels of theory. Thus, the development of ab initio methods appropriate for a new class of compounds focuses primarily on identifying a level of theory and the basis set(s) needed to achieve sufficient accuracy in the electronic energy [67,68]. 

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