Association standard entropy

Entropy of Formation The ideal gas standard entropy of formation (AS°298) of a chemical compound is the increment of entropy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard state definea as the existing phase at a temperature of 298.15 K and one atmosphere (101.325 kPa). Thus ... 

In much the same fashion as the AH° was tabulated, the standard molar entropies (S°) of elements and compounds are tabulated. This is the entropy associated with 1 mol of a substance in its standard state. Unlike the enthalpies, the entropies of elements are not zero. For a reaction, it is possible to calculate the standard entropy change in the same fashion as the enthalpies of reaction ... 

The heat capacity of thiazole was determined by adiabatic calorimetry from 5 to 340°K by Goursot and Westrum (295,296). A glass-type transition occurs between 145 and 175 K. Melting occurs at 239.53°K (-33.62°C) with an enthalpy increment of 2292 cal mole" and an entropy increment of 9.57 cal mole" -"K". Table 1-44 summarizes the variations as a function of temperature of the most important thermodynamic properties of thiazole molar heat capacity Cp, standard entropy S°, and Gibbs function -(G°-The variation of Cp for crystalline thiazole between 145 and 175°K reveals a marked inflection that has been attributed to a gain in molecular freedom within the crystal lattice. The heat capacity of the liquid phase varies nearly linearly with temperature to 310°K, at which temperature it rises more rapidly. This thermal behavior, which is not uncommon for nitrogen compounds, has been attributed to weak intermolecular association. The remarkable agreement of the third-law ideal-gas entropy at... 

The free energy and the entropy changes associated with chemical reactions are similarly defined and manipulated. Thus, AG is the difference between the Gibbs free energy of the products and that of the reactants under standard conditions. The standard entropy change AS is related to AG and A/f by... 

We can also calculate the standard entropy change associated with the reaction as AG° - AH0 - TAS°. 

A least square plot of log Ccsc as a function of number of carbon atoms in the alkyl chain is given in Fig. 4.13. obtained from the slope of this line is —0.95kT and is comparable to the free energy of micellization measured for similar surfactants in solution (Fig. 4.14). Heat and standard entropy changes associated with the adsorption process can be calculated by considering the adsorption of the long-chain molecules, X, as follows (Somasundaran and Fuerstenau, 1972). 

Knowing the standard free energy change in the cell reaction, AH° can be calculated if AS0 is known. From standard entropy values the change H+ to iH2 corresponds to +15.6 e.u. and so only an estimate of the entropy change associated with the valency change of the complex is required to give a value for AH0 and hence I. 

By means of this equation, measured association constant values can be converted directly into a key thermodynamic parameter. Otherbinding thermodynamic parameters maybe obtained through thermodynamic equations or preferentially measured by ITC. The key parameters are standard enthalpy change of binding and standard entropy change of binding... 

We then see that entropy is the thermodynamic function for predicting the spontaneity of a reaction. On a molecular level, the entropy of a system can in principle be calculated from the number of microstates associated with the system. We learn that in practice entropy is determined by calorimehic methods and standard entropy values are known for many substances. (18.3)... 

Positive standard entropy of the ligand-receptor association in this experimental setup means that there is a lot more free ligand and receptor than the R L complex. 

This relationship between entropy and enthalpy has been reported many times in the literature. An example of a graph relating (AH°) to (AS°), produced by Martire and his group [8], is shown in Figure 9. From a theoretical point of view, this relationship between standard enthalpy and standard entropy is to be expected. An increase in enthalpy indicates that more energy is used up in the association of the solute molecule with the molecules of the stationary phase. This means that the... 

We have now introduced the necessary basics for determining the association constant (ATa), standard free energy change (AG°), standard enthalpy change (AH°), standard entropy change (AS°), and standard heat capacity change (ACp for the complexation of a host and a guest. In subsequent sections, we detail practical aspects of these measurements. 

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