Enthalpy entropy changes and

The basicity of pyrazole and its relation with imidazole basicity (due both to enthalpy and entropy changes (77MI40403)) have been discussed on theoretical grounds (Section 4.04.1.2.1). The pK values of 90 pyrazoles have been determined by Gonzalez et al. (68BSF707,68BSF5009) and it is essentially his work that will be discussed below. A selection of pK values are shown in Table 28. The pK values for some other pyrazoles have been measured in connection with nitration studies (Section 4.04.2.1.4(ii)) (71JCS(B)2365). 

It can be shown - p- that if an LFER is observed over a range of temperatures, and if the enthalpy and entropy changes are temperature independent, then the enthalpy changes must be directly proportional to the entropy changes for the reaction series. Let us start with the proposition that a real effect of this type has been demonstrated for a reaction series we write this as... 

Alberty, R. A., 1969. Standard Gibb.s free energy, enthalpy, and entropy change.s a.s a function of pH and pMg for reaction.s involving adeno.sine pho.sphate.s. of Biological Chemistry 244 3290-3302. 

Also of importance is the effect of temperature on the gas solubility. From this information it is possible to determine the enthalpy and entropy change experienced by the gas when it changes from the ideal gas state (/z and ) to the mixed liquid state ( andT,). 

The equilibrium concentrations of many disubstituted benzenes (containing alkyl and halogen substituents) show that the meta isomer is in nearly all cases the most thermodynamically stable. It is not obvious why this should be so. Shine182 had discussed this problem in terms of the relative sizes of the standard enthalpy and entropy changes between any pair of isomers. 

Schematic view of the three phase changes leading from more constrained to less constrained phases, illustrated by the phase changes for water. Each is accompanied by positive enthalpy and entropy changes for the substance.

Solving for In gives an equation relating Tgq to standard enthalpy and entropy changes ... 

Lu, N. Kofke, D. A. Woolf, T. B., Staging is more important than perturbation method for computation of enthalpy and entropy changes in complex systems, J. Phys. Chem. B 2003,107, 5598-5611... 

It is well known that interpretation of structural effects on reactivity in terms of enthalpy and entropy changes is often complicated, or even overwhelmed, by solvation phenomena. Cyclisation reactions are no exception. This is especially so for systems involving large polarity changes on going... 

Enthalpy and entropy changes for the formation of the major species have been determined (16, 29) and are given in Table III. Oligomerization is enthalpy driven as seen, for example, from the thermo-... 

Thermochemical investigations of molybdate solutions have been carried out and reaction heats were measured (108,109). As the interpretation of calorimetric data depends heavily on the correct reaction model, progress in determining reliable enthalpy and entropy changes for condensation reactions have been hampered. However, since there is little doubt that [M07O24]6 is the first polyanion which forms on acidification, the enthalpy and entropy changes obtained for its formation should be meaningful. The values for Eq. (30) are AH° =... 

Equilibrium constants are also dependent on temperature and pressure. The temperature functionality can be predicted from a reaction s enthalpy and entropy changes. The effect of pressure can be significant when comparing speciation at the sea surface to that in the deep sea. Empirical equations are used to adapt equilibrium constants measured at 1 atm for high-pressure conditions. Equilibrium constants can be formulated from solute concentrations in units of molarity, molality, or even moles per kilogram of seawater. 

From the Van t Hoff plot they calculated the enthalpy and entropy change of absorption as AH = -69.5 kJ/molH and AS = -129.6 J/moIH K, respectively. Correspondingly, for desorption they calculated AH = -83.2 kJ/molH and AS = -146.7 J/molH K. It is interesting that the enthalpy of desorption of ball-milled, nanocrystalline Mg CoH is higher than that for absorption. This is a very similar behavior to the one observed for the ball-milled, nanocrystalline MgH as discussed in Sect. 2.1.4 (Fig. 2.43). 

This equilibrium open-circuit potential for a H2/air fuel cell is calculated from thermodynamic data of reaction enthalpy and entropy changes. 

The majority of reported studies of formation of cyclodextrin inclusion complexes in solution have been mainly concerned with determination of the stability constants by using equilibrium spectroscopic techniques, and the measurement of the enthalpy and entropy changes characterizing the complexation reaction. The aim of much of this work has been to determine the driving force of complex-formation. Despite the amount of research in this area, however, no general agreement has been reached, and... 

The thennodynainics of complexation between hard cations and hard (O, N donor) hgands often are characterized by positive values of both the enthalpy and entropy changes. A positive AH value indicates that the products are more stable than the reactants, i.e., destabilizes the reaction, while a positive entropy favors it. If TAS > AH°, AG° will be negative and thus log(3 positive, i.e., the reaction occurs spontaneously. Such reactions are termed entropy driven since the favorable entropy overcomes the unfavorable enthalpy. 

The enthalpy and entropy changes of the total complexation reaction, AH° and AS°, reflect, primarily, step 1 [Eq. (3.18a)], the dehydration snbreaction. 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

Both U02(TTA)2 and Th(TTA)4 have two molecules of hydrate water when extracted in benzene, and these are released when TBP is added in reactions Eqs. (4.11) and (4.12). The release of water means that two reactant molecules (e.g., U02(TTA)2 2H2O and TBP) formed three product molecules (e.g., U02(TTA)2 TBP and 2H2O). Therefore, AS is positive. Since TBP is more basic than H2O, it forms stronger adduct bonds, and, as a consequence, the enthalpy is exothermic. Hence, both the enthalpy and entropy changes favor the reaction, resulting in large values of log K. 

The overall enthalpy and entropy changes for the distribution reaction (i.e., transfer of the metal complex from the aqueous to the organic phase) can be obtained from the temperature dependence of A r according to... 

In the second approach, the chemical equilibrium between the reactant(s) and the transition state is expressed in terms of conventional thermodynamic functions, i.e., enthalpy and entropy changes. This method is easier to implement and provides useful insights for estimating both the preexponential factors and the activation energies. Consequently, we shall utilize the thermodynamic formulation of the TST in this paper. 

---