Thermodynamic parameters, exothermic

Figure 13.10 Calorimetric titration response showing the exothermic raw (downward-projecting peaks, upper panel) heats of the binding reaction over a series of injections titrating 0.061 mM RNase A (sample) with 2.13 mM 2CMP at 30°C. Bottom panel shows the binding isotherm obtained by plotting the areas under the peaks in the upper panel against the molar ratio of titrant added. The thermodynamic parameters were estimated (shown in the inlay of the upper panel) from a fit of the binding isotherm.

The site-selectively derived thermodynamic parameters obtained by adaptation of Equation 1.17 (Table 1.8) clearly revealed that the heat of adsorptions are exothermic on both enantioselective and nonenantioselective sites, and the difference in the adsorption enthalpies on enantioselective and nonenantioselective sites is about 10 and 15 kJ mol for/ - and 5-enantiomers, respectively. The differential enthalpy change upon adsorption of R- and 5-enantiomers at the enantioselective site AAEIg... 

Table IX shows the thermodynamic parameters of micellization, calculated from the cmcs and their temperature dependence, for two homologues of each of these types of nonionics. It can be seen that AHm and ASm progressively decrease in the order usual-type > S > L-A > G type for both homologous series. This suggests that the high polarity of the amido linkage causes either less dehydration or exothermic association of the hydrophilic part on micelle formation.

In this equation, CA is the concentration (in mol m 3) of the rate-limiting component A, k is the nth-order rate constant (with units m3(" lf mol1-" s-1), n is the order of the reaction and rA is the rate of reaction (units, mol m 3 s 1). As already mentioned, in the field of reaction calorimetry, qRe lC is generally defined as positive for an exothermic reaction (negative A rH). The aim of the determination is to calculate the kinetic parameters k and (possibly) n. Some methods also determine the thermodynamic parameter ArH on the basis of this reaction model. 

Formation of an ionic tetracoordinate Si+ complex from an uncharged nucleophile and a functional silane is an exothermic process accompanied by a marked drop in entropy. Many qualitative observations indicated that these complexes are generated more readily at lower temperatures (78,242,252,256). Unfortunately, there are few data on the thermodynamic parameters of complex formation. From the temperature variation of the 29Si resonance position, Bassindale and Stout (252) determined the enthalpy and entropy of the formation of bis(iV,Af-trimethylsilyI)-imidazolium chloride (Table IV, entry 10). A similar procedure permitted Chaudhry and Rummer (242) to determine the enthalpy of formation of complexes of 2-trimethylsilyl-l,l,3,3,-tetramethylguanidine (Table IV, entries 6, 7). 

If a reactant gas is introduced into the collision cell, ion-molecule collisions can lead to the observation of gas-phase reactions. Tandem-in-time instruments facilitate the observation of ion-molecule reactions. Reaction times can be extended over appropriate time periods, typically as long as several seconds. It is also possible to vary easily the reactant ion energy. The evolution of the reaction can be followed as a function of time, and equilibrium can be observed. This allows the determination of kinetic and thermodynamic parameters, and has allowed for example the determination of basicity and acidity scales in the gas phase. In tandem-in-space instruments, the time allowed for reaction will be short and can be varied over only a limited range. Moreover, it is difficult to achieve the very low collision energies that promote exothermic ion-molecule reactions. Nevertheless, product ion spectra arising from ion-molecule reactions can be recorded. These spectra can be an alternative to CID to characterize ions. 

Previtali and Scaiano have attempted to correlate rates of hydrogen abstraction with thermodynamic parameters 69> along the lines of Polanyi 70>. In this approach (AH proportional to AH) the exothermicity of hydrogen abstraction varies with the triplet excitation energy and the carbonyl 51-bond energy, at constant C—H and O—H bond energies. 

The enthalpy of reaction, AH, is the other important thermodynamic parameter to consider. On its own, whether a reaction is exothermic or endothermic will not determine if a reaction is industrially feasible or not. Both exothermic and endothermic processes are known in industry, methanol carbonylation to acetic acid (Equation 3 AH —123 kJ/mol at 200°C), being an example of the former and the steam reforming of methane to synthesis gas, (Equation 4 AH + 227 kJ/mol at 800°C), being an example of the latter. 

Most chain-propagating steps are exothermic and one can use the strength of bonds that are broken and formed as a rough guide to the rate of the reaction (thermodynamic parameter). 

In a study achieved by Memon et al. [16] the sorption of carbofuran and methyl parathion on treated and untreated chestnut shells has been studied using high performance liquid chromatography. In this study, the maximum sorption of methyl parathion and carbofuran onto chestnut shells was achieved at a concentration of 0.38.10 and 0.45.10" mol.dm respectively. Adsorption isotherms depicted a better fitting with the Langmuir isotherm. The results of sorption energy obtained from the Dubinin-Radushkevich isotherm pointed out that adsorption was driven by physical interactions. The kinetics of sorption follows a first-order rate equation. The thermodynamic parameters AS and AG indicate that the sorption process is thermodynamically favourable, and spontaneous, whereas the value of AH shows the exothermic nature of sorption process for methyl parathion and endothermic nature of carbofuran. The developed sorption method has been employed in methyl parathion and carbofuran in real surface and ground water samples. The sorbed amount of methyl parathion and carbofuran may be removed by methanol to the extent of 97-99% from the surface of chestnut shells. 

Stepwise equilibrium constants and derived thermodynamic parameters for the reaction of [M(ttfa)3] with 2,2 -bipyridyl showed that up to two molecules of base could be added. The stepwise heats and entropies of addition depended on the ionic radius of M (La, Nd, Gd, or Lu), and the adducts were stabilized by large exothermic enthalpy changes. One and two molecules of tributyl-phosphate could be added to [Eu(ttfa)3]. ... 

It is interesting to compare the thermodynamic parameters obtained for the 1 1 complexation of Cbz-Gly with y-CyD with those for the 1 1 complexation of Cbz-Gly with j8-CyD. This comparison may also be taken as an independent assessment of the reliability of the above-mentioned four-parametric fit. The equilibrium constant for the 1 1 complex formation of Cbz-Gly with y-CyD is about one-tenth as large as that for Cbz-Gly complexation with fi-CyD [82], and the heat effect obtained for the y-CyD complex is about one third as large as that for the fi-CyD complex. It is reasonable to expect less-pronounced van der Waals interactions and therefore a smaller exothermic heat effect, when the cavity is too large to comfort-... 

Once again the detail needs to be prefaced by some general considerations. The process of chemisorption is in essence a chemical reaction unfortunately, like many chemical reactions, it is not a simple process, as it does not always lead to a well-defined product. The occurrence of the different structures of the hydrogen atom adlayer exemplifies this. Nevertheless the fact that chemisorption takes place means that the Gibbs free energy of the system must decrease, and that because of the loss of translational entropy there has to be a decrease in the system s heat content chemisorption is thus of necessity exothermic. All manner of thermodynamic parameters can therefore be ascribed to the process and to the resulting state. ... 

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