Thermodynamics bond enthalpies

By these methods, one can obtain fairly reliable estimates of enthalpies of formation of many compounds. As the bond enthalpies used are average values, they cannot be expected to result in highly accurate results for enthalpies of formation. More complex procedures also have been developed that will provide greater accuracy [14]. Related methods for estimation of thermodynamic data are discussed in Appendix A. 

The relative bond enthalpies from the photoacoustic calorimetry studies can be placed on an absolute scale by assuming that the value for D//(Et3Si—H) is similar to D/f(Me3Si—H). In Table 2.2 we have converted the D/frei values to absolute T>H values (third column). On the basis of thermodynamic data, an approximate value of D//(Me3SiSiMc2—H) = 378 kJ/mol can be calculated that it is identical to that in Table 2.2 [1]. A recent advancement of photoacoustic calorimetry provides the solvent correction factor for a particular solvent and allows the revision of bond dissociation enthalpies and conversion to an absolute scale, by taking into consideration reaction volume effects and heat of solvation [8]. In the last colunm of Table 2.2 these values are reported and it is gratifying to see the similarities of the two sets of data. 

The BDEs are well-defined thermodynamic quantities that can be employed in rigorous first-law calculations, but their values strongly differ in each of the above steps. Only the average of the four BDE[CH —H] values could be taken as an estimate of bond enthalpy DH° [C—H],... 

If the reactions involve states other than gaseous ones then ancillary thermodynamic data has to be introduced in addition to the bond enthalpies. 

Ligand substitution, oxidative addition reaction, and absolute bond enthalpies afford insight into thermodynamic driving forces behind important processes. One of the most striking among the examples given above... 

EI4.4 The bond enthalpy of a C-F bond is higher than the bond enthalpy of a C-H bond, making C-F bond more difficult to break. Further, the combustion produces not only CO2 but H O as well that is thermodynamically a very stable compound. Formation of both CO2 and H2O thermodynamically drive the combustion of CH4. There is no equivalent to the formation of H2O in the combustion of CF4. 

An ingenious method of preparing a salt of [113804] is to use reaction 15.107 which is thermodynamically driven by the high Si—F bond enthalpy term in Me3SiF (see Table... 

Through these simulations, students appreciate the energetic and kinetic aspects of chemical reactions and why the composition of the atmosphere is critical to the outcome. For example, the class simulates the effect on the yield of amino acids if the atmospheric composition was CO2 instead of CH4 or the nitrogen source was N2 instead of NH3. The simulations also allow students to investigate the consequences if the pre-biotic atmosphere contained free 02. Through these simulations, basic chemical concepts are discussed including thermodynamics, thermochemistry and bond enthalpies, kinetics, and catalysis. 

ICR-FTMS experiments have been employed to derive thermodynamic information using several approaches ( ). These include studies of exothermic and endothermic ion-molecule reactions, equilibration studies, competitive collision-induced dissociation reactions, and photodissociation studies. Exothermic reactions involving therma-lyzed (cooled by an inert buffer gas) ions provide brackets on metal-ligand bond enthalpies, as illustrated by Equation 44 which implies that D(Fe-C5H4 ) > 66 kcal/mol. Endothermic reactions can also be studied in the manner described above for ion beam experi-... 

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