Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Enthalpy contribution

A force field that can produce vibrational spectra has a second advantage in that the Ay// calculations can be put on a much more satisfactory theoretical base by calculating an enthalpy of formation at 0 K as in ab initio procedures and then adding various thermal energies by more r igorous means than simply lumping them in with empirical bond enthalpy contributions to Ay//-. The stronger the theoretical base, the less likely is an unwelcome surprise in the output. [Pg.162]

Summarizing, the greater the forces between the molecules, the greater the energy (enthalpy) contribution, the larger the distribution coefficient, and the greater the retention. Conversely, any reduction in the random nature of the molecules or any increase in the amount of order in the system reduces the distribution coefficient and attenuates the retention. In chromatography, the standard enthalpy and standard entropy oppose one another in their effects on solute retention. Experimentally it has... [Pg.53]

The high enthalpy contribution results from its larger mass and size providing stronger interactions with the stationary phase molecules, and its increased entropy contribution arises from it being a terminal atom, thus prior to interaction with the stationary phase, it has greater freedom. [Pg.60]

Table 6.3 Some enthalpies of atomization (Af/j, 298 K) and comparative bond-enthalpy contributions, E... Table 6.3 Some enthalpies of atomization (Af/j, 298 K) and comparative bond-enthalpy contributions, E...
The ZPE term represents the enthalpy contribution to the isotope effect, and... [Pg.10]

Relative molecular enthalpies then result simply as the sum of the minimum potential energy V°, Z/yj, and the rotational and translational enthalpy contributions (= 3 RT). By experience, the // -contributions of different conformational minima of a molecule rarely differ by more than 1 kcal mole"-1 (see also Section 6.2.4.). Vibrational entropy contributions may be evaluated in a similar way as//vibr. [Pg.183]

As noted above, all radical abstraction reactions can be divided into groups and the activation energy Ee0 for a thermally neutral reaction can be calculated for each group (see Equation [6.11]). This opens up the possibility of calculating of the enthalpy contribution (A h) to the activation energy for the given (z th) reaction and a thermally neutral reaction characterized by the quantity fse0 [4,11] ... [Pg.247]

Still, the strain enthalpy is of particular importance. An elastic continuum model for this size mismatch enthalpy shows that, within the limitations of the model, this enthalpy contribution correlates with the square of the volume difference [41,42], The model furthermore predicts what is often observed experimentally for a given size difference it is easier to put a smaller atom in a larger host than vice versa. Both the excess enthalpy of mixing and the solubility limits are often asymmetric with regard to composition. This elastic contribution to the enthalpy of mixing scales with the two-parameter sub-regular solution model described in Chapter 3 (see eq. 3.74) ... [Pg.219]

Metal-Hydrogen Bond Enthalpy Contributions. Metal Carbonyl... [Pg.75]

Note The bond enthalpy contributions (b.e.cs) are shown in the tables to the nearest whole kilojoule (kJ). [Pg.75]

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq... Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...
Fig. 1. Variation of the terminal metal-CO bond enthalpy contribution, T (M-CO) kj mol as a function of the enthalpy of atomization of the metal, AHf (M, g) kJ mol-1... Fig. 1. Variation of the terminal metal-CO bond enthalpy contribution, T (M-CO) kj mol as a function of the enthalpy of atomization of the metal, AHf (M, g) kJ mol-1...
Table 10. Bond enthalpy contributions, D(M-ArH) kJ mol, in bis(arene) metal compounds... Table 10. Bond enthalpy contributions, D(M-ArH) kJ mol, in bis(arene) metal compounds...
While these disadvantages are severe, electron impact determinations play a useful role in suggesting the pattern of variation in bond enthalpy contributions in molecules which have not been studied by conventional thermochemical techniques. A few examples of this are shown in Table 11. Electron impact measurements also indicate that D (Ru-Cp) in ruthenocene is ca. 100 kJ mol-1 greater than D (Fe-Cp) in ferrocene82). [Pg.95]

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)... Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...
Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]... Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]...
Table 15. Enthalpy of sublimation, formation and disruption (kJ mol-1) for (jr-arene M(CO>3] compounds and arene-M bond enthalpy contributions IT(ArH-M) kJ mol-1... Table 15. Enthalpy of sublimation, formation and disruption (kJ mol-1) for (jr-arene M(CO>3] compounds and arene-M bond enthalpy contributions IT(ArH-M) kJ mol-1...
They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. [Pg.102]

Table 16. Enthalpy of formation, A//f (c) and disruption, AHjy and bond enthalpy contribution, i (Co-L) in LCo4(CO>9. All values in kJ mol-1 (Ref.84))... Table 16. Enthalpy of formation, A//f (c) and disruption, AHjy and bond enthalpy contribution, i (Co-L) in LCo4(CO>9. All values in kJ mol-1 (Ref.84))...
Table 17. Enthalpy of sublimation/vaporization, A//sub/vac, enthalpy of disruption, A/fo, and olefin-iron bond enthalpy contribution for [Fe(CO)n(olefin),j] compounds. All values are in kJ mol-1... Table 17. Enthalpy of sublimation/vaporization, A//sub/vac, enthalpy of disruption, A/fo, and olefin-iron bond enthalpy contribution for [Fe(CO)n(olefin),j] compounds. All values are in kJ mol-1...
Table 18. Comparison of (M-C2H4) and (M-CO) bond enthalpy contributions for M = Fe, Ni and Rh... Table 18. Comparison of (M-C2H4) and (M-CO) bond enthalpy contributions for M = Fe, Ni and Rh...
Table 23. Selected nickel-ligand bond enthalpy contributions, D[(CpNi-L)+] kJ mol-1 determined by ion cyclotron resonance (Ref.93 )... Table 23. Selected nickel-ligand bond enthalpy contributions, D[(CpNi-L)+] kJ mol-1 determined by ion cyclotron resonance (Ref.93 )...
It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. [Pg.109]

Table 26. Selected bond enthalpy contributions, D (M-L) and (M-L) kJ mol-1, in organometallic compounds of chromium, manganese, iron cobalt and nickel and related compounds (An asterisk ( ) denotes the average value in a series)... Table 26. Selected bond enthalpy contributions, D (M-L) and (M-L) kJ mol-1, in organometallic compounds of chromium, manganese, iron cobalt and nickel and related compounds (An asterisk ( ) denotes the average value in a series)...
There are alternative ways of viewing the previous problem that are closer to the idealized concept of chemical bond strength. Consider reaction 5.20, where all the chromium-ligand bonds are cleaved simultaneously. The enthalpy of this disruption reaction at 298.15 K, calculated as 497.9 10.3 kJ mol-1 by using enthalpy of formation data [15-17,31], can be given as a sum of three chromium-carbonyl and one chromium-benzene bond enthalpy contributions (equation 5.21). [Pg.68]

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. [Pg.69]


See other pages where Enthalpy contribution is mentioned: [Pg.298]    [Pg.59]    [Pg.663]    [Pg.128]    [Pg.201]    [Pg.183]    [Pg.59]    [Pg.211]    [Pg.271]    [Pg.75]    [Pg.80]    [Pg.91]    [Pg.94]    [Pg.96]    [Pg.98]    [Pg.101]    [Pg.106]    [Pg.107]    [Pg.68]    [Pg.68]    [Pg.69]    [Pg.69]   
See also in sourсe #XX -- [ Pg.144 ]




SEARCH



Bond enthalpy contribution

Bond enthalpy contributions bonding energetics

Bond enthalpy contributions transferability

Enthalpy and Entropy Contributions

Enthalpy electrostatic contribution

Thermal analysis enthalpy contribution

© 2024 chempedia.info