Entropy, ionization

Assuming that the ionization entropies AS0 and AS0 of AH and AH 8), respectively, are equal, the difference in enthalpies AH0 — AH° can be replaced by the difference in standard free energies AG° — AG°, each of them being given by... 

Forster s cycle (50MI1) (method 1 in Table VIII, also known as the thermodynamic method ). This cycle is particularly important because it can be used even when the protolytic equilibrium is not reached in the excited state. On the other hand, it has two important limitations (i) the frequencies of the 0-0 transitions in absorption or emission are necessary and (ii) ionization entropy changes are assumed to be the same in the ground and in the excited states. The experimental difficulties involved in determining the 0-0 transition frequencies have led to the use of the frequencies of the absorption maxima (procedure a), emission maxima (procedure b), or the average therefrom (procedure c). 

AG°o,v(Ar) and AG°oiv(Ar ) are the free energies of solvation of the neutral molecule and the radical cation, respectively, and F is the Faraday constant. It should be noted that the ionization potential is the enthalpy of ionization at 0 K thus, the ionization entropy and the temperature correction are neglected in equation (3). However, these corrections are assumed to be fairly small. 

It IS a curious fact that substituents affect the entropy of ionization more than they do the enthalpy term m the expression... 

Calculate AS° for ionization of each compound. Comment on the contribution of AH° and AS° to the free energy of ionization. Test the data for linear fiee-energy correlations. Are the linear free-energy correlations dominated by entropy or enthalpy terms ... 

Entropy Factors Arising from Hydrolysis and Ionization... 

For the phosphoric anhydrides, and for most of the high-energy compounds discussed here, there is an additional entropic contribution to the free energy of hydrolysis. Most of the hydrolysis reactions of Table 3.3 result in an increase in the number of molecules in solution. As shown in Figure 3.11, the hydrolysis of ATP (as pH values above 7) creates three species—ADP, inorganic phosphate (Pi), and a hydrogen ion—from only two reactants (ATP and HgO). The entropy of the solution increases because the more particles, the more disordered the system. (This effect is ionization-dependent because, at low pH, the... 

AH and AS to various notional subprocesses such as bond dissociation energies, ionization energies, electron affinities, heats and entropies of hydration, etc., which themselves have empirically observed values that are difficult to compute ab initio. 

The relationship of thermodynamic functions of selective bonding of Hb to a series of carboxylic CP in the variation of the degree of ionization of carboxylic groups is expressed by the effect of enthalpy-entropy compensation (Fig. 18). The compensation effect of enthalpy and entropy components is the most wide-spread characteristic of many reactions in aqueous solutions for systems with a cooperative change in structure [78],... 

To compare the present results with those of DeCarvalho and Choppin, our results in 1 M acid must be corrected for the heat and entropy of HSOiJ ionization. Zebroski et al. (12) determined Ka = 0.084, while Zielen (9) gives the heat of ionization as AHa = +23.2 kj/m. The calculated entropy is ASa - +98 j/m-°K. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Maximum entropy A computer algorithm used to predict the theoretical signal from which that observed in a spectrum has been derived. Used in conjunction with electrospray ionization to enhance the quality of the spectra obtained. 

We shall look more closely at this equation. On one hand, the standard chemical potentials of Ox and Red depend on their standard Gibbs solvation energies, AG ox and AG°Red, and, on the other hand, on the standard Gibbs energy of ionization of Red in the gas phase, AGlon Red. This quantity is connected with the ionization potential of Red, /Rcd, which is, however, a sort of enthalpy so that it must be supplemented by the entropy term, -TA5 on Red. Thus, Eq. (3.1.17) is converted to the form... 

The difference between the electronic energies of the final and initial states must include the energy of ionization of the ion B(z-1)+ in vacuo (where its ionization potential is complemented by the entropy term TA5/), the interaction energy of the ions Bz+ and B(z-1)+ with the surroundings, i.e. the solvation Gibbs energies, and finally the energy of an electron at the Fermi level in the electrode. These quantities can be expressed most simply... 

Solvation of CH3CO2 is stronger => the the solvent molecules become more orderly around it => the entropy change (AS0) for the ionization of acetic acid is negative => the -TAS° makes a positive contribution to AG° => weaker acid. 

Both AH° and -TAS° are more favorable for the ionization of chloroacetic acid. The larger contribution is in the entropy term. 

Table 10 p bh+ values, m slopes, enthalpies and entropies of ionization for some amides in aqueous perchloric acid."... 

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