Free energy, change ionization

Figure 3.7 A diagram comparing the free-energy changes that accompany ionization of acetic acid and ethanol. Ethanol has a larger positive free-energy change and is a weaker acid because its ionization is more unfavorable.

Figure 3.8 Two resonance structures that can be written for acetic acid and two that can be written for acetate ion. According to a resonance explanation of the greater acidity of acetic acid, the equivalent resonance structures for the acetate ion provide it greater resonance stabilization and reduce the positive free-energy change for the ionization.

The greater stabilization of the carboxylate anion (relative to the acid) lowers the free energy of the anion and thereby decreases the positive free-energy change required for the ionization. 

Any factor that makes the free-energy change for the ionization of an acid less positive (or more negative) makes the acid stronger. 

Hence the experimental equilibrium constants of Table IV are proportional to the constants for ionization into ion pairs, and the ratios of the if exp reflect differences in the tendency of different molecules to form ion pairs. Since standard free energies are proportional to the logarithms of equilibrium constants, differences in the tabulated free energies represent differences in the standard free energy change for ionization, even though the individual values represent the standard free energy for the overall process of ionization plus dissociation. 

Calculate the approximate standard free energy change for the ionization of hydrofluoric acid, HF (K,= lx 10 ), at25°C. 

The enthalpies of ionization corresponding to Eqs. (4), (3"), and (11) can be determined by means of the temperature effect on the respective standard free energy changes (70MI3) or by calorimetric techniques. 

Starting from the comparative study of the ionization constants of uracil itself as well as of its several methylated or ethylated derivatives (representing models of tautomeric forms), it may be seen (Table XVII) that uracil and uridine exist in aqueous solution in the diketo form 32. The pX values are not known for the model tautomers 27, 29, and 30, but these forms have been ruled out on the basis of UV studies. Recently the ionization constants of uracil, thymine, their derivatives and nucleotides were determined over the range 10-50°, and thermodynamic enthalpy, entropy, and free energy changes for protonation and depro-tonation of these compounds have been evaluated.93-95,332... 

Pulse electron-beam mass spectrometry was applied by Kebarle, Hiraoka, and co-workers766,772 to study the existence and structure of CH5+(CH4) cluster ions in the gas phase. These CH5+(CH4) clusters were previously observed by mass spectrometry by Field and Beggs.773 The enthalpy and free energy changes measured are compatible with the Cs symmetrical structure. Electron ionization mass spectrometry has been recently used by Jung and co-workers774 to explore ion-molecule reactions within ionized methane clusters. The most abundant CH5+(CH4) cluster is supposed to be the product of the intracluster ion-molecule reaction depicted in Eq. (3.120) involving the methane dimer ion 424. 

Calculate the ionization constant and free energy change at standard condition for the conversion of glycerol to glycerol-1-phosphate by ATP ... 

Several theoretical models have been proposed (31) to rationalize the ionization process in a dielectric medium and to interpret the experimental data (32). Despite serious criticisms (32), the so-called Bom electrostatic model, in the opinion of these authors, is perhaps still the best model. According to this model the free energy change for the ionization of a weak acid in a solvent of bulk dielectric constant c is given by the equation... 

Equation (XVI.3.4) would be expected to hold if the process of ionization were closely parallel to that of formation of the transition state. Since the former involves the transfer from HA of a proton to solvent while the latter involves the partial transfer from HA of a proton to reactant, it is certainly not surprising that the free energy changes in the two processes may be related. That the value of the exponent a lies between 0 and 1 is... 

The interpretation given to the constant a is that it measures the sensitivity of the reaction (catalysis) to the acidity (or basicity) of the catalyst. In terms of the free energy changes we might say that it is a measure of the amount of the free energy change of ionization that occurs in the formation of the transition state. ... 

One may set up the differential free energy change corresponding to the above ionization reaction by... 

Figure 2.8. The standard free energy change of a reaction depends on the temperature and the pressure. (See Table 2.5 for illustrative data on the standard free energy change for the ionization of water.) d G° dT)p = —AS° and (dAG°/dP)r = AV° are the thermodynamic relationships governing the influence of temperature and pressure on free energy of a reaction.

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