Free energy change of ionization

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. ... 

This equation states that the extent to which the free energy change of a particular equilibrium is alerted by adding a substituent R is linearly related to the extent to which the free energy change of ionization of benzoic acid is alerted by putting the same substituent on the benzene ring. 

Each diagonal electrostatic free energy interaction matrix element is the difference between the free energy change of ionization for group i in the otherwise un-ionized protein, and in a model compound in water of p modei,/ ... 

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. 

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.

The standard free-energy change of reaction 17 can be computed from the free energy for dissociating a Hj molecnle in H atoms and the ionization potential of H". Denoting this energy by and snbstituting in Equation 13.16. we have... 

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. 

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. 

The reverse transfer affects each quantity in Eq. (3.382) but mainly in the region of resonance (AG,- 0). This is the favorable interval for reverse transfer which manifests itself by the high but narrow peak in the free energy dependence of Q [solid line in Fig. 3.49(a)]. There Q > Kg because it accounts not only for primary, but all subsequent ionization acts as well. These acts follow reverse electron transfer, which occurs much more often within than outside resonant interval. However, for the same reasons the quantum yield of irrevocable electron transfer dashed line in Figure 3.49(c). 

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

In general terms the relations between possible compound phases in any system are determined by the usual tangent relation between their free energy surfaces. The free energy of any phase is a function of the temperature, activity of the components, number of lattice sites and relative numbers of atoms of each kind in the crystal, concentrations of vacancies, interstitials and substitutions of each kind, concentrations of associated defects, energies of lattice disorder, of defect interactions, of valence change, of ionization, etc. ... 

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