Free energy hydrogen halide

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

The free energy difference is mainly governed by the subtle balance of the two energetic components, the formation energies of hydrogen halides and the solvation ener-... 

Plot a graph of the standard free energy of formation of the hydrogen halides against the period number of the halogens. What conclusions can be drawn from the graph ... 

In this table, the free energy of formation, AGf of the chloride of these metals is listed for four different temperatures. As can be seen, the values are more negative than that of hydrogen chloride. These metals can be used to reduce the halides of titanium, zirconium, or hafnium, whereas hydrogen, as mentioned above, cannot do so readily. In order to be useful in CVD, the by-product chloride must be volatile at the deposition temperature. This may rule out the use of sodium or potassium, which evaporate above 1400°C. 

Hybrid solvation Implicit solvation plus Explicit solvation microsolvation subjected to the continuum method. Here the solute molecule is associated with explicit solvent molecules, usually no more than a few and sometimes as few as one, and with its bound (usually hydrogen-bonded) solvent molecule(s) is subjected to a continuum calculation. Such hybrid calculations have been used in attempts to improve values of solvation free energies in connection with pKp. [42], and also [45] and references therein. Other examples of the use of hybrid solvation are the hydration of the environmentally important hydroxyl radical [52] and of the ubiquitous alkali metal and halide ions [53]. Hybrid solvation has been surveyed in a review oriented toward biomolecular applications [54]. 

MD simulations of halide anions [X]- and their inclusion complexes [X] c [L4+] with a macrotricyclic tetrahedral host built from four quaternary ammonium sites dissolved in [C4mim][PF6] were carried out [118]. In the dry IL the uncomplexed halides were surrounded by four to five [C4mim]+ cations which bond via hydrogen bonding to facial coordination. The first solvation shell of [Cl]-, [Br], and [I] comprised of three to four cations next to four H20 molecules for the humid system. The solvation of the [L4+] host and of its [X]-1 [L4+] complex mainly involved anions in the dry IL, and additional H20 molecules in the humid IL. Free energy perturbation calculations predicted that in the dry liquid [F] is preferred over [Cl] , [Br] and [I]- in contrast to the aqueous solution where [L4+] was selective for [Cl]-. In the humid liquid no [F] /[C1] discrimination was observed, showing the importance of small amounts of water on the complexation selectivity [118]. 

The free energy, enthalpy, entropy, and volume of the hydrated electron are measurable in principle from the temperature and pressure dependencies of the forward and reverse rates of the unimolecular reaction of this species with water to form hydrogen atom and hydroxide ion. Data presently available determine values only for free energies of activation in both directions and for enthalpy and entropy of activation in one direction. Values for the other properties can be predicted if it is assumed that the enthalpy, entropy, and volume of the hydrated electron can be calculated by extrapolating measurements on halide ions to the radius (2.98 A.) necessary to fit the free energy data. The predictions for enthalpy and entropy are thought to be reasonably accurate, but the value for volume change is less reliable. 

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