Enthalpies formation, hydrated

The results for the enthalpies of hydrated hydroxide and chloride anions may then be used to estimate the enthalpies of formation of the cations formed from soluble hydroxides or chlorides. For example, the reaction of sodium with liquid water ... 

The enthalpies of formation of aqueous ions may be estimated in the manner described, but they are all dependent on the assumption of the reference zero that the enthalpy of formation of the hydrated proton is zero. In order to study the effects of the interactions between water and ions, it is helpful to estimate values for the enthalpies of hydration of individual ions, and to compare the results with ionic radii and ionic charges. The standard molar enthalpy of hydration of an ion is defined as the enthalpy change occurring when one mole of the gaseous ion at 100 kPa (1 bar) pressure is hydrated and forms a standard 1 mol dm-3 aqueous solution, i.e. the enthalpy changes for the reactions Mr + (g) — M + (aq) for cations, X (g) — Xr-(aq) for monatomic anions, and XOj (g) —< XO (aq) for oxoanions. M represents an atom of an electropositive element, e.g. Cs or Ca, and X represents an atom of an electronegative element, e.g. Cl or S. 

The cycle allows the overall enthalpy of formation of the aqueous solution of cations and anions to be sub-divided into stages whose enthalpy changes are known except for the two enthalpies of hydration, allowing their sum to be estimated. The equation to be solved is ... 

The overall change of enthalpy represents the enthalpy of formation of the hydrated cation, Mz + (aq), and has a value as given in Tables 2.2 and 2.3 for particular cases. The value consists of contributions from the enthalpy of atomization of element M and the appropriate sum of its ionization enthalpies, and the enthalpy of hydration of the gaseous ion. 

It also includes the enthalpy of ionization of the hydrogen atom (equal to, but opposite in sign to, the electron attachment enthalpy of the gaseous proton), the enthalpy of atomization of dihydrogen and the enthalpy of hydration of the proton. The enthalpy of formation of the cation is estimated by use of the equation ... 

The subsequent additions of water molecules are associated with significantly smaller releases of enthalpy and are consistent with the formation of hydrogen bonds. The total enthalpy change for the production of the ion H+(H20)6(g) is —1116 kJ mol-1, a value not significantly different from that adopted for the enthalpy of hydration of the proton. Further additions of water molecules to the HT(H20)(,(g) ion are associated with enthalpy changes that arc not significantly different from the enthalpy of evaporation of water (44 kJ mol - ) and do not appear to add to the stability of the hydrated proton. 

Estimate the standard enthalpy of hydration of the cyanide ion. given that the lattice enthalpy of potassium cyanide is —692 kJ mol-1 and its standard enthalpy of formation is —113 kJ mol- . 

The basis of the estimations of the absolute enthalpies of hydration of the main group ions is dealt with extensively in Chapter 2. In this section, the same principles are applied to the estimation of the enthalpies of hydration of the monatomic cations of the transition elements, i.e. those of the ions M" +. The standard enthalpies of formation of the aqueous ions are known from experimental measurements and their values, combined with the appropriate number of moles of dihydrogen oxidations to hydrated protons, gives the conventional values for the enthalpies of hydration of the ions concerned. Table 7.4 contains the Gibbs energies of formation and the enthalpies of formation of some ions formed by the first-row transition elements, and includes those formed by Ag, Cd, Hg and Ga. 

Table 7.14 contains the observed and calculated values of Ea for the reductions of the +2 ions to their respective metals. It also contains the thermochemical data related to the transformation of the solid metal atoms to M2 + ions in solution. The calculated values of the potentials are those based upon the standard enthalpies of formation of the + 2 ions in aqueous solution. The entropy terms are neglected and this omission is justified from the close agreement between the calculated and observed values of the potentials given in Table 7.12. The enthalpies of hydration of the +2 ions are those given in Table 7.5.

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

The enthalpy of hydrate formation of simple natural gas hydrate formers (from gas and water or ice) is given in Table 4.7 taken from the dissertation of Kamath (1984). Note that each component has two temperature regions, above and below the ice point, with a A77 difference related by the heat of fusion at the ice point. 

Mehta and Sloan (1996b) present data in Table 4.9 for 19-structure H hydrates formers along uni variant four-phase lines. With only three exceptions, the enthalpy of hydrate formation is 79.5 kJ/mol 7%. In each case, methane occupies the 512 and the 435663 cages while the larger guest occupies the 51268 cage. 

AH Enthalpy of hydrate formation calculated from the above data using the Clapeyron equation assuming ideal hydration. 

Because of the precision of their alkene enthalpy of hydration measurements from Wiberg and coworkers30 31, the authors refined the liquid enthalpies of formation for 1-pentene,... 

In these calculations, the electron affinity of the methyl radical has been taken1 as 27 kcal.mole-1. The other enthalpy terms are all well-known quantities the enthalpies of hydration of individual ions have been assigned as done by Valis ev (see ref. 2) and the enthalpy of hydration of the gaseous methyl anion has been taken as that of the bromide ion. It can be seen from Table 1 that not only is the formation of the methyl anion energetically very unfavoured in the gas phase, but it is also endothermic to the extent of 54 kcal.mole-1 in aqueous solution. A check on this final result can be made by consideration of the standard entropy change for the reaction... 

The enthalpy change for the second step (formation of hydrated ions from the gas phase ions) is designated as the enthalpy of hydration, A//, yd (see Table 8.7 in the text). 

Based on the NBA TDB auxiliary value for Af//° (H2O, 1, 298.15 K), and the selected enthalpy of formation of NiCl2(cr) (-(304.90 0.11) kJ-mol ), Thomsen s values would lead to an enthalpy of hydration of - (2104.9+ 1.8) kJ-mor , where the uncertainty (estimated here) is slightly lower than if the two heat of solution measurements were independent, because it is probable there were compensating errors. 

The crystal structures of the three heavier thallium(I) halides have been established and lattice energies calculated the inert pair of T1+ is apparently insignificant in terms of the stability of the lattice. The enthalpy of hydration of the Tl ion was also derived. Gas phase (TlBr, Til) and matrix isolation studies (TlCl, TlBr, Til) have shown that TlX and TI2X2 species are important.Complex formation in aqueous solution decreases in the order Cl > Br > I, and as with the fluorides, the double salts show no evidence of anionic complex formation by Tl Finally, it is important to note that TII3 is formulated as Tr(lT) from X-ray studies.Thermal decomposition yields TI3I4, whose structure has not been reported. 

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