Hydration enthalpy, molar

In the second hypothetical step, we imagine the gaseous ions plunging into water and forming the final solution. The molar enthalpy of this step is called the enthalpy of hydration, AHhvd, of the compound (Table 8.7). Enthalpies of hydration are negative and comparable in value to the lattice enthalpies of the compounds. For sodium chloride, for instance, the enthalpy of hydration, the molar enthalpy change for the process... 

The derivation of values of conventional and absolute molar enthalpies and molar entropies of hydration of ions... 

The conventional thermodynamic standard state values of the Gibbs energy of formation and standard enthalpy of formation of elements in their standard states are A(G — 0 and ArH = 0. Conventional values of the standard molar Gibbs energy of formation and standard molar enthalpy of formation of the hydrated proton are ArC (H +, aq) = 0 and Ar// (H +, aq) = 0. In addition, the standard molar entropy of the hydrated proton is taken as zero 5 (H+, aq) = 0. This convention produces negative standard entropies for some ions. 

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. 

Table 2.4 Estimated conventional molar enthalpies of hydration for the cations of Group 1 elements (kJ mol-1) /(H, g)= 1312kJ mot. AaH (H. g) = 218kJmor ...

The Absolute Value for the Standard Molar Enthalpy of Hydration of the Proton ... 

Figure 2.10 Plots and trend lines of the conventional molar enthalpies of hydration of the Group 1 cations and Group 17 anions against ineir ionic radii the radii of the Group 1 cations have been increased by 50 pm also included are the values of the conventional enthalpies of hydration of the Group 1 cations minus 1110 kJ mol 1 and the conventional enthalpies o( hydration of the Group 17 anions plus 1110kJ mol-1...

Absolute Values of the Standard Molar Enthalpies of Hydration of Ions... 

Table 2.6 Ionic radii and absolute standard molar enthalpies of hydration lor the Group 1 cations...

Table 2.8 Standard absolute molar enthalpies of hydration of some main group cations at 25 °C (in kJ mol" )...

Note that gw0 is the molar Gibbs energy of formation of the standard hydrate at the reference conditions (To and Pq), ht is the molar enthalpy, and vt is the molar... 

The molar enthalpy of water in the standard hydrate can be expressed as... 

In electrochemistry we make it a rule that the standard chemical potential ju. of hydrogen ions is set zero as the level of reference for the chemical potentials of all other hydrated ions. The standard chemical potentials of various hydrated ions tabulated in electrochemical handbooks are thus relative to the standard chemical potential of hydrogen ions at unit activity in aqueous solutions. Table 9.3 shows the numerical values of the standard chemical potential, the standard partial molar enthalpy h°, and the standard partial molar entropy. 5 ,° for a few of hydrated ions. 

Table 9.3. Standard chemical potential jj°, standard partial molar enthalpy h°, and standard partial molar entropy s,° for a few hydrated ions Standard state 101.3 kPa, 298 K, unit activity in molality scale.

Fig. 5-5. Schematic one-dimensional relative enthalpy diagram for the exothermic bimolecular displacement reaction HO + CH3—Br —> HO—CH3 + Br in the gas phase and at various degrees of hydration of the hydroxide ion [485]. Ordinate standard molar enthalpies of (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). .. (e) as they occur in the chemical reaction. The barrier heights ascribed to the activated complex at intermediate degrees of hydration were chosen to be qualitatively consistent with the experimental rate measurements cf. Table 5-3 [485]. Possible hydration of the neutral reactant and product molecules, CH3—Br and HO—CH3, is ignored. The barrier height ascribed to the activated complex in aqueous solution corresponds to the measured Arrhenius activation energy. A somewhat different picture of this Sn2 reaction in the gas phase, which calls into question the simultaneous solvent-transfer from HO to Br , is given in reference [487].

So-called absolute standard molar enthalpies of hydration of a number of individual ions, including Th", have been obtained by using a combination of experimental data and estimates based on the extra-thermodynamic assumptions that... 

The thermodynamic parameters of hydration for many ions have been determined [121,125,126]. Table 3 gives the values of the standard molar Gibbs energy of hydration AGh and standard molar enthalpy of hydration AH , at 25°C for the alkali metal cations. The tabulated values are based on the respective choices A= - 1056 kj/mol and AHh(H ) = -1103 kj/mol, which result from the extrathermodynamic assumption that the thermodynamic parameters of the tetraphenylarsonium cation and tetraphen-ylborate anion are equal [127]. This reasonable and useful assumption, often... 

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