Enthalpy of dissolution

Since the enthalpy of dissolution of sodium hydroxide is very high, when hydroxide in the solid state is introduced into water this can cause the water to boil violently. 

However, usually the solubility increases with higher temperature. The temperature coefficent of solubility is dependent on the enthalpy of dissolution. An exothermic enthalpy of dissolution causes a decrease in solubility with increased temperature, and vice versa. 

Before we deal with these molecular aspects in detail, it is instructive to inspect the enthalpic () and entropic (-T Sj,j,) contributions to the excess free energies of various organic compounds in aqueous solution (Table 5.3). Values representative of saturated aqueous solutions of the compounds have been derived from measurements of the enthalpies of dissolution of the liquids (i.e., = AwJ //, Fig. 5.1) or solids (// ... 

The above conclusion is unfortunate for the case of polymeric solutes, because then-entropies of dissolution are unusually small. The repeat units can not become as disordered as can the corresponding monomer molecules since they are constrained to be part of a chain-like structure. Such disordering is particularly difficult if the chain is stiff. Thus, in this situation dissolution is even less likely. Crystalline polymers are also more difficult to dissolve than are their amorphous counterparts since the enthalpy of dissolution also contains a large, positive contribution from the latent heat of fusion. 

Solubility is a function of temperature because both P and y are temperature dependent. Usually y falls with increasing temperature thus solubility increases. This implies that the process of dissolution is endothermic. Exceptions are frequent and in some cases, such as benzene, there may be a solubility minimum as a function of the temperature at which the enthalpy of dissolution is zero. 

The enthalpies for the reactions of cis- and frans-[Pt(NH3)2X2] (X = Br or I) with 9.4% aqueous NH3 to yield [Pt(NH3)4X2] have been measured calorimetri-cally.122,123 The results have been used to calculate the standard enthalpies of formation of the solid cis- and trans- isomers the enthalpy of cis - trans isomerization has also been determined. The enthalpies of dissolution of the complexes K2[PtBr4], Rb2[PtBr4], and K[PtBr3(NH3)] have been measured the enthalpy of formation of Rb2[PtBr4] was calculated to be —224.4 kcal mol-1.124... 

The enthalpy of dissolution of gaseous nonpolar molecules into water is always negative at room temperature, and its absolute value is proportional to the accessible surface area of the solute molecule (Frank and Evans, 1945 Tanford, 1980 Dec and Gill, 1984, 1985a,b Olofsson et al., 1984). 

The most reliable calorimetric data on the enthalpy of dissolution of various nonpolar gases, the noble gases and hydrocarbons, are collected in Table II. The very small gaseous molecules (helium and neon) were not included. The surface area of the considered molecules, As, have been calculated from the known spatial structure of the molecule and represented either in terms of A2 or the number of contacting water molecules, Nt, assuming that each water molecule occupies an area of about 9 A2 (Hermann, 1972). The direct correlation between the enthalpy of dissolution of a gas in water, A H, and the surface area of the solute molecule, At, or the number of water molecules contacting the solute molecule, Ns, is seen from column flve in Table II. 

The enthalpy of dissolution of different liquid hydrocarbons in water provides additional information. The transfer of a nonpolar molecule from the pure liquid phase (1) to water (w) can be represented by two steps (1) transfer from the liquid phase to the gaseous phase, i.e., the vaporization, and (2) transfer from the gaseous phase into water (Fig. 8). Consequently, the enthalpy of transfer can be presented as... 

The relationship between activation energies and the expected effects of substituents has also been examined (63JA329). Lack of correlation indicates reaction on the conjugate acid, so that the activation energy includes the enthalpy of dissolution and the activation enthalpy of exchange. Appropriate correction for the former then leads to satisfactory Hammett plots. 

For the determination of the enthalpies of dissolution of solids in HF, a 5. 5 ml aliquot of 25% HF was placed in the sample cell and the acid was covered with a thin layer ( h) of paraffin oil ( Pro-labo. Rectapur) which is inert to HF. The Kel-F capsule (g) containing the solid was placed on the oil layer. Sufficient buoyancy was ensured by the latter any attack of the sample powder by HF vapor or by the solution was thus avoided. The reference cell contained the same volume of HF than the sample cell. 

These conclusions are in agreement with DTA and TG data. Endo-effect at 365°C, which is due to dehydration Mg(OH)2, is observed for activated sample, too. Vice versa, it is absent for the activated mixture of Mg(OH>2 with Si02, starting from 1 h. This means that dehydration occurs as a result of chemical interaction. The enthalpy of dissolution of this mixture passes through a maximum versus activation time, as in the case of activated talc, Mg3Si70,o(OH)2. 

Enthalpy of dissolution is the enthalpy connected with the dissolution of the given solution it depends on the composition and can be calculated from the integral dissolution enthalpies of solutions in the final and starting states ... 

Integral enthalpy of dissolution is the same as in the previous case, when one of the components is in the solid or gaseous state it depends on the composition of the solution and for dilute solutions, it approaches a certain limiting value ... 

Utilization of the dissolution enthalpies in Eq. (4.36) is justified when the individual phases are adequately diluted in an amount of the chosen solvent. With regard to the error in the solution calorimetry, the enthalpy of mixing and the enthalpy of dissolution could be neglected only when the amount of the solvent in the solution formed has not changed. 

The sum of the two values, the enthalpy of cooling, Acooi f, and the enthalpy of dissolution, AgoiT/, is the so-called relative enthalpy, Hrei, of the sample. From the temperature dependence of the relative enthalpy, the heat capacity as well as all the following enthalpy, changes could be calculated. 

The enthalpies of dissolution of Se02(cr) and Sep4(l) in aqueous 1 M NaOH were measured by Carre et al. [79CAR/GER]. The calorimetric results thus obtained were combined with pertinent literature data on standard enthalpies of formation and heats of dilution to yield the standard enthalpy of formation for Sep4(l), Af//°(SeF4, 1,... 

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