Solution enthalpies oxides

The enthalpy of formation of UOCl has been calculated from the heat of solution, with oxidation, in aqueous sulphuric acid, UOCl (298 K) = -833.87 4,23 kJ mol but the UOCl sample used was impure, so the value should be regarded as tentative (198,199). 

A general conclusion is that organic compounds are not adsorbed from aqueous solution by oxides when Eh,o differs from a characteristic resonance frequency, E es, of the oxide by more than about 0.1 eV. When adsorption occurs it is probably accompanied by the formation of a chemical bond as a result of the coincidence of donor levels of the adsorbate and acceptor levels of the adsorbent. This behaviour in adsorption from solution contrasts with adsorption from the gas phase where no correlation has been found between the enthalpy of adsorption, or the shift of vibration frequency of the OH groups accompanying adsorption, with the electronic properties of oxides. No explanation of this difference is offered. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

In recent years, aqueous solutions of Xe03 have been used to oxidize a species in solution, from which A[H°m can be calculated when AH for the oxidation reaction is combined with AH for other reactions. The noble gas oxide Xe03 is used as an oxidant because of its stability and the fact that the final reaction product is Xe(g), which has a zero enthalpy of formation and is easily removed from the reaction mixture. As an example, O Hare4 has reported AfHcm for UCI4. We will not go through the details of his procedure, but the critical step involved measuring A TH for the reaction... 

M HNO3) and from 3 M HNO3 using dilute solutions of DHDECMP, HHDECMP, 0(j)D[IB]CMP0 in o-xylene showed that the increase in D/ in the series phosphone-phosphinate-phosphine oxide was due primarily to an increase in the enthalpy of extraction. 

Because reactions in the body take place in aqueous solution, this value is not the same as the enthalpy change for the reaction in the body. However, the two values are fairly close. Therefore, the oxidation of glycine, which we have found to be exothermic, is a potential source of energy in the body. 

Figure 7.4 Peak potential values of adatom redox processes on Pt(l 11) and Pt(lOO) electrodes in 0.5 M H2SO4 solution, as labeled, plotted against the enthalpy of formation of the corresponding bulk oxide. Lines are included to indicate the tendency (the full line corresponds to the filled squares, and the dashed line to the open circles).

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

The Frost diagrams for the first series of the d block elements in acidic solution, pH = 0, given in Figure 7.11(b) show many similarities with the variation of the enthalpy of formation of the oxides. Only the oxidation states observed for solid oxides are included. 

The importance of the size of the solute relative to that of the solvent mentioned above is evident also from experimental determinations of the extent of solid solubility in complex oxides and from theoretical evaluations of the enthalpy of solution of large ranges of solutes in a given solvent (e.g. a mineral). The enthalpy of solution for mono-, di- and trivalent trace elements in pyrope and similar systems shows an approximately parabolic variation with ionic radius [44], For the pure mineral, the calculated solution energies always show a minimum at a radius close to that of the host cation. 

In the ideal solid solution model used, the enthalpy and entropy of oxidation are independent of composition. 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

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