Enthalpies of Formation from the Oxides

The enthalpies of formation from the oxides of thirty-two 2-3 and 2-4 spinels, among others Ni2Ge04, have been determined by solution calorimetry in molten oxide solvents at 970 K. Regularities in the thermodynamics of spinel formation and the olivine -spinel transition for orthosilicates of Mg, Fe, Mn, Co, and Ni have been discussed. 

The enthalpy of formation from the oxides of Mg2Sn04 and Co2Sn04 were found by oxide solution calorimetry. Using thermochemical data for the formation of olivine, for olivine - spinel transitions and for the transformation of quartz to stishovite, pressures for the disproportionation of silicate spinels were calculated to be in the range 150 - 200 kbar. 

A convention in common use as an alternative to energies and enthalpies of formation from the elements is energies and enthalpies of formation from the oxides. Thus instead of having... 

Enthalpies of formation (from the oxides) of MgAl204 and AlgSiOs (kyanite and sillimanite) have been determined by oxide melt solution calorimetry at 965 and 1173... 

A formalism defining the enthalpies of formation from the oxides of anhydrous zeohtes was proposed by [lOMlj. The formalism was based on (1) a relationship between the measured enthalpies of formation of zeosils and a parameter characterizing the natnre of the zeolite framework represented by the ntrmber of tetrahedral atoms per 1000 (FD) (2) electronegativity difference. For a constant framework (or same structrrral zeolite... 

Calculate the enthalpies of formation from the oxides of Mg2Sn0i and Co2Sn0l spinels. Would both these compounds be stable to low temperatures at atmospheric pressure ... 

Whereas the enthalpy of formation of A12Si(>5 from the elements is large and negative, the enthalpy of formation from the binary oxides is much less so. Af ox m is furthermore comparable to the enthalpy of transition between the different polymorphs, as shown for A SiOs in Table 1.5 [3], The enthalpy of fusion is also of similar magnitude. 

For instance, let us consider the compound clinoenstatite. Its enthalpy of formation from the constituent oxides is... 

Although in crystalline phases determination of the enthalpy of formation from the constituent elements (or from constituent oxides) may be carried out directly through calorimetric measurements, this is not possible for molten components. If we adopt as standard state the condition of pure component at T = 298.15 K and P = bar , it is obvious that this condition is purely hypothetical and not directly measurable. If we adopt the standard state of pure component at P and T of interest , the measurement is equally difficult, because of the high melting temperature of silicates. 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

Finally, in Table I, Column Qq refers to the heat of dissociation of 02 from the oxide to the next lower oxidation state, calculated per half mole 02. The necessary data for the enthalpies of formation (Hf) were derived from Handbook of Chemistry and Physics (45th ed., 1964-1965). 

Because most substances cannot be prepared directly from their elements, heats of formation of compounds are seldom determined by direct measurement. Instead, Hess law is employed to calculate enthalpies of formation from more accessible data. The most important of these are the standard enthalpies of combustion. Most elements and compounds combine with oxygen, and many of these oxidations are highly exothermic, making the measurement of their heats relatively easy. 

We have already mentioned the use of N2H4 in rocket fuels. The stored energy in explosives and propellants ( high energy density materials ) usually arises either from oxidation of an organic framework, or from an inherent high positive enthalpy of formation. For the hydrazinium salt [N2H5]2... 

In some sources, the enthalpies of formation or the Gibbs energies of oxide compounds with several cations (silicates, aluminates etc.) are reported in terms of the reactions of formation from the constituent oxides. In such cases the data have been corrected with respect to the corresponding basic data of the constituent oxides presented in this work. 

A/J°(CaFe204) = -17500 ( 400) J-mol The standard enthalpy of formation from oxides [2002For]... 

The natural paulingites were studied calorimetrically [0801] the enthalpies of formation from oxides and elements were determined - Table 8. 

Standard enthalpies of formation from oxides (A,JT°) and from elements H°), and partial molar enthalpy of oxygen, evaluated for 298 K and the listed exact compositions, according to drop-calorimetry for units see the... 

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