Enthalpy from binary oxides

Af,ox m standard molar enthalpy of formation (of a ternary oxide) from (binary) oxides... 

Kubaschewski (1972) collected and compared the enthalpies of formation of complex oxides from binary oxides. He did not offer any systematic correlation of these enthalpies with structural properties. Hoppe (1966,1970a, b, 1975) developed the MAPLE concept (Madelung part of lattice energy) as a tool to guide the structural interpretation of bonding in complex oxides and halides. It requires as input parameters the unit cell of a compound and positions of all atoms, and it treats the crystal as an ionic array of point charges. If crystal structure determinations have been properly done, MAPLE calculations for a complex compound are within 2% (sometimes larger, sometimes smaller) of the MAPLE values of the binary (parent) compounds. Therefore, purely ionic-model calculations are not suflftciently sensitive to correlate quantitatively with the relatively small enthalpies of solid-state complexation. 

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. 

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,... 

As the two binary compounds approach each other in acid/base character, the enthalpy of formation gets less negative. For ternary compounds formed from very similar binary oxides (for which the data are not shown), such as (AI2O3 + SiC>2), (CuO + Fe2C>3), and (Fe2C>3 + TiC>2), the AtH values are, in fact, endothermic. 

Conclusions These experiments support a hypothesis that acid/base considerations determine the energetics and stabilities of ternary oxide formation. The enthalpies of formation of the ternary oxides from the constituent binary oxides support this hypothesis, since the enthalpies get increasingly more exothermic with increasing strength of the acids and bases that react. Across the... 

This table lists standard enthalpies of formation AH°, standard third-law entropies S°, standard free energies of formation AG°, and molar heat capacities at constant pressure, Cp, for a variety of substances, all at 25 C (298.15 K) and 1 atm. The table proceeds from the left side to the right side of the periodic table. Binary compounds are listed under the element that occurs to the left in the periodic table, except that binary oxides and hydrides are listed with the other element. Thus, KCl is listed with potassium and its compounds, but CIO2 is listed with chlorine and its compounds. 

Gryzbowska et al. [106] compared the reaction products formed when pulses of allyl iodide or propene were passed over bismuth oxide or molybdenum oxide. A clear limitation of these experiments is that even the simplest bismuth molybdate catalysts contain neither bismuth oxide nor molybdenum oxide, but instead are made up of a binary oxide of bismuth and molybdenum, whose structure is different to that of bismuth oxide and molybdenum oxide. Gryzbowska et al. selected allyl iodide because of the very low bond dissociation enthalpy associated with the C-I bond, implying that a surface allyl species would readily form from this starting material. In addition, a lower reaction temperature was required for the reaction of allyl iodide than for propene reflecting the greater inherent reactivity of the former. 

There are no experimental data for enthalpy of formation of BaThOsCcr), but there are two measurements of the Gibbs energy of formation of BaThOsCcr). Mishra et al. [1999M1S/AL1] have studied the decomposition pressnre of BaO(g) from the reaction BaThOsCcr) Th02(cr) -i-BaO(g) by Knudsen effusion from 1770 to 2136 K. As discussed in Appendix A, these resnlts correspond to a Gibbs energy of the formation reaction from the binary oxides ... 

The authors also discuss the relationship between the standard enthalpies of the formation reactions of the various zircon-type orthosilicates MSi04 from the binary oxides and the ionic radii of the tetravalent M cations. 

All of the actinides from Th through Cf form dioxides but several of these have not been studied thermodynamically, due in part to their instability and to limited availability (e.g., it is very difficult to prepare multi-milligrams of Cf02 even though such quantities of the isotope are available). Plots of enthalpy of solution for the f elements have been established (Morss 1986) which permit estimating values for the other actinide dioxides. Although binary oxides above the dioxide stoichiometry are known for some of the actinides (Pa, U, Np), little thermodynamic data are available for these oxides. 

Although periodic trends in enthalpies of formation are often striking, these trends can in general not be used to estimate accurate data for compounds where experimental data are not available. Other schemes are frequently used and these estimates are often based on atomic size and electronegativity-related arguments. As an example, the enthalpy of formation of a ternary oxide from the binary constituent oxides, i.e. the enthalpy of a reaction like... 

Figure 7.15 (a) Enthalpy of formation of ternary oxides and nitrides from their binary constituent compounds as a function of the ratio of ionic potential [16]. Reprinted with permission from [16] Copyright (1997) American Chemical Society, (b) Gibbs energy of the oxide-sulfide equilibrium for group 1 and 2 metals at 1773 K as a function of the optical basicity of the metal. 

The only stable binary fluoride of krypton is the difluoride, KrF2, and all the known chemistry derives from this molecule. Early reports of the preparation of KrF4 have not been substantiated, and no simple stable oxides or oxide fluorides have been isolated. However, small amounts of the violet free radical, KrF, have been observed following y-irradiation of KrF2 (.2—4). There has also now been direct observation of [KrO ]+ (n = 1, 2) and [KrOH]+ by Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, for which ab initio theoretical calculations suggest that the two species are covalently bonded with bond dissociation enthalpies of 237.7 and 169.5 kJ mol1 respectively (21). 

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