Thermodynamic higher oxides

The endothermic radical lO has also been studied in the gas phase the interatomic distance is 186.7 pm and the bond dissociation energy 175 20kJmol . It thus appears that, although the higher oxides of iodine are much more stable than any oxide of Cl or Br, nevertheless, lO is much less stable than CIO (p. 849) or BrO (p. 851). Its enthalpy of formation and other thermodynamic properties are A//f(298K) 175.1 kJmol", AGf(298 K) 149.8 kJmol-, 5°(298 K) 245.5 J K- mor . 

Electrode potentials are determined by the affinities of the electrode reactions. As the affinities are changes in thermodynamic functions of state, they are additive. The affinity of a given reaction can be obtained by linear combination of the affinities for a sequence of reactions proceeding from the same initial to the same final state as the direct reaction. Thus, the principle of linear combination must also be valid for electrode potentials. The electrode oxidation of metal Me to a higher oxidation state z+>2 can be separated into oxidation to a lower oxidation state z+>1 and subsequent oxidation to the oxidation state z+>2. The affinities of the particular oxidation processes are equivalent to the electrode potentials 2 0, i-o> and E2-. ... 

Oxidation-reduction potentials for complexes in solution are determined by the relative stabilities of the complexes of the metal ion in the lower and higher oxidation states. The thermodynamic cycle connecting redox potentials and stabifity constants is shown in Fig. 7. This cycle can be useful both in rationalizing aspects of aqueous solution chemistry of complexes and in predicting or estimating values for stabifity constants or redox potentials for systems which are difficult or impossible to access experimentally. Thus knowledge of stabifity... 

Comparisons can be drawn between the chemistry of [Ni,v(dmg)3]2-and that of the sexidentate bis oxime imine complex [Ni,vMezL]2+, which is much better characterized from a thermodynamic point of view (45, 56). It can be optically resolved (54) and shows no indications of protonation above pH 0. The isostructural nickel(III) and nickel(II) complexes are subject to protonation and are much more labile to substitution. Protonation of the oxime-imine chromophore destabilizes the higher oxidation states. 

Based on the fluorite-type module theory the thermodynamic properties, hysteresis, and reactions between the homologous series can be elucidated and the structures of homologous series experimentally discovered may be modeled. Using these principles a wide range of non-stoichiometric ternary lanthanide higher oxides from RO2 to R2O3 were founded. 

Several review articles and books on the lanthanide higher oxides, which include thermodynamic properties, have been published (Eyring, 1979 Haire and Eyring, 1994 Trovarelli, 2002 Adachi and Imanaka, 1998 Adachi et al., 2005). The systematic thermodynamic data of the cerium, praseodymium, and terbium oxides can be found in Bevan s and Eyring s papers (Hyde et al., 1966 Hyde and Eyring, 1965 Bevan and Kordis, 1964). 

Of the 15 experimentally known phases of the higher oxides only five of them have been determined by X-ray and neutron diffraction using the Rietveld refinements method. To understand the thermodynamic behavior and phase reactions it is helpful to have a model of the undetermined structures. Using the experimental electron diffraction data it is possible to determine the symmetry of the unit cell and develops a transformation matrix between the fluorite and ten of the intermediate phases as shown in Table 2. The module theory provides a method for modeling the unknown structures of the homologous series of the lanthanide... 

The lanthanide higher oxides have not only peculiar thermodynamic properties, but also unique physical and chemical properties. The physical and chemical properties are presented as a macroscopic parameter, such as the electrical conductivity, the coefficient of expansion, and the conversion rate of a catalysis process. Due to the lack of knowledge of the wide range of non-stoichiometry of the oxygen-deficient fluorite-related homologous series of the lanthanide higher oxides, the macroscopically measured data of the physical and chemical properties are scattered, and therefore, based on the structural principle of the module ideas a deep understanding the relationship between the properties and structures is needed. 

In mechanism (c), oxidative transformation of the metal species takes place by holes or hydroxyl radicals (or other reactive oxygen species, ROS) attack (Figure 3). This occurs according to reaction (12) when the oxidation of the metal or metalloid to a higher oxidation state is thermodynamically possible (cases of Pb(II), Mn(II), T1(I), and As(III)). 

Thermodynamically relevant oxidation potentials of enolates were recently obtained from cyclic voltammetry studies on 60-63. Since the a-carbonyl radicals proved to be sufficiently stable, also their oxidation potentials were determined. They are much higher than the ones from the corresponding enolates and agree qualitatively with the reduction potentials of three related a-carbonyl cations as determined by Okamoto [157,158], Thus, depending on the oxidation power of the used oxidant either a-carbonyl radical or a-carbonyl cation chemistry can be triggered from enolates as demonstrated above. 

Reaction (5) and similar ones may occur at a relatively low redox potential of metal ions [109] (typically, the electrode potentials, E°, should not exceed 0.7 V). Thermodynamically favored oxidation of carbon to form surface oxides, CO or CO2, does not occur as yet under these conditions, probably because of a considerable overvoltage of the carbon corrosion. Metal ions with higher oxidation potentials may oxidize a support surface to produce various oxygen-containing carbon compounds. The latter is always accompanied by a pH shift, while the pH is practically constant in the former case [109,111]. 

At active electrodes there is a strong electrode (M)-hydroxyl radical ( OH) interaction. In this case, the adsorbed hydroxyl radicals may interact with the anode with possible transition of the oxygen from the hydroxyl radical to the anode surface, forming the so-called higher oxide (Eq. 78). This may be the case when higher oxidation states on the surface electrode are available above the thermodynamic potential for oxygen evolution (1.23 V/RHE). 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

Plutonium can exist simultaneously in four oxidation states in acidic, aqueous solutions. All four oxidation state species [Pu(III), Pu(IV), Pu(V), and Pu(VI)] can be present in solution to give thermodynamically stable systems (17). At environmental acidities, Pu(III) is probably unstable to oxidation (20) thus, the three higher oxidation states are more likely to E encountered, with Pu(IV) being the most stable (17). Because of the relatively slow oxidation-reduction reaction (Hue to the Pu-0 bond) occurring between Pu4+ and Pu022+, these two species should be the easiest to assay for by radiochemical methods. The presence of Pu02+... 

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