Alkaline oxides, thermodynamics

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

The data given in Tables 1.9 and 1.10 have been based on the assumption that metal cations are the sole species formed, but at higher pH values oxides, hydrated oxides or hydroxides may be formed, and the relevant half reactions will be of the form shown in equations 2(a) and 2(b) (Table 1.7). In these circumstances the a + will be governed by the solubility product of the solid compound and the pH of the solution. At higher pH values the solid compound may become unstable with respect to metal anions (equations 3(a) and 3(b), Table 1.7), and metals like aluminium, zinc, tin and lead, which form amphoteric oxides, corrode in alkaline solutions. It is evident, therefore, that the equilibrium between a metal and an aqueous solution is far more complex than that illustrated in Tables 1.9 and 1.10. Nevertheless, as will be discussed subsequently, a similar thermodynamic approach is possible. 

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

Figure 13.19 summarizes the reaction mechanism starting from Sn(OH) + or Sn(OH)e in acidic media in alkaline media. As in the case of Pd, Sn02 oxide is spontaneously formed by dehydration due to an internal oxolation reaction promoted by a strong polarization of the O-H bond of the hydroxide. Thermodynamically stable species with respect to pH are presented in Fig. 13.20. Various molecular cationic species with different hydroxylated levels are possible in an acidic medium, whereas only Sn(OH)g is expected for a basic pH.

The products are thermodynamically favored under the oxic alkaline conditions that are characteristic of most of the ocean. Reaction rates are slow, so metal oxides tend to precipitate onto detritus or preexisting nodules because of the catalytic effect of the surfaces. The Fe and Mn are supplied by both river and hydrothermal sources. For Mn, these two sources are about equal. 

Many of these vapours will break down spontaneously to atoms in the flame. Others, particularly diatomic species such as metal monoxides (e g. alkaline earth and rare earth oxides), are more refractory. Monohydroxides which can form in the flame can also give problems. The high temperature and enthalpy of the flame aid dissociation thermodynamically, as does a reducing environment. The role of flame chemistry is also important. Atoms, both ground state and excited, may be produced by radical reactions in the primary reaction zone. If we take the simplest flame (a hydrogen-oxygen flame), some possible reactions are the following ... 

Examples of aqua cations containing transition elements are given in Table 7.2. The ions in Table 7.2 are those that are thermodynamically stable in acidic aqueous solutions except for Co3 +, which oxidizes water slowly. In alkaline solutions they are precipitated as hydroxides, oxides or hydrated oxides. 

They represent the thermodynamic data for the stability of the + 5 oxides with respect to their formation from their elements. The Nb3+ ion is not well characterized. In alkaline solutions, both Nb and Ta form polymeric anions of the formulae [H VM60 9](K v) with values for x of 0, 1, 2 or 3. 

With the smaller cyclic peptides the lack of flexibility combined with the lack of an amino anchor point means that deprotonation of the peptide groups requires more strongly alkaline conditions than for their linear counterparts. However, once formed the cyclic complexes can be thermodynamically and kinetically more stable.97 This also applies to Cu in oxidation state III. 

This is achieved in the electrochemical C02 sensor to be described now. The gas sensitive electrode in the emf sensors for C02 detection is based on an alkaline or alkaline earth carbonate.41"43 If Na2C03 is to be used, Na- (5" -alumina or Nasicon (Part I2) are suitable electrolytes. A thermodynamically well-defined sensor is obtained by using a mixture of a binary and a ternary oxide such as42 Na2C03, C02 Na+ -conductorj Na2M03, M02... 

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