Energy of oxidation

If this energy is positive, the material is stable if negative, it will oxidise. The bar-chart of Fig. 21.1 shows the energies of oxide formation for our four categories of materials numerical values are given in Table 21.1. 

Ceramics themselves are sometimes protected in this way. Silicon carbide, SiC, and silicon nitride, Si3N4 both have large negative energies of oxidation (meaning that they oxidise easily). But when they do, the silicon in them turns to Si02 which quickly forms a protective skin and prevents further attack. 

Figure 23.3 shows the voltage differences that would just stop various metals oxidising in aerated water. As we should expect, the information in the figure is similar to that in our previous bar-chart (see Chapter 21) for the energies of oxidation. There are some differences in ranking, however, due to the differences between the detailed reactions that go on in dry and wet oxidation. 

The bonding agent technique is usually not applicable to the metal particles in the composite. However, the surface of the metal is almost invariably covered by a thin (40-80 A) oxide layer [50]. The free energy of oxide surfaces is normally quite large (10 mJ/m ) to allow quick wetting by most organic polymers (40-60 mJ/m ). Additionally, the metal surface may provide two... 

In summary for non-metal transfer situations chemical thermodynamics is a useful guide to probable behaviour. The transfer of a non-metal, X, dissolved in a molten metal, M to another metal M", will depend on the relative free energies of formation of M X and M X (see Section 7.6). Thus sodium will give up oxygen to Zr, Nb, Ti and U, as the free energy of oxide formation of these metals is greater than that for sodium on the other hand, sodium will remove oxygen from oxides of Fe, Mo and Cu unless double oxides are formed. 

Thermoanalytical techniques such as differential scanning calorimetry (DSC) and thermogravi-metric analysis (TGA) have also been widely used to study rubber oxidation [24—27]. The oxidative stability of mbbers and the effectiveness of various antioxidants can be evaluated with DSC based on the heat change (oxidation exotherm) during oxidation, the activation energy of oxidation, the isothermal induction time, the onset temperamre of oxidation, and the oxidation peak temperature. 

The theory of the accumulation of the Gibbs energy of oxidation of carbonaceous substrates in the form of the bond of the phosphate ion to... 

The first term on the right-hand side is in this ideal solution approach given by the standard Gibbs energy of oxidation... 

The formation of an oxide layer is thermodynamically favourable and kinetically rapid at room temperature, but as the temperature rises, the free energy of oxide formation (originally negative) increases to the point where the metal, oxide and oxygen are in equilibrium. At temperatures above this equilibrium value, and if the oxygen partial pressure is low enough, the oxide can decompose. 

In general, in ICP-MS, metal argide ions (MAr+) are observed at lower intensities compared to the dimeric metal oxide ions (MO+). Both ionic species correlate with the bond dissociation energies in the ICP or by the expansion of plasma in the vacuum. A correlation of measured oxide ion intensities (MO+) and experimentally determined or theoretically calculated bond dissociation energies of oxides has been found in laser mass spectra using a LAMMA 500 (laser microprobe mass analyzer, Leybold Hereaus AG, Cologne) by Michiels and Gijbels.52... 

In many organisms, a central energy-conserving process is the stepwise oxidation of glucose to C02, in which some of the energy of oxidation is conserved in ATP as electrons are passed to 02. 

The next step is another oxidative decarboxylation, in which a-ketoglutarate is converted to succinyl-CoA and C02 by the action of the a-ketoglutarate dehydrogenase complex NAD+ serves as electron acceptor and CoA as the carrier of the succinyl group. The energy of oxidation of a-ketoglutarate is conserved in the formation of the thioester bond of succinyl-CoA ... 

The citric acid cycle (Krebs cycle, TCA cycle) is a nearly universal central catabolic pathway in which compounds derived from the breakdown of carbohydrates, fats, and proteins are oxidized to C02, with most of the energy of oxidation temporarily held in the electron carriers FADH2 and NADH. During aerobic metabolism, these electrons are transferred to 02 and the energy of electron flow is trapped as ATP. 

As a result of this short-circuiting of protons, the energy of oxidation is not conserved by ATP formation but is dissipated as heat, which contributes to maintaining the body temperature of the newborn. Hibernating animals also depend on uncoupled mitochondria of brown fat to generate heat during their long dormancy (see Box 17-1). 

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